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PTFE and Hybrid Multilayer Bonding and Fabrication
The reasons for selecting circuit board are threefold. First, to allow the
integration of digital and Radio Frequency (RF) circuitry into a single
assembly. Second, to decrease overall printed circuit board (PCB) size.
Third to provide additional thermal paths and increase the rigidity of the
final assembly.
There are basically two types of PTFE multilayers. One is an all PTFE substrate
multilayer board (MLB) which is typically bonded with a low dielectric constant,
low loss sheet adhesive or bonding film. The other is a hybrid or composite
multilayer consisting of a layer (or layers) of PTFE and a layer (or layers) of
a different material typically FR-4 bonded with a thermosetting prepeg.
Material Selection
The first part of the process is the selection of the PTFE material for the RF
signal layer(s). Taconic TLC and TLE materials were specifically designed for
multilayer applications. Their combination of low loss (<0.003 @ 10 GHz) and
low coefficient of thermal expansion (CTE) in the Z-axis (50-70 ppm/ºC 25-
250°C) make them well suited to 4-10 layer multilayers. Taconic TLY and TLX
materials are often used in multilayer applications due to their extremely low
loss (<0.001 and <0.002 @ 10 GHz respectively) but layer count is typically
limited to a maximum of 4 layers due to the higher coefficient of thermal
expansion in the Z-axis (280ppm/ºC and 140ppm ºC 25-250ºC respectively).
Artwork Sizing and Compensation
Woven glass reinforced PTFE laminates will have positive dimensional change
in the X and Y axis after removing most of the surface copper to produce a
circuit layer. This growth can be compensated for by shrinking the inner layer
artwork prior to imaging. Exact values depend on the amount of copper
removed from a layer, circuit pattern and layer thickness. Figure #1 offers
some basic scale factor values for the different material types.
| Laminate |
Positive Dimensional Change in Parts Per Million (PPM) |
| TLC |
200-400 |
| TLE |
220-400 |
| TLT |
400-600 |
| TLX |
400-600 |
| TLY |
400-800 |
|
Innerlayer Preclean, Dry Film Lamination and Imaging
Preclean
As with thin core or flexible substrates mechanical scrubbing or machine
scrubbing should be avoided. These processes will stretch or deform the
material as it passes through the pinch rollers and rotary scrub brush
Chemical preclean should be used exclusively on PTFE laminates prior to
dry film lamination.
Lamination and Imaging
Laminate, image and develop per standart innerlayer procedures. Follow
manufactures directions for lamination, exposure and development.
Innerlayer Preparation
To ensure a surface that is readily bondable, care should be taken to avoid
disturbance of the PTFE surface after removing copper during etching. To
provide a suitable finish on the copper surfaces proper cleaning and
preparation should be performed. Examples of some surface finishes that
have been used successfully are a light brown oxide, microtech and
electroless nickel followed by an immersion gold.
While it is true oxide coatings and untreated copper surfaces will decompose
and oxidize during lamination at temperatures suitable for Fluorinated
Ethylene Propylene (FEP) film adhesives, the thermoplastic bond achieved
should still exceed IPC specifications.
Thermoplastic Sheet Adhesive/Bonding Films
Typical bonding film materials are FEP and HT 1.5 based film. Figure #2
illustrates their performance at 1 MHz and 10 MHz and recommended
bonding temperature.
| Typical Properties of Thermoplastic Bonding Films |
| Bond Film |
FEP |
HT 1.5 |
| Dielectric Constant |
2.00 @ 1 MHz |
2.35 @ 10 GHz |
| Dissipation Factor |
0.0007 @ 1 MHz |
0.0025 @ 10 GHz |
| Recommended Bonding Temperature |
282 - 296 ºC
540 - 565 ºF |
218 - 232 ºC
425 - 450 ºF |
|
Thermoplastic Sheet/Bonding Film Lamination
Due to the temperature required to melt thermoplastic bond films and sheet
adhesives they are not well suited for use in hybrid multilayers where FR-4 or
other similar thermosetting materials are used as dielectric layers. The lower
thermal degradation point of thermosetting materials will cause them to
oxidize and decompose.
Typical lamination pressures range from 100 - 200 PSI at temperature.
Adequate press padding should be used to allow for even distribution of
pressure. This should yield a uniform bond line thickness and adequate
adhesion.
Please note that a high rate of cool down may cause excessive layer to layer
stress which may result in delamination. Adequate thermal lagging/padding
and a controlled cooling rate should insure a successful bond.
