Laser based mid manufacturing processes and qualification methods regarding their metallization

Laser Based MID Manufacturing Processes and Qualification
Methods Regarding Their Metallization
Jamshed Karim Babu and Andreas Brose
Otto-von-Guericke University of Magdeburg
Institute for Micro and Sensor System
Universitätsplatz 2, 39106 Magdeburg
jamshed.babu@st.ovgu.de
Abstract— Molded Interconnect Device (MID) has added a
new dimension to different engineering sectors which is in constant
need of improvement in functionality, integration density and
miniaturization. Due to their versatile possibilities for functional
integration and spatial design freedom, MID offers a reasonable
solution and limitless possibilities. The fields of application of MID
is increasing day by day and it is frequently being used in crucial
fields like safety and security relevant systems where reliability is
an important issue. This reliability issue is highly influenced by the
adhesion strength of the MID. However, there are no general
standards or guidelines that can test or evaluate the Molded
Interconnect Devices. To determine the adhesion strength of the
metallization on the Molded Interconnect Devices a limited
number of effective and suitable test methods are available. In this
paper, firstly laser-based MID structuring processes are described
keeping the focus on LDS process. In addition, suitable LDS
parameters for metallization, importance of adhesion and
frequently used test methods to determine the Adhesion strength
are presented along with their assessment regarding their
suitability in different cases along with focusing on the suitability
of Laser Direct Structuring process among the available MID
structuring processes. Additionally, new hot pin pull test is
described in detail. The importance of testing standards is
illustrated by the impact of influencing factors on the test results,
such as the wetting of the test structures with solder and the
temperature. Finally, the results of the comparative investigations
has been described which show promising conclusion concerning
a low standard deviation and reproducibility.
Keywords—Adhesion, Molded Interconnect Device, Adhesion
Strength Test, Hot Pin Pull Test, Peel Test, Shear Test, Laser Direct
Structuring.
I. INTRODUCTION
3-Dimensional Molded Interconnect Devices are a fast
emerging technology. It combines traditional injection molding
of plastic products with 3-dimensional conductor pattern
generation through selective metallization of the surface for
electrical or electronic circuit interconnection. On a more formal
definition, the Molded Interconnect Device (MID) is injection
moulded (plastic) part with electrically conductive circuit paths
or interconnects on the surface of the substrate. Through this
specific technology, electrical and mechanical functions are
integrated into the same component which enables the product
size, complexity and manufacturing process to be significantly
reduced. By this technology we are provided with an electro-
mechanical method of interconnecting electronic circuitry
through precise integration of connection media, such as holes,
connectors and tracks. Increased design flexibility, high volume
of production capabilities, reduced component inventory and
reliable process control e.g. use of MID in flow sensor, OLED,
multiband antennas for smartphone, ACC position sensor,
pressure sensor, insulin pump, 3D switching module, solar
sensor - these benefits are offered as a result of injection molding
of plastic parts. Again, like injection molding, metallization of
plastic by means of chemical deposition technique is a mature
technology. By combining both of the technologies and
introducing two or three dimensional conductive patterns, MID
manufacturing on a volume scale has been enabled in successful
manner.
The manufacturing processes for MID are many and varied.
Mainly the process which will be selected depends on the basis
of stated criteria. Laser structuring (additive and subtractive),
two-shot molding, hot embossing and film insert molding are the
most important processes. Plasma structuring and printing
technologies are also making their mark and being valued
gradually.
Fig. 1: Reference process steps for MID [1].
There are other minor significant technologies for now such as
masking and primer technology and physical processes of
metallization. Needless to say, all of these processes are oriented
towards the reference process of higher order MID shown in Fig.
1. The production of the MID blank which is a three-step
process, is followed up by various connection techniques to
complete the whole procedure. Soldering, conductive-adhesion
bonding, wire bonding and press fitting are the primary
connection techniques. As Fig. 1 depicts, apart from the basic
structuring processes, other structuring processes like Primer
Technology, Tampon Printing and Plasma Technology are also
used. While applying one-shot injection molding, depending on
the necessity and suitability of the situation one can go for LPKF
Laser Direct Structuring Process, ADDIMID Technology or
other alternative laser structuring processes and choose different
printing techniques such as Aerosol Jet Printing, Inkjet Printing
or Hot Embossing. Similarly a lot of options are available while

