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QFP rework

Jimmy

#27856

QFP rework | 1 April, 2004

I have some fine pitch QFP's to rework with our smt rework system (forced air). There are no intitial concerns in removing the component (we havent actually begun the rework process) however, installing a new chip presents a problem.

The QFP has a ground plane on the bottom of the chip that must be soldered to the pad of the PCB. After removal of the component, we would clean the pads to remove excess solder and wick the excess solder from the ground pad on the PCB. A new component would be used for installation and prior to placement, a dab of solder paste must be placed on the ground pad of the PCB. After placement of the QFP, reflow would begin.

The problem that I foresee is with reflow. If the solder paste melts underneath the package prior to the leads, wouldn't it cause the component to drift hence, get the leads out of alignment? Do I need to modify my profile such that leads reflow first, followed by the ground pad (the idea is that the surface tension of the leads would keep the component from drifting)? Any thoughts as to how I may rework this component?

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RDR

#27857

QFP rework | 1 April, 2004

You may want to tack a coup[le of leads first, making sure that the component is fully seated against the PCBA. Or what we do is apply solder to the E-Pad and the lead pads and then hit it with a "heat gun" until the solder melts on the E-Pad and then we place the part onto the board and reflow all the solder connections at once using tweezers to "nudge" the part onto the pads. The E-pad will solidify again just prior to putting the part on but what we found is that by preheating in effect the E-Pad the process sped up and the component sees a lower temp overall. This works very well. We perform this rework on approxiamtely 100-200 boards per week on a relatively ongoing basis from a small 64 pin QFP at 25 mil pitch to a 216 pin 16mil pitch QFP. (Good old ECOs for new code)

Another idea Russ

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Steven

#27858

QFP rework | 1 April, 2004

Heat guns scare me, hot air scares me too. Just the heck of it why not apply high temp paste to the center and eutectic 63/37 standard solder to the outside pads. The outside reflows first allowing proper seating of the component and with a proper profile the center high temp solder will reflow later on. Just a thought.

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RDR

#27860

QFP rework | 1 April, 2004

one more thought on this, When these boards are built new do we not have solder paste on the ground pad and then reflow? Has this caused any misregistration for you? I am assuming that you are not pasting the leads with the ground pad correct? I would also believe that it would be hard to get the ground pad to reflow first since the leads are exposed more directly to the heat being applied. I would bet (probably lose) that if you just stick that part on with the leads aligned that you are going to be fine.

Russ

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RDR

#27861

QFP rework | 1 April, 2004

Steve, why the fear of hot air?

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Jimmy

#27863

QFP rework | 1 April, 2004

Thanks for the input. I dont have a component specific stencil thus, cannot paste the lead pads. I would apply no clean flux to the pads on the PCB and dispense a dab of paste on the ground pad of the PCB.

>>When these boards are built new do we not have solder >>paste on the ground pad and then reflow? Has this caused >>any misregistration for you? We didnt assemble these boards so I have no knowledge of the manufacturing process that was implemented however, paste was cerainly applied to the ground pad. This is the primary reason why I think a reflow profile should be such that leads reflow first followed by the ground pad.

As far as using a higher melting temperature solder paste for the ground pad, it certainly sounds logical. My concern would be to keep the package temperature below the danger mark of 250 deg.C. Also, the component is originally reflowed using 63/37 alloy at both the leads and the ground pad . . . so there is a proper technique, I am just not sure about it. The problem is that I don't have ample parts (PCB's and components) to find a solution. Two trials at the most, and we must get the technique down.

>>I would bet (probably lose) that if you just stick that >>part on with the leads aligned that you are going to be >>fine. I think you might be right. Hopefully, my profile is such that the leads will reflow first and once the ground pad reflows, it will not shift the part out of alignmnet.

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