Thermosetting Prepeg Lamination
Hybrid multilayers can be manufactured using conventional FR-4 or similar
thermosetting prepeg systems. The lamination cycle, pressure and copper
surface preparation should be the same as that used for standard prepegs.
Consult the prepeg manufacture for the processing parameters recommended
for use with the particular resin system.
| Drill Size (mm) |
SFPM |
Chip Load (mm/rev.) |
Feed Rate (mm/sec.) |
Speed KRPM |
| 0.45 |
223 |
0.0538 |
43 |
48.0 |
| 0.65 |
269 |
0.0628 |
42 |
40.1 |
| 0.80 |
305 |
0.0665 |
41 |
37.0 |
| 1.20 |
317 |
0.0773 |
33 |
25.6 |
| 1.35 |
348 |
0.0768 |
32 |
25.0 |
| 2.00 |
412 |
0.0570 |
19 |
20.0 |
| 2.35 |
484 |
0.0480 |
16 |
20.0 |
| 3.20 |
660 |
0.0450 |
15 |
20.0 |
|
Post Lamnination Processing
Drilling
Entry material should be a non metallic phenolic coated paper product.
Backup phenolic coated hard board material.
Standard FR-4 drill bits with a drill point of 130º will produce acceptable hole
quality. Adequate vacuum should be maintained to ensure proper removal of
drilling debris to prevent drill wrap or birdnesting of the PTFE material (see
drilling equipment manufacturer for recommended vacuum levels).
Hole Preparation for Plating
It is necessary to desmear the through holes of hybrid multilayer PCB´s
manufactured with FR-4 prepegs to ensure good mechanical and electrical
contact with the innerconnects. This procedure should be performed prior to
treatment of the PTFE hole wall surfaces. Desmear does not have to be
performed on PTFE multilayers bonded with thermoplastic sheet adhesives,
but a plasma treatment or sodium etch is still needed to provide a wetable
surface for the plated through hole chemistry.
Plasma Desmear of Hybrid Multilayers
Epoxy resin smear may be removed using standard plasma techniques for
FR-4 laminates. Please consult your plasma equipment vendor or FR-4
laminate supplier.
Permanganate or Sulfuric Desmear of Hybrid Multilayers
Chemical desmear may be performed using standard processing techniques
for FR-4 laminates. The PTFE will not be effected by either plasma or sulfuric
desmear.
PTFE Hole Wall Treatment
A plasma treatment or sodium etch cycle may be used prior to plated through
hole (PTH). These treatments modify the PTFE in the trough hole allowing the
PTH chemistries to wet out the hole wall surface. This process should be
performed after desmear on hybrid multilayer PCB´s. The FR-4 will be
unaffected by either process. Please contact your sodium enchant
manufacturer for information regarding processing guidelines.
Plasma desmear and PTFE treatment may be performed in consecutive cycles.
First, perform the desmear cycle to remove FR-4 resin smear. Second,
perform a PTFE treatment cycle to modify the PTFE through hole surfaces.
Please note that maximum hold times after plasma treatment may be as
short as one hour prior to PTH. The maximum hold time should be arrived
at using empirical data.
The following are two plasma cycles which have been used successfully.
Please note that all plasma equipment and circuit designs will be different.
This factor combined with variations in laminate thickness, resin content and
hole diameter make it necessary to experiment with different cycles and
anode placements to achieve fool-proof results.
PTFE Plasma Treatment Cycles
Cycle A
| Stage 1 |
O2 50%. He 50% at 4300 watts Rf and 250 psi for 10 minutes
|
| Stage 2 |
He 100% at 2575 watts RF and 250 psi for 15 minutes
End point temperature is 175ºF |
|
Cycle B
| Stage 1 |
O2 90%, N2 10%
End point temperature is 75ºC.
Use temperature to establish power levels
|
| Stage 2 |
CF4 10-15%, 02 - balance
End point temperature is 110ºC |
| Stage 3 |
02 100%
One half power used in Stage 1 |
|
Plated Through Hole
Once the PTFE is treated, the through holes may be metallized using either
electroloss copper or direct metalization process. Depending on multilayer
complexity and thickness. the finished product should meet all IPC
requirements for copper plating thickness and reliability.
Summary
PTFE multilayer circuit boards and hybrid circuit boards may be manufactured
both efficiently and consistently using today´s more rigid and reliable PTFE
materials. Advances in fabrication and the understanding of woven PTFE
products have greatly improved the success when building a PTFE of hybrid
multilayer of current high power, high density designs. As with any process,
the level of understanding and control are the ultimate factors in determining
the success of a material within that process.
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