Need Expert Support

Here is some general things I usually ask myself when having trouble with BGA's

What type of BGA ?

Pitch/Bump Dia etc. Smaller bumps sizes & tighter pitches require more attention. This also impacts stencil aperature design & paste selection.

Was the paste inspected in these areas after printing ?

Look for adequate coverage/general paste deposit quality.

What kind of paste was used ?

The aqueous cleaning process (For water soluble pastes) can cause problems with residues left under BGA's. This is a component of a low stand-off height. No-clean flux chemistry is usually not a problem.

Did you get X-ray's of bad & good one's ?

This helps to determine placement quality and finds gross opens/shorts. Also helps in finding voiding in the joints.

Did they run a profile with a thermocouple attached near the BGA ?

Placing the thermocouple underneath the device would be ideal but, this is not always possible.

reply »

Hi, can you definitely attribute the 30%yield to poor bga soldering? What is your high flyer defect encountered on the failed boards.Is this ICT ,flying probe ,functional test yield? Can you determine a defect pattern? Did your subcontractor provided you with quality data collected @every step of the process?what process controls do they have in place? most people will agree that 90 %of defects could be attributted to solder printing process and the variables involved (stencil design ,solder paste used ,environment,etc)I think the stencil design would definitly be critical.Without seing the actual files I can only tell you generalities .did anyone do a DFM/DFT analisys?Are the bga's on topside only?Is it a mixed technology board?(SMTtop/smtbot/th/wave?)how many reflow cycles?In what order?Are the vias under the bga's tented ? (on both sides if boards are going over the wave)Are the bga pads isolated with solder mask? If this is a subcontractor you worked with before with good results do a quality run to find out where the trouble spot is. The bga's pitch and other mechanical caracteristics are available on line from most manufacturers .Look under data sheet or specifications.About stencil design:Intel and Tessera have extensive info on their web sites.For example on a 5 mils stencil for a topside board with FP and BGA's (not micro or mini)appertures would equal the bga pads size (no apperture reduction).In certain cases one would make the stencil apperture square and equal in size with the bga pad diameter(oversized appertures).Your subcontractor should be able to give you exact resons why a stencil is designed in a certain way.If the copper distribution is fairly equal across the board and the boards didn't dryout before they made it into the oven and they use any decent watersoluble solder paste and they have succesfully reflowed qfp's in a similar board configuration you shouldn't have any problems.Look at their quality data during the run and board and stencil design.I totally agree that cleaning is extremely important but probably your boards would failed later in the field if the cleaning was inadequate.Did they measured paste heights at all bga locations on all boards .Are the failed boards bga x-rays looking worse than the ones that passed? Solder voids could be attributted to various factors and one of them is bad profile.If you have a similar weight component in a qfp package in the close vicinity of a bga somewhere in the middle of the board ask them to attach some thermocouple to it and this would give you a fairly accurate picture regarding the bga solder joints temp.I don't remember exacly but I think it is in the +/- 5 degrees celsius range. Good luck. Rob

reply »

I am currently in San Jose working on a BGA assembly problem in a customers subcontract house. I do consulting in this exact kind of work with years of experience trouble shooting PC board assembly problems especially with BGAs. I am booked up until late next week. You can call me on my cell phone at 512-636-7986.

reply »