Vinit,

The "raised" area around the vias/PTHs is probably due to damage to the laminates when the holes were
made.
You did not say if the blistered/discolored areas were around the holes.

If possible try to post some pictures at steve's web site:  http://stevezeva.homestead.com/index.html

You may need to do a cross section of the damaged area.

David A. Douthit
Manager
LoCan LLC


Vinit Verma wrote:

> Probably the following message did not get delivered to this forum a few
> days before since I did not get ANY reply hence posting it again.
>
> Hi Technetters,
>
> We are currently running an 8 layer PCB, 1.6mm thickness. In the last run of
> approx. 400 nos. we found approx. 10 nos PCBs with blisters in some specific
> areas. In approx. 40-50 nos. the blistering is not there but the laminate
> has discoloration.  More specifically, the blistering is seen in a
> particular area with a ground plane just beneath. In some PCBs the
> blistering is there on both sides of the PCB (just mirrored on both sides)
> and in some cases just on one side.
>
> Looking carefully at the bare PCBs I observed that there is a small ring
> around all the vias/PTHs with a width of approx. 0.2-0.3mm. This ring is
> basically the laminate itself but slightly raised and hence looking like a
> small ring. Is this normal? Can this be causing the blistering? OR is this
> an indication of some other phenomenon which is causing the blistering?
> Actually this ring is visible in ALL vias and PTHs all over the board, but
> blistering is happening ONLY in one particular area. What bothers me the
> most is that  ALL PCBs have this ring around the vias/PTHs but the
> blistering has happened in approx. only 10 out of 400 PCBs.
>
> Somebody pointed it out to the reflow profile, which actually is withing the
> the paste manufacturer's specified range. Even if it IS due to the profile
> then why ONLY 10/400? Why not more?
>
> Can anybody suggest any possible causes of the same.
>
> We are not baking the PCBs prior to processing, but the boards are in vacuum
> packing with 10 PCBs per bag and the packing is opened only at the Stencil
> printing stage.
>
> Any suggestions welcome.
>
> Regards
> Vinit Verma
> Flextronics Technologies (India) Pvt. Ltd.
> 88A, Electronic City, Hosur Road,
> Bangalore-561 229
> Tel: +91-80-852 0022, Ext. 161 ; Fax: +91-80-852-1022
> E-Mail : [log in to unmask]
>
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