Hans; is there any chance that these plane layers have copper extending out to the edge of the panel? i.e. is there any exposed copper on the sheared or routed edge of the panel that potentially can make contact with your electroless basket?  Is there any chance of stray voltage (from a leaky heater, for example)?   I have seen several times where this current sets up a higher electroglavanic potential that creates this exact problem.

It's not such a major problem with the surface copper foil since there is just so much of it compared to the amount on the inner layers that's exposed either in the holes or on the edges...

SOLUTIONS:  make sure that there's no exposed copper planes on the edge of the panel and make sure you don't have any leaky heaters.  Teflon heaters are notorious for pinholes that leak current.

Regards, 
Fred J.

>>> "Hinners, Hans - LYPME" <[log in to unmask]> 04/20/99 01:15PM >>>
Hi All,

I have a mystery on my hands and I hope somebody out in cyberland can help.

We have a 10 layer board made out of 1 oz. FR-4 material. Tg = 140 degrees C
which is intermittently showing negative copper etchback during the
electroless process (using Atotech chemistry) but, only on a few layers.

For instance, layers 2,4, and 6 have negative etchback but layers 1,3,5,7-10
are fine.  This etchback feature is not apparent in the (plasma) desmear
coupon, the process in-between being drilled and electroless copper plating.
The only distinguishing feature of these layers is that they are all thermal
layers (which are mostly solid copper).  I'm at a loss to explain how the
layers being solid relates to their etch characteristics but it's the only
thing that jumps out as being unique.  To make it even more confusing
another thermal layer came out okay, layer 9.

We have also seen this on a 17 layer board where a thermal layer has
negative etchback but everything else is okay.

E-mail me directly and I will send you two jpegs of my cross-sections (only
200k!) so you can see what I am talking about.

Anybody seen this before?  Suggestions?   Comments?  Questions?

Thanks a bunch!

Hans

Air Force Material Command - We keep them flying!
~~~~~~~~
Hans M. Hinners                                 WR-ALC/LYPME Bldg. 640
Materials Engineer                                      380 Second Street,
Suite 104
Manufacturing Eng. Sec.                         Robins AFB GA 31098-1638
912-926-1970 (Voice) 468 - 1970 (DSN)   912-926-7974 (Fax)
[log in to unmask] 

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