Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


BGA Short

Yngwie

#22153

BGA Short | 25 October, 2002

Hi Experts,

I'm facing short problem with BGA at SRT rework in one particular BGA.

It is BGA, with a metal piece at the center like a coin. The BGA is located in between two high profile press fit connectors and the short always observed at the sides of these 2 connectors. Th enozzle clearance to the connectors is only about 3 mm. No short observed on the other 2 free sides. Could the short was due to the high hot air jet created due to the blockage by the 2 connectors which prevents free flow of the hot air that came out of the rework nozzle ? Can this created higher temperature than it should be ?

If my assumption is wrong, what would be the area I should look at? package balls coplanarity ? Board warpage ? rework profile

The BGA is not a full column, rework stencil is 6 mils, no shorts observes during the assembly. these BGA need to be replaced due to electrical defect detected at Functional test. REwork profile is as per recommended by teh paste supplier ( ramp 1-3 Deg C, 120 - 150 Deg C for 70 sec, above 183 Deg C,45 sec and peak temp is 207 Deg C.

Pls help and thank you in advance.

/yngwie

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Yngwie

#22172

BGA Short | 29 October, 2002

Ok..I will refer to the past thread myself....but, just an update, after getting the BGA rebaked ( as I didn't see an evidence from Production saying that they hv baked these batch of BGAs). The shorts are all gone. But one thing thou', if the problem was due to the pop corn effect, why is the the problem happened at these specific location ? shouldn't it scattered all over ?

thank you..

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Steven

#22179

BGA Short | 29 October, 2002

If you are using a low end hot air system IE: Weller, Hakko or Pace then your problems are with the system and not necessarily with your process. However process is important. We have found by inspection with an Ersa Scope (BGA inspection system) that the lower cost hot air systems do not create a consistent burn pattern across the surface of the BGA. With the more expensive and better units this has been aleviated. However your concerns regarding the hot air nozzle not seated correctly is a problem. If you have a low end system may I suggest and IR rework system, where nozzles dont come into play. Good luck.

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Yngwie

#22180

BGA Short | 29 October, 2002

Thank for you input. I'm using the SRT 1000 with the low air flow options. This suppose to be a good machine, rite ?

cheers.. zambri

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RDR

#22189

BGA Short | 30 October, 2002

Low air flow huh. This is interesting, I have a low end machine that has adjustable air flow and I was having some shorting problems awhile back (corners mostly) and my first attempt was to preheat the board up to 170C and then use lower temp to put into reflow. I was using a lower airflow. Since then I have done more playing around and found that If I increased the airflow I could run a more traditional profile with even better results! Could it be possible that this low flow was not heating the package evenly causing some locations to get very hot while others remained cooler causing the package to warp? Since you noticed it was the sides that had "blockage" of escaping air I think that this might be the case. My "theory" would be that increasing air flow might allow the nozzle chamber to become more stable in regards to heating the package evenly. OR it would really make a mess!

just a thought on the situation

Russ

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dragonslayr

#22198

BGA Short | 30 October, 2002

Have you tried doing a reflow of the BGA with the connectors removed? This should solve the question of connector proximity giving you a problem.

Also, try rotating the board 90 deg in your rework station before reflowing a BGA. Be sure to rotate the BGA the same direction. This will help you find out if a nozzle problem exists. If the defect still occurs on the same sides of the BGA (closest to the connector) with the BGA and board rotated, there is most likely a nozzle flow issue. If the defect follows and is now rotated with the chip I would suspect a component issue.

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Yngwie

#22220

BGA Short | 31 October, 2002

Thank you very much for the valueble inputs...

cheers..

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