BGA Rework

Thanks for your replies guys, there's some good info there.

Dave, answers to your questions:

The stencil was initially a big problem as with the Egineering Model's (EM's) and Engineering Qualifing model (EQM's) we kept getting short circuits due to too much paste. To fix this we reduced the pad size (these are all teardrop shape pads). We then found that the reliabily of the print went right down because the stencil was right on the limits of what could be done. We introducted a critical inspection of the solder paste print at at least times ten mag. This was unfortunatly ignored in build of approximately 30 boards. All these board have been brought into dispute and we've reworked several.

The boards can not be point to point tested on the outputs/inputs of the BGA's so we run a boundary scan on the boards which should pick up any faults. We have since found that this DOES NOT pick up ALL faults. Big problem.

These are all space boards and have the ESA spec on them, they can't be reworked too many times, and on most of the boards a bunch of FPGA's and line drivers have to be taken off then put back on after rework.

Another problem is that the boards are double sided and therefore each of the BGA's have already offically been though one rework. The boards (polyamide) are a bit notorious for the pads leaping off anyway without rework.

ANOTHER problem is that the board are super-duper space hardware and therefore dissapate heat so quick you can hardly measure it, this makes sure you cannot put the BGA's back on locally as the heat profile cannot be achieved and is not space qualified. We have just about managed to qualify taking the BGA's off locally and thankfully this seems to work, and we even have evidance in some cases which clearly shows there was no solder paste present.

The bottom line of the problem is the operator of the SMT machine who was trusted to follow the work innstructions didn't, and we have no recovery plan! Every board we have reworked has had more than one failure than went undetected from the boundary scan.

I even went as far as putting the board inthe X-Tec x-raying machine to see if you could see the change in shape of the solder fillet, which you can, but only if you know what you are looking for, on which ball, on which BGA, which when we have 240 connections on each BGA and 22 BGA's on a board - it doesn't work out well to spent all that time trying to get the right angle in the x-ray machine!

The only conclusion i could come to was the removal of all BGA's to ensure that i have enough confidence in the board to tell the customer it will survive launch and then work for 15 years!

I do believe there are machines that measure the solder paste once it is printed, and some even measure the hight of the solder paste, had a quick look but haven't checked with Siemens, who made our SMT machine.

Any thoughts on the above welcome!! Chrissie

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Two weeks!!! Wow, what a place to work. Any openings???

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This may be a little too late, but ever heard of a missing "silver". Had one one time (missing ground plane)and wow look at those TANT caps explode...might need some safety glasses! If they are all open look towards the design a little closer just to be sure that you can rule it out.

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I think i am just going to try and rework this one board. As you can well imagine we are going to have some interesting meetings trying to get the customer to accept it. The other boards actually have replacements (of the ones that have failed, and please don't even mention Thermal Vac to me as i'm sure some the results from there will be intersting!). This one didn't have a replacement and had a lead time of 7 weeks just for the PEC. Not to mention the 14 man days it then takes to build it up to test!

Chrissie

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Chrissie,

I'm glad you (and your people) made it through the painful process of reworking all those BGA's. Hope all your boards sucessfully pass test. Now on to the larger issue.

In my experience, the solder paste screen printing process is (along with Reflow) the most important part of the whole surface mount process. After all, if the boards (prints) don't look good coming out of the screen printer, they certainly aren't going to get any better on down the line !!

I'm actually shocked to hear that you aren't doing anything but a visual inspection - post print, given the nature of the products you're building.

As to your comment "I do believe there are machines that measure the solder paste once it is printed, and some even measure the height of the solder paste, had a quick look but haven't checked with Siemens, who made our SMT machine." There are many such machines available.

With this market you could pick up used equipment very cheap. I have had significant experience with the Cyberoptics (www.cyberoptics.com) Laser Section Microscopes (LSM). These are stand alone units (LSM 3) that are very operator dependent.

What you might want to look into is the Cyberoptics SE200 (Cybersentry), which is an inline system which you could program to measure paste height and volume on your most critical areas around the board. Such as, oh I don't know, BGA locations maybe ? This could take the operators out of the equation by providing you with a "Go, No Go" capability. The Cybersentry's are plentiful on the used market now.

There are of course much more expensive options avialable to you (Inline AOI) depending on your needs and budget.

Hope this helped in some way. Good luck.

DaveM

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Nice sugestions from all.. but one simple solution that you should practice from here on out.... CFM... If you would have built up your process, you would have found the flawes there early allowing you to correct the process flow. I would use CFM on a job worth a dollar much less 25 million. Good luck in the future! Whatever those boards goes in hopefully will stay clear of my house.

Kevin Simpson Technical Services Manager Qualcon www.qualcon1.com

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