Dear Teknets,

We would appreciate your help for solving a though case leading to high
costs in the field. To our knowledge this is not very common.

A customer have machines that break down due to an electric short on axis
motor drivers. The shorts show up at random on the board surface, and are burns.
They are not necessarily located on holes.

The boards are in machines and a growing percentage is coming back from the
field. Some are 3 years old now. Older boards from the same manufacturer don't
have this problem. The design being the same.

It happens onto two different boards of different building and thicknesses. 

In both cases, part of the production is made pin-lam in house by the
manufacturer,
the other is made mass-lam by a Mass-lam manufacturer.

REF - 125
4 layers - thickness 2.4 mm - inner + outer copper 1 oz
FR4 core is 0.56 mm thick
80 VDC - 10 A
5 VDC - 240 mA
+- 12 VDC - 140 mA

REF - 126
4 layers - thickness 2.4 mm - inner + outer copper 2 oz
FR4 core is 0.41 mm thick
200 VDC - 16.5 A
  5 VDC - 10 A
110 VAC - 5 A
124 VDC - 1 A
  5 VDC - 1 A
 18 VDC - 1 A
 24 VDC - 1 A

     -  -              ---------- copper outer layer 1
\/\/\]  [\/\/\/\/\/\~/\/\/\/\/\/\ FR4 Preg 7628      2 several
        ~~ ~~ ~~~~ ~ ~~~~~o~~~o~~                    x
\/\/\]  [\/\/\/\/\/\/\/\/\/\/\/\/ FR4 Preg 7628      3 several
     ]  [   --------------------- copper innerlayer  4
=====]  [======================== FR4 core           5
     ]  [   --------------------- copper innerlayer  6
\/\/\]  [\/\/\/\/\/\/\/\/\/\/\/\/ FR4 Preg 7628      7 several
\/\/\]  [\/\/\/\/\/\/\/\/\/\/\/\/ FR4 Preg 7628      8 deveral
     -  -              ---------- copper outer layer 9

x = path used by the short on a long distance, 2 + 3 splitted
    and we can see dark veins (burns)containing copper residues...
    inside the length of the board. We can see cavities + copper
    particules. It looks like veines of a tree.
o = looks like copper residues trapped between the prepregs.

There is no dammage between 4 + 6.

An explanation could be that, the prepreg resine is removing
oxydated copper fragments and they concentrate in the middle
of the two prepregs. Then aging (temperature of the working
board) the shorts are possible within those small residues.

I don't dare sending you all pictures in .jpg format as an
attachment, but if you do like to have a look on the microsections
let me know. They are stored in my PC and I will send it to you
ASAP.

Thank you for your help
Roland



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Henri Jaquet SA
Geneva Switzerland
tel. xx41-22-794-7878
fx. xx41-22-794-3052