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Subject:
From:
Thad McMillan <[log in to unmask]>
Date:
16 Jul 97 13:41 CDT
Content-Type:
text/plain
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text/plain (117 lines)
     Wave can definately reflow BGA solderjoints.  There are several design 
     and process things you can do to reduce this problem.
     
     Design tricks:
     
        1.  Use a dumbell BGA footprint.  One end of the dumbell is a round 
     BGA SMT pad.  The other end is the via pad.  Make sure that the trace 
     connecting the two round pads is as narrow as possible.
     
        2.  Cover both sides of the connected via with LPI soldermask.
     
        3.  Plug the WAVE side of the connected via with a secondary 
     soldermask.  Contact your local PCB fabricator to find the best way to 
     do this.
     
        4.  Maximize the distance between the BGA SMT pad and associated 
     BGA via.  Fan the vias that connect the BGA pads away from the BGA SMT 
     pads.  It is not possible to do this in the middle of the array, but 
     the outermost BGA rows are definately possible.  You can also do this 
     on the inner rows for BGA patterns that are hollow in the center.
     
        5.  Use thermals for the BGA vias.  There is an industry trend to 
     not use thermals for  microvias, since they are not pin in hole.  This 
     is normally a good thing.  However when a BGA goes through the IR oven 
     the vias connected to power and ground can act as a heat sink and 
     cause a possible cold joint.  This is not a wave problem, but IR.
     
     Process tricks:
     
        1.  Use Kapton tape on the wave side vias.  This is a pain in the 
     neck to do in volume,  but it works.
     
        2.  Use a wave fixture to shield the Bottom of the BGA.
     
     The goal of all of these tricks is to reduce heat transfer up the via 
     to the BGA ball during wave.
     
     
     My two cents.  I have the standard cautionary note that these tricks 
     may not work for your particular design/application and should be 
     investigated before applying.  For example there have been industry 
     concerns that plugging vias can later trap flux or other process 
     chemicals.  Haven't seen this in my applications however.
     
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Re: Wave and BGA
Author:  [log in to unmask] at dell_unix
Date:    7/16/97 9:39 AM


                      RE>Wave and BGA                              7/16/97
     
Hi Andre,
>From our experience, we found that we have to mask the via array pattern 
beneath the BGA devices prior to wave soldering.  I believe that our 
assemblers use Kapton tape.  There are many solder joint problems if this 
isn't done.  Our vias are also partially covered with mask, but I believe 
that our vias are slightly closer to the BGA SMT pads.
     
We also went to a high Tg (~180C) FR4 and found a noticeable reduction in via 
problems on boards that went through this process.
     
Best regards,
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
[log in to unmask]
     
-------------------------------------- 
Date: 7/16/97 10:37 AM
To: Greg Bartlett
From: Andre Bisson
Hello all,
     
I will like to have comment on any effect that a wave can do on a BGA.
     
We have some boards with BGA on component side and many component on the 
solder
side. Our process will be reflow component side, glue solder side, install 
all TH
parts and go to wave.
     
Is anyone had or have  problems with BGA (solder joints problem) after wave? 
Our
via's are half tented with solder masks and the trace connecting the via to 
the
BGA pad is approx. 0.020"-0.030".
     
My concerns are:
     
- Re-reflow of BGA solder ball causing cold solder or any solder defect? 
- Reliability problems?
- Etc.
     
Any advice will be well appreciated.
     
Thank you.
     
Andre Bisson
CAE Electronics
[log in to unmask]
     

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