following the Film Insert Molding structuring. One can choose
from Thermoplastic Foam Molding, Injection Compression
Molding, Press Insert Molding or other varieties of Film Insert
Molding. From selecting proper thermoplastics to executing all
the processes in between, a number of key factors play an
important role that decide the quality of the final outcome in the
procedure. In this paper, different laser-based MID structuring
techniques are briefly discussed along with an enhanced
description of LDS process. Later on different LDS parameters
are discussed for creating fine pitch metallization. And lastly
various existing adhesion strength measuring tests including the
hot pin pull test were discussed and compared to highlight their
usefulness and effectiveness.
II. LASER-BASED DIFFERENT STRUCTURING
TECHNIQUES
Specifically when it comes to laser structuring, various
techniques are commonly classified as additive, semi-additive or
subtractive technique. Laser direct structuring is an additive
technique and the most important manifestations are LPKF-
LDS® and ADDIMID. Among the semi-additive techniques
MIPTEC, a process developed by Panasonic is mention-worthy.
On ceramic substrate materials subtractive laser structuring
techniques are applied commonly. A general overview of the
process steps is shown in Fig. 2.
Additive Semi-additive Subtractive
Fig. 2: Process steps in additive, semi-additive and subtractive structuring [1].
II(a). LPKF-LDS® PROCESS
More than 50% of the market is accounted by the Laser
Direct Structuring (LDS) process when it comes to structuring
MID [1]. It has become vastly popular in MID production over
the last decade due to various reasons such as offering high level
of versatility, possibilities for prototyping, series production,
low tool costs, micro-fine structures and high 3D design
freedom. In addition to that it is now being possible that a wide
range of thermoplastics can be used in this structuring technique
which widens its reach even more. As the development and
diversity regarding color and type of plastic, materials has been
increasing enormously over recent years, the LDS process holds
limitless possibilities. The following section contains a brief
overview of currently popular materials and overall process
steps of LDS.
LDS Substrates: Different types of plastics are available for
their application varieties and corresponding sets of
requirements. The selection may range from run-of-the mill
plastics such as ABS (Acrylonitrile Butadiene Styrene) through
high-specification polyamides to high-temperature plastics such
as LCP (Liquid Crystal Polymer) and PEEK (Polyether Ether
Ketone). According to the properties of the material the price
range also varies from one to another. However, due to excessive
demand in telecommunication sectors and comparatively
cheaper price, PC (Polycarbonate) and ABS are the most
commonly used material. There is also a very wide range of
plastic materials available for the LPKF-LDS® process. In most
of the cases, the plastics which are suitable for laser direct
structuring require a specific additive that is added to the blend
during compounding. The compounding is made possible by the
extreme heat resistance of this additive which also prevents
nucleation in the injection-molding process. It affects specific
properties of the plastics by only a little margin. There are
chemical substances in the additive which gets activated by the
laser irradiation and also gets exposed by ablation of the molding
skin. To prepare plastics for laser direct structuring without the
specific LDS additive, a product akin to an LDS painting system
is made available by LPKF [1]. A two-component primer/curing
agent system named ProtoPaint LDS is used for coating plastic
injection moldings with an LDS-compatible skin. In general, the
agents which can be applied to improve the adhesion are called
primers. In MID technology, materials with paint-like properties
are called primers and curing agents are those materials which
helps to separate liquid carriers from metallic constituents.
Application procedure is followed by standard spray gun or
spray coating technique where in both the cases a coat
approximately of 30 to 40 µm thickness has to be applied in two
steps and then cured. Then the plastic parts can be structured.
The results can also be comparable to those achieved with LDS
plastics. However, this process is only applied primarily for
prototyping as the constant-use properties are not comparable
with those of plastics with LDS additive in the compound.
Major Steps of LPKF-LDS® Process: The major four steps of
LPKF-LDS® method are: Injection molding, laser structuring of
the plastic, metallization and surface finishing shown in Fig. 3.
Plastics which get compounded with the LDS additive are fully
compatible with the injection-molding process. The principle of
ablation and nucleation by laser irradiation mostly determines
the basis of Laser Direct Structuring. According to [1], for the
patterning Nd:YAG laser, about 1 to 2 µm of material from the
surface gets ablated while simultaneously activating the additive
necessary for metallization creating a microscopically rough
Mold plastic body
Laser-structure
plastic
Surface finishing
Chemical copper
Mold plastic body
Surface activation
Chemical copper
Apply photoresist
Laser structuring
of photoresist
Galvanic copper
and surface
finishing
Etch away photoresist
and metallization
Mold plastic body
Surface activation
Chemical copper
Electrolytic
copper
Galvanically build
etch resist
Laser structuring
of etch resist
Etch away copper
Surface finishing

