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Date: | Tue, 19 Sep 1995 14:08:04 -0500 |
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James Moffit commented about my reply to Kevin Di Tondo:
>Kevin asked how to perform SMT mods using haywire/jumper-wire fixes. Jerry
>Cupples provided a real good answer. I would only take exception to Jerry's
>statement: "use the smallest wire you can". If a circuit handles digital
>(logic) signals you may be OK that way. If the circuit handles analog or
>unknown signals you need to assure that the cross-sectional area of the wire
>(circular mils) being installed provides sufficient current carrying
>capacity to do the job (Hint - compare with the cross sectional area of the
>original trace). Note that the (recently published) Navy repair manual
>(Recommended Requirements and Procedures for Repair of Electronic
>Assemblies, by the Naval Air Warfare Center, Aircraft Division Lakehurst,
>their Document NAWCADLKE-MISC-05-MT-0001) provides a table of solid wire
>equivalents to various (2 oz.) conductor widths. Enjoy :) J. H. Moffitt,
>EMPF/SAIC
Well, James; thanks for the kind words about my soemwhat acerbic answer.
Kevin asked specifically regarding fine-pitch SMT leads. I don't know many
power or analog devices in that style package.
Since I don't have my copy of NAWCADLKE-MISC-05-MT-0001 handy, let give you
what my HP27S-sci-calc-rev-1987 said about copper cross sectional area of
wire and traces:
cross sectional area, square mils for common PWB traces
copper weight (oz): 0.5 1.0 1.5 2.0
---------------------------
trace width (mils)
6 4.2 8.4 12.6 16.8
8 5.6 11.2 16.8 22.4
10 7.0 14.0 21.0 28.0
wire gage vs. cross sectional area of common wire
AWG cross sectional area
(square mils)
--- --------------------
26 199
28 124
30 79
32 50
34 31
A typical QFP (.65 mm pitch) SMT lead cross sectional area is about 0.004"
x 0.008" or 32 square mils. A 0.5mm pitch is
I would further state, just to be technical as all get out, that for a fine
pitch QFP mod wire, one should use 32 or 34 AWG tin-plated copper wire.
>From the above charts, you can see that a 30 AWG wire has more cross
sectional area than any common digital signal trace, or even thatn the 5V
device supplies for SMT parts. I believe lots of people use a bigger wire
than is necessary.
You should be aware that the stripline impedance characteristics of a mod
wire are far different from a trace, due to the relative spacing from the
ground plane. This is more complex to discuss, but may affect a logic
device with fast rise time. In the digital world I live in, it's a much
more important issue than the current carrying ability of a mod wire.
regards
Jerry Cupples
Interphase Corp
Dallas, TX
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