We have experienced voiding with via in pad, I know it ws stated that the pads don't have vias in them but if they did they can somtimes gass through the plating, its not the air traped by the paste. I have investigated this with my old outgassing oil test which showes up the problem and will contribute to voiding.

Bob Willis

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  ----- Original Message ----- 
  From: Peter George [log in to unmask] 
  To: [log in to unmask] 
  Sent: Friday, November 16, 2001 8:52 AM
  Subject: Re: [TN] BGA voids


  Hi, Peter,

  I was at a seminar earlier this year where BGA's and their solder joints
  were discussed at some length. The received wisdom is that voids in solder
  joints are not of themselves a 'defect' unless they break out through the
  side of the ball. In fact, they make the joint more "flexible" - better
  able to withstand creep fatigue - so are beneficial, up to a point.

  What causes them is often outgassing of volatiles in the solder paste flux
  during the reflow process, though the size and shape of voids increases and
  changes when there are via-in-pad features as well. About the only way to
  detect them is by X-ray, and to judge from previous TechNet postings, there
  are some pretty fancy machines around now that can give you a very much
  clearer view of the balls than is normally the case ... if you have cash to
  splash!

  Peter Duncan




                      Peter Lee
                      <ppwlee@YAHOO        To:     [log in to unmask]
                      .COM>                cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST
                      Sent by:             Aero/ST Group)
                      TechNet              Subject:     [TN] BGA voids
                      <[log in to unmask]
                      ORG>


                      11/16/01
                      04:25 PM
                      Please
                      respond to
                      "TechNet
                      E-Mail
                      Forum."






  Hello,





  I am encountering voids after reflow on several micro BGAs on one PCA
  product. The voids that I $B!G (Bve observed (with Xray) contribute to as high as
  60-70% of the joint area. My reflow profile was designed by running a
  sample board with dummy components and thermalcouples attached right at the
  joint. I also inspected the PCB lands and didn $B!G (Bt see any via on pad.





  Does anyone know the possible reasons for voids formation in BGA (micro)
  reflow?





  In general, in step by step how would one trouble shoot such a defect?





  Rgds,


  Peter






















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