surface. This surface contains nuclei embedded in the micro-
cavities produced by the laser which creates an adhesion bond
between the plastic and the metallized layer without any need
for additional post-ablation treatment. These nuclei are
catalytically active as well. At this time processing rates can get
as high as 4000 mm/s which can be constantly diminishing with
increasing three-dimensionality and the complexity of part. This
happens because of the limitation of scanning speed for focus
tracking in Z axis. The more wide the angle to the target surface,
the more rapidly the focus has to be adjusted. After the laser
structuring of the substrate, metallization is done with Cu-Ni-Au
buildup. Metallization step is generally followed by couple of
extra cleaning steps which are necessary for activation, pre-
treatment and coating. After this, surface finishing is done which
makes the MID ready to go through the necessary assembly
procedures.
Fig. 3: Process steps provided by LPKF
LDS is environmentally friendly because components are easy
to sort for recycling as no etching or pickling chemicals are
required. It offers high level of functionality and low
construction volume.
II(b). ADDIMID PROCESS
Another additive laser structuring technique is ADDIMID.
It can be applied to stereo-lithographic structure components or
injection-molded plastic bodies. It bears a similar resemblance
not only in the process chains but also in modification of a
plastic by the addition of special fillers with the LPKF-LDS®
process [1]. This particular technology involves upgrading the
plastic with an additive consisting of a metal powder with an
electrically insulating cladding material. This technology is still
under development. In trials, microfine materials Cu, Ni and Al
powders with particle diameter from range of 0.5 to 1.2 µm
have been used up to date [1]. Being similar to the LPKF-LDS®
technique, in ADDIMID technique also after the raw material
has been compounded the plastic bodies are injection-molded.
In this process just like LDS technique a Nd:YAG laser with a
wavelength of 1064 nm gets used for structuring [1]. For this
purpose a 𝐶𝑂2 laser with 10.6 µm wavelength can also be used.
As a crucial process advantage both methods exploit the duality
of lasering. While the surface for structuring is partially ablated,
the fillers which are mixed through the plastic matrix gets
activated which subsequently act as the catalyst for chemical
metallization. This technique offers high flexibility, suitability
for prototyping along with series production, high 3d design
freedom and also low tooling costs. From economical point of
view it also offers the advantage of low material costs for the
additive. Regarding the structure widths of conductor traces and
spacing, nothing specific can be said as the research is still
ongoing [1].
II(c). MIPTEC PROCESS
In terms of process chains, Panasonic’s semi-additive
MIPTEC (Microscopic Integrated Processing Technology)
technique of structuring differs widely from the LPKF-LDS®
and ADDIMID. As shown in Fig. 2, after injection-molding of
the plastic body, copper plating is required [1]. After that
removal of unwanted metal is done by UV or IR lasering
followed by galvanic reinforcement of copper layers. Finally,
the etching of unwanted copper is done and the body gets ready
for surface finishing. Multiple parts are produced in a single
sheet and are then diced. Removal of the metallization takes
place in this technique meaning that with inter-conductor
spacing of the same order, very fine conductor structures of 50
µm can be produced using this process. This happens because
after the full-coverage plating, the laser structuring takes place
and a very high quality surface with very low roughness are
produced by metallization. As a result, without any post-
working steps semiconductor chips can be bonded. However,
the only materials currently compatible with the MIPTEC
process are PPA as a thermoplastic substrate and the two
ceramics AI203 and AIN. But new scopes regarding the
ceramics are opening up as with very good thermal conductivity
and minimal thermally induced expansion, ceramics have a
crucial advantage for MID LED applications in particular.
II(d). LSS PROCESS
LSS (Laser Subtractive Structuring) process has more steps
than additive or semi-additive processes (Fig. 2). After
injection-molding step, short surface activation takes place to
permit electroless copper or nickel plating. This is a chemical
pre-metallization which is then followed by a galvanic process
to build up the plating to target thickness. By application of an
activable etch resist which can be of photoresist or galvanoresist
type, the structuring takes place. Galvanoresists are removed by
laser energy whole photoresists react chemically to UV energy.
Photoresists can be further categorized as positive or negative.
After exposure, positive photoresist is soluble and it washes off.
On the other hand, solubility of the negative photoresist
decreases after the exposure. With a conductor width of 30 µm,
resist technology can be used to produce structures at a
patterning speed up to 2000 mm/s [1]. Breakage of the resist at
the edges of the structures can occur after the post-exposure
processes, which may result in high fluctuations in the width of
the insulating channels between the tracks. Another problem is
the presence of contaminants in the resist as a single grain of
dust is enough to prevent activation of resist it shadows.
Incomplete insulation results in shorting as a possible fault.
Galvanoresists can be a good alternative to avoid these
problems but it limits the patterning speed to 600 mm/s [1].
Higher patterning speed relates to major risk of incomplete
ablation of the resist with the resulting fault of shorting. Post-
working of areas which are not ablated is possible in principle,
but it adds further steps making the process complicated.
Another alternative procedure is repeated exposure of the same
areas at a higher throughput rate, which is a trade-off between
higher speed and the extra time needed for the repeat passes.

