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BGA relow without solder paste?

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hly

#36733

BGA relow without solder paste? | 20 September, 2005

I have a situation with hot boards with fine pitch BGAs that I need to build. However, there is a big problem--the auto screen printer is down.

I decided that we would use the stencil and manually line up the pads and do a manual print. I can't accurately control the thickness of the solder paste and alignment is not very accurate either. I have therefore decided to mask off the BGA (fine pitch also) locations on the stencil to do the manual print. Now instead of the solder, I have applied a gel flux to the BGA sites and process the board as normal. After reflow, the attament of the BGA seems o.k. Bridging is not much of a problem but there is the risk of opens now.

Does anyone else have expierence with reflowing BGAs this way without solder paste? Any comments on the joint quality would be appreciated.

Thanks HLY

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#36734

BGA relow without solder paste? | 20 September, 2005

Hello I'll be the one to ask a dumb question--Why not fix the screen printer? If it is a MPM you can call me and I'll help if I can. And no I'm not trying to sell any thing. Better yet --why is the printer down? Wasn't thinking that you may be out of the country (USA). Harry

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K

#36735

BGA relow without solder paste? | 20 September, 2005

HLY: If you must build product while you wait to get the printer fixed, and if the BGA balls are eutectic solder, your approach is a good one. All solder paste does in this assembly application is supply the flux. Additional solder volume is not necessary.

I've done quite a few BGA repairs using a Metcal BGA rework station. In those cases, I would apply the "gel" flux to the BGA balls by pressing them into a well of "gel" flux of a controlled depth (approx 14 mils). I would then place the BGA and reflow. Works great. Rather than stenciling the flux onto the board you may want to consider applying the flux to the BGA balls.

Kelly

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RDR

#36736

BGA relow without solder paste? | 20 September, 2005

If the balls are eutectic (63/37) You shouldn't have any problems. All of our BGA rework is performed without paste. You have plenty of solder to make the joint with the balls themselves. Be wary of cleaning however since the standoff is less.

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hly

#36743

BGA relow without solder paste? | 21 September, 2005

Cognex board is bad. Waiting to get a replacement. Thanks for offering to help

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hly

#36744

BGA relow without solder paste? | 21 September, 2005

Hi K, Thanks for your reply. I was wondering if there is an industry spec that specifies a minimum required solder volume for BGA joints?

Thanks HLY

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hly

#36745

BGA relow without solder paste? | 21 September, 2005

Thank you for your reply.

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GS

#36766

BGA relow without solder paste? | 21 September, 2005

IPC-7525 tells you how to make stencil apertures for BGA

IPC-7095 tells you how much should be the paste volume in order to get a reliable BGA solder joint.

How are your PCB finisced? (HASL, ENIG, OSP, etc?) In case of not HASL, it could be necessary to do BGA pad dressing first, then gel flux on BGA balls (by dipping balls into a levelled flux container ), place and then reflow.

If you what to be sure about correct solder joint obtained, do a X-Ray (voids?!)and why not a BGA solder joint microsection.

Regards...........GS

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