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