Chemical or electrochemical finishing of the parts can also be
an alternative, but this too adds complicated steps to the
process. LSS process is particularly efficient in providing 3D
injection molding parts with extensive strip conductors. Major
drawbacks of the subtractive technique are the extensiveness
and complexity in the process chain and the severe fluctuations
in the thickness of the plating.
After the overall structuring process is done different
suitable printing process is required to print the desired layout
which is followed by the metallization step. Once the
metallization takes place, assembly of the components is
required. Finishing all these steps properly, the reliability and
quality of the final MID product gets tested which is done by
different conventional test methods.
III. OPTIMIZED LDS PROCESS PARAMETERS FOR
CREATING FINE PITCH METALLIZATION
When it comes to high accuracy, Laser Direct Structuring
process has already gained quite a good reputation. Very
recently some experimental tests were carried out according to
[2] in order to study and observe the effect of the LDS
parameters on the dimensions of the micro groove. According
to [2], in the experimental work a polymer plate having
dimensions of 60x60x2 mm have been used. The material is
VESTAMID® HT plus LDS 3031 black. It is a mineral
reinforced Polyphthalamide (PPA) with glass fiber. According
to LPKF-LDS technology, this compound is designed to be
used in the production work of 3D MID. To perform the tests a
laser machine Nd:YAG laser having 1064 nm wavelength,
beam diameter of 65 µm, power in the range from 1 to 17 W,
maximum frequency 200 kHz and maximum pulse duration
23.7 ns was used. Several important parameters of LDS process
like laser power, laser scan speed, laser frequency and the width
space between the two circuit lines are studied and different
tests were performed to observe the actual effect on the MID.
The quality of the MID depends hugely upon the LDS
structuring and metallization step and the dimensions and the
profile of the laser groove plays a very important role to
determine and ensure the desired quality. It is mention-worthy
that for each and specific LDS parameters there is a different
groove dimension and profile. For achieving best groove
dimensions and profile suitable optimum laser parameters must
be obtained. 3D laser microscope scan at the surface of the
polymer has been shown in Figure 4(a) and Figure 4(b) depicts
the cross sectional area of the groove.
Fig. 4(a): 3D laser scan image of the groove profile [2].
Fig. 4(b): Important groove dimensions [2].
Fig. 4(c): Metallization thickness.
Fig. 4(c) shows the metallization thickness in the different zone,
the maximum being at the center and the minimum at the edge.
But interesting fact is that the above behavior between the
groove and metallization profile changes after two lines are
created together maintaining a specific distance between them.
And this change occurs due to heat interaction between the
grooves of the two lines, which may produce defects between
the created circuit lines depending on their distance values. A
very important relationship appeared to be present between the
minimum distance achieved between the two circuit lines and
the interactive zone width. To calculate the interactive zone
width (I.Z.W.), the minimum space (Ms) or distance between
two circuit lines and the proposal distance (Ps) of the two
grooves an equation has been proposed-
Ms = Ps – I.Z.W. (1)
During the LDS process when there is no interaction between
the heat transfers for the two grooves, equation 1 can be used.
Normally when the proposal distance (Ps) is high enough, this
effect does not occur. But whenever it requires low Ps, such as
micro MID products, it is a serious problem. By changing the
sequence of the laser structuring for the circuit lines to give
enough time for cooling and thus preventing the heat interaction
between the grooves, this effect can be reduced to a minimum
amount. To avoid metallization defect, it is very important to
use specific types of LDS parameters to produce one width of
the circuit lines. LDS parameters including laser power of 12
W, laser speed of 1000 mm/s and laser frequency of 70 kHz can
be used for the interior area of the circuit line as shown in Fig.
5. At the boundary of the circuit line another set of parameter
can be used including, laser power in range from 3 to 9 W, laser
speed of 2200 mm/s and laser frequency in range from 110 to
130 kHz [2]. These parameters were tested and under these
parameters metallization defect will not occur.
Half width of the interactive zone
Metallizationthickness

Fig. 5: The metallization process for, (a) Defect, laser power of 12 W, laser
speed of 1000 mm/s and laser frequency of 70 kHz, and (b) No defect, laser
power of 9 W, laser speed of 2200 mm/s and frequency of 90 kHz [2].
According to [2], the minimum distance (Ms) always increases
with the laser speed and decreases with the laser power. Also
Ms increases with the laser frequency. Depending on the value
of Ms, laser diameter D and minimum space distance of
metallization process (MSM), the value of Ps can be readjusted.
Mainly depending on two conditions the value of Ps can be
changed accordingly.
First: Ms – D > MSM
In this case, metallization defect will not occur. Also the value
of Ps can be reduced to the value equal to or more than the
laser beam diameter (D). So 𝑃𝑠 𝑛𝑒𝑤 = Ps – D
Second: Ms – D ≤ MSM
Metallization defect will occur in this case and the Ps value
cannot be reduced. So 𝑃𝑠 𝑛𝑒𝑤 = Ps
IV. IMPORTANCE OF ADHESION
Before moving on to different test methods to measure the
adhesion strength of MID, the influence and importance of
adhesion should be emphasized properly as one of the key
performance indicators in MID technology is the adhesion of
the metallization with the thermoplastic substrate. The
mechanical stability of the conductors and the components on
the carrier both depend on the strength of the bond between
metallization and the substrate. Adhesion is the bond between
metallization and the substrate whereas cohesion is the inner
strength of the adhesive.
Fig. 6: Cross section of a bond [4].
The adhesive stays in its normal state in the cohesion zone
whereas in the adhesion zone the adhesive has a modified
structure and composition due to its adhesion to the surface of
the substrates. In the adhesion zone the molecular interaction
between the substrate surface and adhesive takes place altering
the macroscopic properties of the adhesive in the adhesion zone.
In the transition zone between the adhesion zone and the
cohesion zone the structure, composition and macroscopic
properties of adhesive continuously changes. The adhesion
strength tests discussed below are of destructive type. These
tests are performed for qualification during the development
phase of the MID and maybe randomly during serial production
of parts which cannot be sold afterwards. Formation of strong
adhesion bond forces between molecules and an optimal wetting
behavior are very important in order to maintain the bonding
strength between metallization and the plastic substrate.
Fig. 7 (a): Creating micro-rough surface [4].
Fig. 7 (b): Adhesion bond between metallization and substrate [4].
Fig. 7 (c): Actual illustration of metallization on plastic substrate [4].
Adhesive
Substrate
Adhesion
zone
Substrate
Transition
zone
Cohesion
zone
Adhesion
zone

