Hello Hans,
I have actually had the same problem before.  I was using
permanganate desmear chemistry as a hole cleanup.   Before the
desmear process I had no evidence of negative etchback, but after
up to 1.5 mils.  The root cause was a short from the electric heater
grounding through the rack.  After it was repaired the problem went
away.
Joe Dickson
Dir of Engineering
Tyco Santa Clara
>
> >>> "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]
>
> ################################################################
> TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
> ################################################################
> To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
> To subscribe:   SUBSCRIBE TechNet <your full name>
> To unsubscribe:   SIGNOFF TechNet
> ################################################################
> Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section fo
r additional information.
> For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
> ################################################################
>
> ################################################################
> TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
> ################################################################
> To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
> To subscribe:   SUBSCRIBE TechNet <your full name>
> To unsubscribe:   SIGNOFF TechNet
> ################################################################
> Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
> For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
> ################################################################
>
>

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################