FOR 77/79 GHZ SAFETY AND RELIABILITY

APPLICATIONS THE MOST RELIABLE LAMINATES

ARE USED

Manfred Huschka

Taconic Advanced Dielectric Division, Mullingar, Ireland

Keywords: ADAS, PTFE, NON-REINFORCED, COPPER FOIL, INSERTION LOSS

Abstract

Due to the expected potential growth of 77/79 GHz ADAS

(Advanced Ariver-Assistance System) several non-PTFE

(Polytetra Fluoro Ethylene) laminates are trying to find their

way into designs. However from electrical and mechanical

reliability points of view non-reinforced PTFE/Ceramic

laminates still provide a leading edge.

The high consistency of dielectric constant and loss factor of

the thermoplastic material PTFE over the required operating

temperature range at resonance frequency of the sensors has

been recognized by the RF and microwave industry since tens

of years. This is also one of the main reasons why such types

of laminates are being used for every ADAS generation up to

now.

For many years only one kind of such laminate was available

and therefore is widely used in the industry. 2017 saw the

emergence of another such type of laminate, whose tested

reliability data confirm that the selection of non-reinforced

PTFE laminates for 77/79 GHz is the correct one. In fact,

certain features demonstrate a second generation laminate is

needed in order to meet the requirements of next generation

77/79 GHz ADAS.

The market introduction of almost no profile ED (Electro-

Deposited) copper foil provides an even improved insertion

loss over rolled annealed copper foil, in addition to its lower

cost. Only PTFE laminates result in high enough copper peel

strength even at repeated rework cycles.

Fig 1: ADAS applications

The Case for ADAS

Why are we now so interested in 77 GHz ADAS? It has been

around for some time, although in small volumes. The market

acceptance is becoming quite high, and in a similar way as

ABS years ago it gets cascaded down from luxury cars to

almost every car. Exponential growth rates in the next few

years are expected – and will happen, because all the car

makers want to introduce ADAS.

Safety and reliability have become key words in the industry.

Fig. 2: 77 GHz ADAS Sensor (Courtesy of Robert Bosch GmbH)

Fig. 3: 77 GHz ADAS Sensor (courtesy of Robert Bosch GmbH)

Base Material Characteristics

The RF laminate used in the printed circuit board is required

to be very stable over a very wide temperature range. PTFE

laminates show a very tight DK/DK(@23C) (Dielectric

Constant/Loss Factor) behaviour from -50 to +150 °C (Fig.

4):

Fig. 4: PTFE laminates show a very tight DK/DK(@23C) behaviour from -

50 to +150 °C

Likewise the gradient of DF over the same temperature range

is very low (Fig. 5):

Fig. 5: DF of PTFE laminates from -50 to +150 °C

Non-PTFE organic resins are bound to have higher levels of

natural electrical polarity - the Carbon-Fluorine bond in

PTFE is short and not very polarizable in an electrical field,

while Carbon-Oxygen bonds are naturally more polarizable.

This means a lesser degree of stability over frequency

NF-30 is a non-reinforced PTFE laminate, just containing

ceramic fillers. Same as the laminate being used for current

77 GHz designs, it is electrically very homogeneous in all 3

directions. A woven fiberglass reinforced resin will naturally

have some electrical anisotropy due to the fiberglass weave

structure, which in particular high mmWave frequencies are

sensitive to. It is possible to flatten the fiberglass however the

warp yarns never get flat. There will always be a higher level

of anisotropy in these type of resin systems.

Recent developments in copper foil technology have resulted

in the introduction of a so-called “Almost No Profile” ED

copper foil [Taconic grade name for 0.5 oz is ULPH] (Fig. 6).

Compared with a traditional Very Low Profile copper foil

this foil leads to better defined and much steeper sidewalls of

traces due to less copper treatment which has to get etched

out of the substrate; in other words, finer features with tighter

tolerances are possible.

Fig. 6: Comparison VLP copper foil with ULPH copper foil (courtesy of

Circuit Foil Luxembourg Sàrl)

And what’s of significance is that the insertion loss gets

improved considerably – the higher the frequency the better

the insertion loss (Fig 7.):

Fig. 7: NF-30 Insertion Loss Comparison of 0.5 oz “Almost No Profile”

copper foil (ULPH), reverse treatment copper foil (CLHN), and Very Low

Profile copper foil (CH)

Fig. 8: Insertion Loss Comparison of 1 oz “Almost No Profile” copper foil

(ULP), Rolled Annealed copper foil (RA), and Flat Profile copper foil

(HVLP) on a different substrate, measured up to 110 GHz