The most important factor that plays a key role in forming the
adhesion bond is the macromolecular construction and the
surface energy of polymer [3]. The polarities of the molecule
surface structure caused by dipole, dispersion forces or
ancillary valence, considerably affect the strength of the
adhesion bond. The enrichment of the polar groups of polymer
which can be achieved physically or chemically ensures
improved adhesion bonding between metallization and the
substrate. As mentioned earlier, while laser-activating the
substrate a micro-rough surface (Fig. 7) is created with nuclei
in it. During the metallization process Copper (Cu) is deposited
to this nuclei in plastic substrate and metallization gets firmly
mechanically anchored by the adhesion bond.
As metallization is considered the most critical step of MID
manufacturing process, adhesion bond between metallization
and polymer substrate is one of the most important factor of this
step. Adhesion bond plays an important role and contributes
deeply as it holds the metallization against the ultra-sonic power
in wire bonding, compensates thermo-mechanical stress caused
by manufacturing processes like soldering or curing and by
environmental conditions during product lifetime, creates
stability during reliability testing in the electrical connection [4].
However, different types of defects (Fig. 8) can already occur
due to the metallization which has a valuable impact on the
adhesion bonding afterwards [4]. The bond strength needs to be
in such a reliable manner that the whole device can withstand
the thermo-mechanical and mechanical loads. However, the
real challenge is determining the Adhesion strength of the bond
between the plastic substrate and the thin layers of metal.
Fig. 8: Different types of defects in Metallization of MID [4].
The structure sizes involved and the three-dimensional layout
of the MID contributes to a certain proportion to hamper the
reproducibility of adhesion tests, which is why practicality of
testing needs to be considered. One good feature of a reliable
mechatronic system is that it presupposes an adhesion bonding
of the conductor tracks on the substrate. Especially when it
comes to MID, the quality gets significantly determined by the
adhesion strength of the metallization to the substrate.
Numerous factors along the entire manufacturing chain
influence the initial adhesion strength of the MID. The adhesion
of the metallization can be significantly affected by the decision
in each process step. Thus it can influence factors like selection
of the substrate material through the design of the molding tool,
different parameters of the injection molding tool, chemistry
used for cleaning, structuring and metallization. All the
processes that come one after another such as reflow soldering,
transmission of pins by press-fit technology or wire bonding do
require a sufficient adhesion strength which depends on the
individual process and hence, further influences the initial
adhesion. Moreover in rough environmental conditions, the
adhesion strength decreases during product lifetime due to
thermal and mechanical loads as well as by chemical and
climatic influences. So, this issues also need to be taken into
account. For MID, during laser structuring or etching, a micro-
rough surface is accomplished on the basis of which mainly the
bonding mechanism between metallization and substrate
depends on. With different polymer and different laser
parameters the roughness can change. Layers with a typical
thickness of 5-15 μm copper, 5-15 μm nickel and 0.1 μm gold
are build up. And one of the toughest challenges during the
processing step is to determine the adhesion strength of these
thin metallized layers on thermoplastic substrate. To determine
the adhesion strength of MID, different conventional test
methods are applied which are mainly adopted from the existing
testing standards from PCB technology e.g. pull-off/pull out
test, peel test, tape test, shear force measurement test.
V. CONVENTIONAL TEST METHODS
Most commonly used methods which are applied to
determine the adhesion strength of MID are known as Shear
Force Measurement, Pull-off Test and Peel Test. As the test
methods differ considerably from one another in their procedure
and set-up, a direct comparison is not possible among them. The
type of failure modes also differ from each other because the
type of loading applied in order to determine the adhesion also
varies. These three methods are described below along with their
specific criteria and applicability on MID.
Pull-off Test: A very well-known method used to determine the
adhesion strength, mostly of thin surface coating is the pull-off
test [5]. Tensile forces act vertically on the test area in this test
procedure and those particular forces are measured. A dolly
needs to be soldered or glued to the composite panel to perform
this test. Once the prepared specimen is fixed in the device
which is being tested, an increasing amount of tensile force
applied to the dolly until the coating detaches from the substrate
as shown in Fig. 9.
Fig. 9: Schematic illustration of Pull-off test [5].

Finally, from the size of the detached area and the maximum
force applied to the dolly, the Adhesion strength of the bond is
calculated.
In case of other failures occurring elsewhere than between the
desired substrate and metallization, it is very much necessary to
add a reference to the fracture pattern as a supplement to the
original measured value. As MID technology differs a lot from
the conventional PCB technology due to its spatial design
freedom, a test dolly (diameter 7 mm) is recommended for
testing adhesion from one side only [2].
Fig. 10: Different failures during unsuccessful Pull-off test [5]
Due to lack of having large and planar surfaces on MID, the
continuous change in size for dolly and test structure has to be
considered as well. As a result, the positioning of the dolly and
the manual application of the adhesions become significantly
difficult which can eventually result in an increased standard
deviation. In Fig. 10, too less adhesion was applied in section 1,
excessive amount of adhesion was used in section 2 and the
position of the dolly was decentralized on the test structure in
section 3, as all of these are the results of failures performing
pull-off test on MID.
Advantage:
 Easy procedure.
 Standard tensile test equipment can be used.
Disadvantage:
 Large test pad (at least of ø 7 mm).
 High temperature impact due to soldering process if the
dolly is soldered, not glued.
 Influence by manual handling.
Peel Test: To determine the adhesion strength of conductor
tracks on printed circuit boards another common test method is
used named peel test. In order to perform the test, a pull-off
object is needed to be affixed to the conductor track by gluing,
soldering or clamping and by peeling the conductor tracks off
the substrate the adhesion strength is determined. At a constant
speed of 50±5 mm/min. a steadily increasing tensile force is
applied orthogonally to the conductor track in order to peel off
the conductor as shown in Fig. 11. For ensuring the constant
peeling angle the substrate is moved contrary to the direction of
peeling. As the peel-off force is the lowest force measured per
conductor, the adhesion strength can be calculated as quotient of
the minimum force measured and the width of the conductor
track. A conductor track with a width of 3±0.2 mm, a thickness
of at least 35 µm and a length of 65 mm are recommended in
order to perform the peel test [5]. As this requirement of
conductor tracks usually do not meet with the conductor tracks
on MID, the peel test is only used in exceptional cases for testing
MID.
Fig. 11: Schematic illustration of Peel test [5]
Common MID would need a galvanic reinforcement to meet the
necessary requirement of the thickness to perform usual peel test
because normally they are often too brittle. Eventually this
would influence the results of the test. Other than tested, the
result cannot be applied one-to-one to combinations of
materials.
Advantage:
 Already known from PCB technology.
Disadvantage:
 Dimensions of circuit paths are too large to be used in
LDS- MID.
 Minimum layer thickness ≥ 35 µm (otherwise galvanic
reinforcement is necessary).
Shear Force Measurement test: Though the shear force
measurement is particularly used in the field of PCB technology
to measure the mechanical strength of the interconnection
technology between the applied electronic components and
conductor track this method can also be applied conditionally to
measure the adhesion strength of the metallization on the
substrate. A chisel shears off components parallel to the
conductor track and thus measures the necessary force as shown
in Fig. 12.
Fig. 12: Schematic illustration of Shear force measurement [5]

