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The Use of PTFE/Woven Glass Base Material in the PCB Industry
Manfred Huschka, Taconic Advanced Dielectric Division
It is amazing to hear comments in the pcb industry, such as „We do not use Teflon base
materials, because they are difficult to process“, „If Teflon base material could be processed like
FR4, then …“, „The surface preparation of Teflon base material is too complicated, too
expensive, too awkward to handle, too …“, „We have no special process line for Teflon pcbs“,
„We are not geared for speciality materials“, etc.
What is really hidden behind those statements? Are they justified or just based upon insecurities
regarding PTFE/woven glass base materials?
If we look back not even ten years, PTFE (= polytetrafluoroethylene, Teflon) base materials were
really exotic materials with limited availability, and which could be processed with difficulties only.
Mainly in the last few years tremendous efforts have been undertaken to wipe out those myths. In
the past these types of base materials were mainly used in military applications, and therefore only
required small volumes. However, since the first large volume commercial pcb application, LNBs
for satellite television receivers, the PTFE world has changed dramatically.
Processing
PTFE is used as non-stick coating for commercial applications due to its inert molecular structure.
Its surface can be „roughened“ only through drastic measures. For years a hole wall roughening of
drilled PTFE/glass fabric base material for better adhesion of the through-hole plating was only
possible through sodium naphthalene treatment. During this chemical reaction explosions of the
released hydrogen can occur upon improper handling. Also the Effluent Treatment Manager of a
pcb shop has „better“ chemicals on his wish list. Since plasma etch chambers are used in pcb
manufacture, hole walls can be roughened without effluent and by taking care of the environment.
With many pcb manufacturers also manufacturing flexible and/or rigid flex pcbs, plasma etch
chambers are widespread throughout the industry, and do not require to be purchased specifically
for PTFE/glass fabric. Plasma etch cycles using either nitrogen only, followed by an oxygen purge,
or using Helium/Tetrachloromethane, make PTFE/woven glass base materials a material as
problem-free as FR4 in this respect.
In order to roughen hole walls, holes have to be generated in the previous step. With PTFE being
relatively soft in comparison to FR4, special drilling parameter are required for each PTFE/woven
glass base material type. Not in the (currently inactive for new designs) MIL-S-13949S covered
base materials with DK values between 2.95 and 3.50 can even be drilled using parameter almost
identical to FR4: Cutting speeds of approximately 140 m/min, with higher feed rates for smaller
hole diameters lift the insecurity of the unknown off a pcb manufacturer.
Both manufacturers of base materials and drill bits have jointly developed detailed drilling
parameter which only require being programmed into the CNC drilling machines. The frequent
demand that base materials for microwave applications should be drillable like FR4 for allegedly
easier processing cannot be achieved in practise. It is a myth that base materials made of glass-
reinforced thermoset plastics (these are resin system cured under heat) can be drilled like FR4.
Each pcb manufacturer is also aware today of the fact that not even FR4s can be drilled the same:
Due to the introduction of multifunctional FR4 systems with higher, but different glass transition
temperatures depending on manufacturer, different drilling parameter might be required. Even
slightly different parameters mean different settings at the drilling machine. But – what difference is
there during programming to alter a number by 1 or 10?
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The main differences to FR4 have now been addressed. Further process steps, such as photo
resist processing, etching, through-hole plating, application of surface protection, etc. use pcb
industry parameter.
Base Material Type Dielectric Constant at
10 GHz
Tolerance Dielectric Loss
Factor at 10 GHz
RF-35 3.50 ± 0.07 0.0018 **
TLC 2.75; 3.0; 3.20 ± 0.05 0.0030
TLE 2.95; 3.0 ± 0.05 0.0028
TLT 2,45; 2.50; 2.55; 2.60;
2.65 *
± 0.04 0.0006 *
TLX 2,45; 2.50; 2.55; 2.60;
2.65
± 0.04 0.0019
TLY 2.17; 2.20; 2.33 ± 0.02 0.0009
CER-10 10 0.0035
* at 1 MHz
** at 1.9 GHz
Table 1: PTFE/Glass Fabric Base Materials (dielectric constant, dielectric loss factor)
Multilayer
Depending on the requirement there are several options to manufacture multilayers: All-PTFE
multilayers are only possible through sweat bonding at very high temperatures. If RF properties are
required for the dielectric to be generated, FEP or CTFE bonding films will be used. The most
common option however is the hybrid multilayer, whereby the PTFE/woven glass is bonded to
either double-sided FR4 or an FR4 multilayer using standard FR4 prepregs and press cycle (fig. 1).
Since the oxidised copper-clad surface of PTFE/woven glass is bonded to the FR4 prepreg,
guidelines for all-RF multilayer are not required.
HF
0.51 mm TLC-30
Prepreg
0.76 mm FR4
Circuitry
Plated-through holes
Fig. 1: 4 Layer Hybrid Multilayer
Considerable cost savings can be achieved this way when the total system is cost-analysed:
Instead of several digital pcbs and one RF pcb, all being connected by connectors, cables, etc., it