Page 64 - Troubleshooting Analog Circuits

Avoid PC-Board Problems at the Outset 51

usually provide some useful advice. (See Table 5.1 for a comparison of PC-board
materials.)
In some RF applications, phenolic material has advantages over glass epoxy: it
has a lower dielectric constant and superior dimensional stability. And for ultra-
broadband oscilloscope probes. some types of glass epoxy have a definite disadvun-
tage due to mediocre dielectric absorption, especially if the epoxy has not been
properly cured.
As for quality, there is almost never an excuse for buying your boards from a
vendor whose products are of unknown quality. "Low cost" would be one poor alibi;
"Can't get acceptable delivery time from our normal vendor" would be another. One
time. to meet a rush contract, we had to build circuits on boards made in our own lab
facilities. I had never had trouble before using these boards for prototyping. so I was
surprised when I began the troubleshooting and found that an apparently good board
occasionally had a short between two busses.
Close inspection with a magnifying glass showed a hairline short about 3 mil5
wide, which was caused by a hair that fell onto the artwork. You would never ask ;I
printed-circuit foil that narrow to carry 20 mA, but this narrow short would carry 200
mA before blowing out. Similarly. we found hairline opens: The ground bus was
broken in two or three places by a tiny 4-mil gap, just barely visible to the naked eye.
Of course these "opens" were caused by the image of a hair, during a negative pro-
cess. After several hours of fiddling around, opening shorts, and shorting opens. we
vowed not to be caught by such poor workmanship again.
As for the third problem, don't let ham-fisted engineers or technicians beat up a
good PC board with the overenthusiastic or misguided application of a soldering iron.
That's sure to lift the foil. Use an iron that's hot enough so you can get in and get our.
If the iron's not hot enough and it's taking too long, that's when the foil will lift . . .
Solder mask, the subject of the fourth problem, is almost always worthwhile. a5
many people have learned. Without it, the admirable tendency of solder to bridge
things together. which is wonderful in most instances. becomes disastrous.

Table 5. I PC-Board Laminate Materials
Dielectric Dissipation Volume Surface Maximum
Type Manufacturer Constant Factor Resistivity Resistivity Temperature Comments
(at I MHz) (at I MHz) (Mn-cm) (Mn) ("C)
NXXK Generic 4. I IM?? 5 x IO6 5 x In' +I3 Low cost. paper-haw.i
phenolic: poor meshan-
icd strength
CEM- I Generic 1.5 0.015 I x IIIX 5 x IO' +I30 Standard. econoniicdl
CEM-3 Generic 17 0.010 I x IO8 5 x 10' +I30 Similar 10 CEM-I hur
can be punched
G- IO Generic 4.7 0.023 5 x ID% 1 x 108 + I30 Comparable IO CEhl-3
F44K6 Oak 3.5 0.02 I x IO' 3 x IO6 N/ .4 Flexible
FR-4 Genenc 4.9 0.OIX 1 x I(l* 5 x 10' + I3O Similar to CEM-.? but
tire retardant per I'L~
94-v-0
GT-572 Keene 2.5 n.ooio I x 11)' I x IO' +2fa Tetlon is good for high
temp. high speed
GX-527 Keene 2.5 o.w19(at IOGHzi I X IO- I x 10: +>fa Characterized ior htsh
frequency
HI-3Wi Technoply 4.5 0.1320 3 x 10: 5 x IO" +?XI 1O.GiMJ hours) Polyimide
I
3003-quan7 Technoply 1.6 0.004 5 x IO' R x 107 +?SO1 1O.OOO hours1 Comparable to CEhl-3