One important parameter is that the width of the shear chisel
should be narrower than the distance between the junctions of
conductor track and component. Without contacting the surface
of the substrate, the shearing tool should get in touch with the
component as far at the bottom as possible. The shear chisel
must be placed parallel to the component, otherwise a uniform
force distribution will not be present over the component. In this
case, not only the shear force but also a peeling force is recorded
which eventually distorts the result.
Fig. 13: Different fractures during shear force measurement on MID [5].
Apart from the shear force measurement, the fracture faces have
to be analyzed. In Fig. 13 different types of fracture faces after
shear force measurement on MID has been shown where in
section 1, a fracture joint is shown, in section 2, the wrong
application of the procedure occurred. It can also be observed
that while shearing off the component, the chisel moved into the
substrate material. In section 3, a direct fracture in the
component is shown. In section 4, a mixed fracture is shown as
on the right side fracture occurred between metallization and
substrate while on the left side in the solder joint. If the occurred
fracture is between metallization and substrate, as shown in
section 5, a direct conclusion can be made regarding the
metallization adhesion. When it comes to MID, this type of
failure or fracture occurs more often. However, the shear test can
also be performed in a more direct method without electronic
components. This variant of the test is very difficult to proceed
due to the thin metallization layers and the beads caused by the
laser structuring process. Another special version of this method
is used to measure the shear force named micro chisel test where
conductor tracks are peeled off using a micro chisel while forces
in X and Z direction are recorded.
Advantage:
 During analysis quantifiable values occur.
Disadvantage:
 Difficult procedure to proceed because of thin
metallization layers and beads of laser structuring
process.
 High investment in equipment.
Hot Pin Pull test: This new method for determining the
adhesion strength is easy to use and also used for the
characterization of PCB pad cratering. This test can be carried
out with Nordson DAGE’s micro material testing system Dage
Fig. 14: Hot pin pull test using the testing system Dage 4000Plus.
4000Plus as shown in Fig. 14. Straight copper pins with a
diameter of 900 μm are used for this test. Either plain copper
pins or pins with tinned end both can be used. Test pins are
widely available with a tip radius of 100 μm, 300 μm or 450 μm
as shown in Fig. 15. There is a special heater cartridge into
which the test pins are vertically inserted and they are held in
place by a spring-loaded mechanism. A user-defined time-
temperature profile can be assigned to the heater cartridge in
which the setup for the profile can be defined by temperature
and time criteria for the reflow in six consecutive stages shown
as Fig. 16.
Fig. 15: Schematic illustration of hot pin pull test [5].
The inserted pin is positioned either into a previously dispensed
solder paste on the test structure contacting the metallized
surface (untinned pin) or on the test structure (tinned pin).
According to this temperature profile, the pin temperature
ramps up once the test is started. In this method, the pin is
soldered to the metallized test structure. By pulsing compressed
air along the pin and onto the test sample the cooling process is
handled. During the running process when the temperature
reaches T6, the clamping mechanism of the cartridge fixes the
pin and an increasing tensile force is applied by the pull-off
process to the pin. Hence, after applying particular amount of
force the detachment of metallization from the substrate occurs
and it gets recorded and logged. From the size of the detached
area and the maximum force applied, the adhesion strength can
be calculated.

