« back to results for ""
Below is a cache of http://extra.ivf.se/ngl/documents/ChapterC/ChapterC2.pdf. It's a snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
Chapter C: Conductive Polymers Chapter C: Conductive Polymers
LEVEL 2: CONCLUSIONS AND GUIDELINES
2.1

DESIGN AND PRODUCTION ISSUES
Figure C1 Production flow chart
*
Not all electrically conductive adhesives provide sufficient mechanical strength so this can be achieved by
additional use of a non-conductive adhesive positioned under the centre of the component. This is especially
appropriate for the first generation of electrically conductive adhesives.
**
This step is only appropriate if the adhesive is not fully cured in the first curing process. This may be
advantageous in order to facilitate rework [C14b].
When compared to the process for assembly using solder it can be seen that removal of flux residuals and
also prebake may be eliminated by use of adhesives instead of solder. Furthermore, the board shall not be
solder plated, and pre-tinned component are not needed. In fact, this last mentioned aspect may be a
problem because the supply of components, which are not pre-tinned is limited on the market today.
Screen print of
adhesive to board
* Dispense dot of
non-conductive adhesive
Pick and place mounting
of components
Curing
Test/rework
** Postcure 2.1.1 Applying the adhesive to the board
It seems that the most frequently used method for applying the adhesive is by screen printing or stencil.
The layer thickness of the adhesive may be dependent on the rype of components used [C17] but lies, in
general, between 20-50 AM [C18]. In [C17] the redommended pad thickness is higher, i.e. between 140
and 180 lam in order to take into account the high tolerance of the different components. By evaluation of
data sheets it seem that the normal recommended thickness most closely equals that reported in [18]. For
anisotropic adhesives a thinner layer may be optimal for use. An example is given in [C3] where a 15-20
AM layer is applied.
2.1.2 The pick and place operation
The components can be placed by normal pick and place operations but component alignment must be very
precise. This is necessary because adhesives do not have the self-aligning feature that solder provides [C3].
In [C10] it is said that pick and place machines provide sufficiently precise placement. Smaller
manufacturing operations which may use manual or semiautomatic placement could have epoxy smearing
problems.
2.1.3 The curing operation
Electrically conductive adhesives can be cured in conventional IR ovens, in hot belt ovens or in vapour
phase equipment 2,3). Thermoplastic adhesives must not be cured in vapour phase ovens: The organic
vapours used in these ovens not only will prevent evaporation of the contained solvent but will attack the
organic solder and dramatically change its properties for the worse [C1].
The curing time and temperature are very dependent on the material and the properties required. Normally
curing temperature lies between room temperature and 180°C (16,1). For anisotropy adhesives pressure is
also needed during curing and values from 0.2 kg/cm2 [C49] to 2-50 kg/cm2 [C49] are reported. Curing
time may be from seconds to about half an hour or more (C18,3). Posture is typically one hour at 80°C
[C3].
2.1.4 Bleed out
Bleed-out is a phenomenon where the resin is not stable enough to stay positioned on the footprints on the
board. According to [C26] this is especially a problem with thermosetting materials where it is said that:
during the curing process, the bulk viscosity of the material is initially reduced due to the temperature rise.
Then the viscosity will slowly build up because of the molecular weight increase. Conductimer*P-86 is a
fully reacted high molecular weight thermoplastic. The viscosity of this thermoplastic adhesive
continuously increases as solvent is lost.
In [C43] it is said that bleed-out especially is a problem, if the substrate surface is rough or contaminated
with organic material.
2.2 BASIC FACTS ABOUT ELECTRICALLY CONDUCTIVE ADHESIVES
As the name indicates electrically conductive adhesives are adhesives which are made electrically
conductive in one way or another. Most often this is done by incorporation electrically conductive
particles (metal or plated particles) into the adhesive a polymer structure. There are several
basically different ECAs, different in the way they conduct or in the construction of the polymer
structure. In addition to these main types there are variants within each group due to choice of
material, modifications in particle size and shape, curing conditions etc. Below in Figure C2 the main types and their subgroups are listed:
Figure C2 Showing a survey of the main types of electrically conductive adhesives and their
subgroups
2.2.1 Isotropic electrically conductive adhesives
The main characteristic of this type of adhesives is that it conducts in all directions in the joint as a normal
solder joint.
2.2.1.1 Filled isotropic electrically conductive adhesives
Filled isotropic electrically conductive adhesives consist, in general, of about 70-80% filler particles. When
the adhesive is cured the filler particles are uniformly distributed and form a network within the polymer
structure - the non-conductive binder. By this network electrons can flow across the particle contact points
making the mixture electrically conductive. Due to the nature of the network formed, paths are made
available for the current to flow in all directions.
Electrical
conductive
adhesives
Anisotropic
Isotropic
Filled
Thermosetting
Non-filled
Thermoplastic
1-component
2-component
E.g. polyethenes
Polyamides
PVC
Polyimidosilozane
E.g. epoxy, silicone, polyimid
polyurethanes Filled isotropic electrically conductive adhesives are available both as thermosetting and thermoplastic
materials.
2.2.2 Non-filled isotropic electrically conductive adhesives
This kind of adhesives are based on polymers that are inherently electrically conductive (also called doped).
Most of the inherently conductive polymer materials are polymerised aromatic compounds that derive their
electrical properties from their molecular structure. The carbon structure has very long arrays of
alternating double and single bonds resembling the structure in graphite. Examples of such materials are
Polyacetyline, polypyrrole, polyparaphenylene, polyaniline and many others [C28].
In [C28] it is said that "Emerson & Cuming's senior scientist, Dr. Justin C. Bolger, decribes inherently
conductive polymer technology as "very long range". He characterises these materials as extremely brittle
and sensitive to oxidation. He also suggests, "They will become useful for thin film deposition but never as
a replacement for solder"."
Non-filled electrically conductive adhesives will not be further discussed in this project.
2.2.3 Anisotropic electrically conductive adhesives
The special characteristics for this type of adhesives are their ability to conduct in only one direction -
along the Z-axis. This special effect is achieved by the formulation of the adhesive and the curing
conditions.
Anisotropic electrically conductive adhesives are all filled adhesives but the content of electrically
conductive filler is reduced compared to the amount used in isotropic electrically conductive adhesives. The
curing process involves the use of moderate pressure. When the component terminations are pressed
against the corresponding circuit pads, an amount of unfilled resin is squeezed out, leaving a particle rich
layer between termination and substrate [C41].
Figure C3 An illustration of the mechanism of anisotropic electrically conductive adhesives
Other systems are available, where the filler particles are coated with a nonconducting coating that breaks
when exposed to pressure. This results in an amount of filler in the matrix between two pads, where the
coating is intact and a few particles pressed between termination and pad that now are able to conduct. This means that the adhesive cannot conduct in either X or Y directions, although filler particles touch each
other.
As for filled isotropic electrically conductive adhesives the anisotropic adhesives exist both as
thermosetting and thermoplastic materials.
Anisotropic conductive adhesives are available in liquid form for screen printing and as an adhesive film,
especially suitable for connecting flex circuits to LCDs or to PCBs.
The electrically conductive particles used in anisotropic conductive adhesives are often deformable spheres
coated with e.g. gold or silver, also nickel is used. The advantages of deformable spheres are the
availability of a larger contact area when pressure is applied.
Clearly, one great advantage of these adhesives that only conduct in one direction