[SI-LIST] Re: Voids in Solder Joints

Voids of the kind you describe may be caused by a number of things.
Here are some thoughts about possible causes:
1.Solderpaste has volatile solvents. A normal solder temperature profile should 
ramp slowly to a peak temperature then hold then ramp down. If the leading edge 
is to steep, volatile cause to rapid outgassing and voids may appear.
2. Runoff: Your top side dogbones are not via filled.
3. The board/vias are outgassing due to process entrapment of liquids,moisture.
4. Poor size control on solder mesh size in the solder paste. Ratio of flux to 
solder inconsistent in paste. 

Run the bare board through without the BGA installed. Examine the pads. ( From 
experience, about the last thing I would suspect is the BGA itself.)

Here some additional thoughts
Now that all being said, if you are "connected to each pin" however marginally, 
the cross sectional area being so small, it is highly unlikely that enough 
resistance develops to be a problem. You may want to rexamine you initial 
premise that it is a board assembly process. (microsection board?)
1.What can you do to prove your theory?
2.Is slowing the clock possible
3. Is in circuit reprogramming possible so most all pins can be made to drive 
I/O pins as outputs?
4. Can you get a FPGA part with the same or near same PINOUTS to allow a test 
case for #3 above?
 
Steve Salkow

-----Original Message-----
From: Gupta, Deepali [mailto:Deepali.Gupta@xxxxxxxxx]
Sent: Wednesday, September 24, 2003 3:44 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Voids in Solder Joints


Hi,

I have been troubleshooting a board related issue. I have a BGA device
soldered on the board which is giving me functional failures. After running
x-rays on the device I found some voids in the solder jointon most of the
solder balls. 

I am not sure what kind of problems arise due to voids. Voids does not mean
that there is an open joint. I have curve traced al the pins and they give
me the correct characeristics so I guess it is making contact. But what
would a void do when passing a signal is what I am not sure about.

If anyone has dealt with similar issue your feedback would be very helpful.

Thanks

Regards,
Deepali


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