Fig. 16: Temperature profile of hot pin pull test [5].
In Fig. 17, the hot pin pull method has been shown in total of
eight sections. The test was performed with an untinned pin
with a tip of radius of 450 µm and low melting SnBi solder paste
according to [5]. LDS process laser structure has been used in
the test structure with the metallized circles having a diameter
of 1mm. In section 1, the solder paste that has been dispensed
on the metallization pad and the pin positioned over the test
structure is shown. Needless to say, according to the size of the
test area, the amount of solder was adapted. In section 2, the
sample has been shown right after the pin dipped into the solder.
Section 3 shows the picture during the heating process. As the
metallized test structure is wetted by the flux, a transition is
built up by the solder from pin to test structure in conical shape.
As the process continues in section 4, the single solder particles
start to melt when the melting temperature of the solder is
reached (melting temperature of SnBi is 137 degree Celsius).
As seen in section 5, the solder being completely melted covers
the test structure in a cone shape surrounding the pin. After that
when the cooling process takes place the pin gets completely
covered by the circular structure of the solidified solder as
shown in section 6.
Fig. 17: Process steps of hot pin pull test [5].
In section 7, the exposed surface of the substrate is shown after
the test metallization is pulled-off. In section 8, the pin with the
detached metallization is connected by the solder joint [6].
Advantage:
 A low standard deviation (below 10%) can be obtained.
 Different pad sizes can be used on the substrate.
 This method allows the direct use of serial parts.
 The process changes can be detected by the resolution
of test method as it is sensitive enough to do so.
Disadvantage:
 Special set of apparatus is required to perform the test.
Conditions:
 As low as possible temperature profile.
 Test should be continued with completely wetted
structures for reproducible test conditions.
 Machine parameters such as take-off speed should be
set properly.
 Amount of solder paste must be taken into account with
the change of test pad.
 Different test geometries can be set only if other
parameters like temperature profile does not have a
massive impact.
 According to the change of pad and solder paste the
temperature profile must be taken into account.
Major investigations on hot pin pull test: According to [5],
all investigations regarding the hot pin pull test were performed
with low melting SnBi solder with melting point of 138 ºC and
blank copper pins and LDS process have been used to produce
all test structures. The influence of the temperature on the test
results was one of main focuses of the hot pin pull test. Samples
which were used as test samples were made of liquid crystal
polymer (LCP, Vectra E840i LDS) and had circles with a
diameter of 1 mm. Depending on the test structure size the
amount of solder paste was adapted accordingly and depending
on the amount of solder paste the necessary peak temperature
and time period for the reflow process were fixed. In pre trials,
such a temperature profile was determined so that the substrate
is not damaged or the adhesion strength is not influenced and
also a complete wetting of the test circles got ensured. For a
period of 20 second with a peak temperature of 170 °C, this
profile was stepped up increasing the peak temperature in two
steps by 80 °C each and the results show a significant impact on
the measured adhesion strength on Fig. 18. As the peak
temperature increases the adhesion strength considerably
decreases. With a peak temperature of 330 °C, the fracture face
and the cross section show a damage on the substrate material in
contrast to a peak temperature of 170 °C. So, temperature profile
should be determined as soft as possible to avoid influencing the
results of adhesion test [5].
Fig. 18: Adhesion strengths at different peak temperatures of hot pin pull test
[5].
Other main focuses on the investigation is the complete wetting
of the test structure with solder paste as it ensures the
reproducible application of the hot pin pull test. For the complete
wetting of the test structure the properties and condition of the
metallization e.g. roughness, impurities plays an important role
as well. In Fig. 19 detached test circles having a diameter of 2
mm in different solder wetting conditions are shown. As shown
in section 1 and 2 of Fig. 19, there is a possibility of uncontrolled
rip out of the whole metallized area (section 1) and a complete

detachment of the test structure due to lack of complete wetting
with solder paste on those two sections. As shown in section 3,
a completely wetted test structure with a fully detached
metallization should be used for the proper analysis of the test.
Fig. 19: In hot pin pull test different detached structures with different solder
wetting conditions [5].
Significant differences were seen while investigating the solder
paste amount used for test circles with diameter of 1 mm and 3
mm as shown in Fig. 19. According to [5], a volume of
0.045±0.01 mm³ solder paste was required to completely wet the
1 mm circle and the same amount was used for 3 mm circles as
well. It is been calculated that the metallization with 0.79 mm²
of area was totally pulled off for the 1 mm circles (Fig. 18,
section 3) while on the 3 mm circles, a partial rip out took place
to the metallized areas from 0.53 mm² until 0.86 mm² with a
mean of 0.62 mm² (Fig. 19, section 1). As a matter of fact the
more the uncontrolled rip out the higher the standard deviation.
And additional forces are required to rip out a part of
metallization which results in the higher adhesion strength. The
results could vary to certain extent depending on the layer
properties and thicknesses. It is also known that the smaller the
test areas are the more the adhesion strength increases.
Fig. 20: In hot pin pull test result of using different detached structures with
different solder wetting conditions [5].
VI. COMPARISON OF TEST METHODS
According to [5], four different samples (V1 - V4) were used
where substrate material LCP Vectra E840i LDS was used as
V1, V2 and PA4T/X Vestamid HTplus TGP 3586 was used as
V3, V4. V1 and V3 samples had comparatively smoother
structure while V2 and V4 had comparatively rougher ones.
With a diameter of 1 mm, the hot pin pull test was performed on
test circles while test circles with a diameter of 3 mm was used
to perform the pull-off test, the dolly being bonded with an
adhesive onto the test structures.
Fig. 21: Comparison of hot pin pull test, pull-off test and shear force
measurement test [5].
The temperature profile is shown in Fig. 16 for hot pin pull test.
On the corresponding land patterns for the measurement of shear
force, CR 0402 components were applied using SnAgCu solder.
For assuring comparability, according to [5], all tests were
performed after vapor phase soldering at a temperature of 230ºC.
Unfortunately due to different test methods of these different
tests, a direct comparison of adhesion strength values cannot be
Pull-off Test
Shear force measurement Test
V3 - PPA Vestamid HTplus TGP3586 𝑅 𝑧 = 13.98 µ𝑚
V2 - LCP Vectra E840i LDS 𝑅 𝑧 = 13.98 µ𝑚
V4 - PPA Vestamid HTplus TGP3586 𝑅 𝑧 = 32.73 µ𝑚
V1 - LCP Vectra E84i LDS 𝑅 𝑧 = 11.03 µ𝑚
Hot pin pull Test

