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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