10 mils (0.25mm) via in pad is too big diameter for this (main reason for voids), why not laser drilling.



Best Regards



Reuven ROKAH 

Think green before printing this mail... Thanks



-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas McCall

Sent: Tuesday, October 13, 2009 1:07 PM

To: [log in to unmask]

Subject: [TN] BGA voids in filled & capped micro-via PWB



Hi,

We have non-conductive resin filled and capped micro-via (via-in-pad) through the complete PWB thickness (1.6mm FR4) 8-layer, we have seen considerable voiding in the BGA balls following assembly:

 

Q1) What is the chance of out-gassing from the PWB board (or specifically the filled capped via) going into the BGA ball and creating a void during reflow. 

 

Q2) What is adequate flatness on the PWB surface considering “dish-down” of the filled resin during via filling and before Cu plating.  I think the IPC spec.  is 70% but does anyone have differing experience of this.

 

Q3) Would a fairly “rough” topology on the surface of the capping plating (i.e. not totally flat, bumps etc) cause any difference in the solderability,

 

I believe that:

·         The via drilled diameter is 0.25mm

·         PWB finish ENIG,

·         Cu/Ni/Au plating thickness is 20um

·         The PWB thickness is 1.6mm

·         The PWB Tg is approx 170 deg-C

·         The paste is Pb-free,

·         The reflow profile is good to the paste suppliers recommended profile,

·         The PWB’s have been baked-out 130 deg-c 24-hours before assembly,

·         Reflow peak reached 242 deg-C

·         Reflow soak 170 deg-C for 170 seconds,

 

There are large and small voids in the BGA balls during X-Ray and cross sectioning. Some of the BGA balls apear slightly elongated not correct shape.

 

In essence would you think that it would be a PWB issue or assembly problem.

 

Any feedback would be much appreciated.

 

Thanks,

D. McCall.

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