drawn. However, Fig. 21 implies to the similar trends shown by
the results of the tests of measuring the adhesion strength of the
investigated samples. For a basic evaluation of the adhesion
strength of metallized structures on MID, all regarded test
methods are suitable. But when it comes to the standard
deviation however, it can be seen that significant differences
appear on them. With hot pin pull test the lowest standard
deviation in a range of about 5% could be achieved while the
values achieved with the shear force measurement are twice as
high compared to that. With pull-off test, standard deviation
reached to more than 25%. Important facts to be mentioned here
that the efforts for performing these test vary widely. The
preparation efforts for the pull-off test are comparatively high
with the application of the adhesive, the positioning of the dolly
and the curing process and also it includes the manual process
which increases the influence of the user even furthermore. For
shear force measurement, the test cannot be used to determine
the initial adhesion of a metallization and the test can only be
performed quite fast if the components are already applied on
the specimen but due to different fracture faces the usable
results can be very much limited. On the other hand, hot pin pull
test has advantage on the preparation effort over other test
methods mentioned above, but only if the required apparatus
are present. There are also some significant differences between
hot pin pull test and conventional pull-off test. As the MID
structures don’t offer much planar surface so the large size of
the dolly and manual application of adhesives are not effective
whereas hot pin pull test offers smaller test pad usage, less
manual activity and partly automated process.
VII. CONCLUSION
In this study, the advantages of choosing Laser Direct
Structuring process over other processes, optimum LDS
parameters with its effects on groove dimensions and quality
have been investigated. Also the importance of adhesion and
comparison between different adhesion strength measuring tests
have been mentioned and investigated. We can summarize and
conclude notable investigations as given below-
 There are sets of LDS parameters to produce certain
thickness of circuit line and minimum space between two
circuit lines. The minimum space distance between two
circuit lines can be achieved in the range of 50 to 60 µm
while the minimum width of the circuit line of 48.5 µm can
also be achieved. Metallization defects may start to occur if
the minimum space between the two lines are reduced to less
than 50 µm.
 The edge dimension namely the width needs to be reduced to
the lowest possible value to avoid or reduce the metallization
defects. As a result it increases the quality of the
metallization, hence the quality of MID products.
 If the necessary apparatus are present then the hot pin pull
test can be a very good alternative to the conventional
methods for testing the adhesion strength. The method itself
being partly automated makes the influence of the user
correspondingly limited thus enabling the reproducibility of
the test to be quite high. For instance, the fact that the pin is
soldered to the test metallization while it is inserted in the
cartridge, minimized the risk that the tensile force is not
applied vertically. Also on account of the controlled heating
a reproducible temperature treatment is ensured and cooling
process gets executed by the heater cartridge.
 In hot pin pull test, low standard deviation is achievable
compared to other test methods which makes it very
promising and also relatively low amount of efforts are
required for the preparation and performing the test.
 Hot pin pull test also provides the possibility to use small test
structures e.g. circles with a diameter of 1mm, which can be
very helpful especially on three-dimensional MID parts,
where planar areas for larger test structure are quite rare.
However, as there are various influencing factors exist which
bear a considerable impact on the results, further
investigations in detailed manner and necessary standards
and guidelines are required that enable a uniform
performance of the test.
REFERENCES
[1] Joerg Franke, “Three Dimensional Molded Interconnect Devices (3D-
MID), April 2013
[2] Bassim Bachy and Joerg Franke, “Experimental Investigation and
Optimization for the Effective Parameter in the Laser Direct Structuring
Process”, JLMN- Journal of Laser Micro/Nanoengineering 2015.
[3] Angelika Paproth, Klaus-Jürgen Wolter, and Rumen Deltschew,
“Adhesion of Polymer/Metal Bonds for Molded Interconnect Devices
(MID)”, June 2005.
[4] Klaus Zeh, “Selection and Qualification of a Test Method for Determining
Adhesion of MID Metallization in the LDS Technology”, 2012.
[5] Thomas Kuhn and Joerg Franke, “Test Methods and Influencing Factors
for the Adhesion Strength Measurement of Metallized Structures on
Thermoplastic Substrates”, EPTC IEEE 2014.
[6] HARTING Mitronics- Christian Goth, “Hot Pin Pull Method – Mew
Test Procedure for the Adhesion Measurement for 3D- MID”, 11th
International Congress MID 2014.

Laser based mid manufacturing processes and qualification methods regarding their metallization

More Related Content

Similar to Laser based mid manufacturing processes and qualification methods regarding their metallization (20)

Recently uploaded (20)

Laser based mid manufacturing processes and qualification methods regarding their metallization