|Ask the Experts|
August 15, 2018
Opens at BGA Component Corner Balls
We have been stumped by occasional opens at the corner balls of some BGA components after reflow. We have experimented with many minor adjustments, yet this occasional problem continues. Do you have any suggestions?
|Expert Panel Responses|
Based upon the warp and twist characteristics of the printed board laminate materials and the BGA this could be the major starting point for investigating this condition. It has been seen over the years that the BGA component does warp and many times creates shorts in the center of the device itself. Based upon the size of the component, when this happens the edges of the components are rising up off the surface of the PCB. The same condition can occur if the PCB itself warps and twist during the soldering process. So I would recommend reviewing the thermal profile, the warp and twist characteristics of both the board and the components. Secondly as a minor issue is the paste deposition process to make sure the paste is sufficient on the outer pads of the component.
Vice President, Technical Director
Have you tried making the stencil aperture larger for the 4 corners?
Vice President Technology
This is a classic example of Solderability-related Head-in-Pillow (SR-HiP). There is usually not a 100% fix, however, the best way to approach this problem is to locate the cause of warping and find the point in reflow where it occurs. In this type of HiP, component warpage is a possibility. In component warpage, the BGA warps during thermal profile causing the lifting of the BGA from out of the solder paste. As the solder paste and the solder ball oxidize, the solder paste does not have enough flux activation to wet the ball. Often, a longer cool down side of the profile will help. This gives a longer time above liquidus, allowing the solder paste and solder ball to form a proper solder joint. If the problem still exists, using a solder paste with uniform volume transfer during printing, as well as using an extended wetting time paste, could be a possible solution. If the BGA is warped prior to reflow, the defect is CP-HiP (Co-planarity Head-in-Pillow). CP-HiP occurs when some of the BGA balls do not contact the paste. In this situation, some BGA will arrive with a co-planarity of .008 mils while the printer might only be depositing .005 mils of solder paste. This combination leaves some of the BGA balls hanging in the air, therefore they will not wet. Printing a slightly thicker paste deposit may eliminate this defect. It is also a good idea to check the co-planarity tolerance on the parts you are receiving. If it is within the limit of your print height, this is probably not the issue. If all modifications fail, the problem ultimately lies within reflow. The best way to approach this is to minimize warping during reflow or change to a more robust paste formula.
Deck Street Consultants
The most likely cause is probably warpage of the BGA Package, this seems to be the reason for the opens as it appears always at the corner balls. On inspection does it look as though the corner paste has melted and that the reflow is in that case OK? If it all looks good, then the likely explanation is warpage where the device is curling up or smiling at the corners. You could look at reducing the ramp rate of the reflow profile to try to minimize the cte differences that might cause the warpage. As this effect happens only occasionally it could need just a minor change. The other explanation could be Head in pillow, this is where the paste at the corner of the device is exposed to most heat and therefore uses up the most flux during preheat. This leaves insufficient flux left to achieve good reflow and the solder does not wet to the ball correctly. On inspection you will see a line between the ball of the device and the solder or the solder will look dull and lumpy. If this is the case, then a more active or robust flux might be the answer. Also check after printing to make sure you are getting good consistent print deposits as a small reduction in volume can cause this effect. If you cannot change the paste or improve the print, can you reflow in nitrogen?
Global Product Champion
There couple good feedback, apart I recommendcheck is Board finish HASL, ENIG. HASL finish has more prone of such defect duecoplanirity issue, switch to ENIG finish PCB.second- PCB warpage: check PCBcomes form supplier preferable to be in sealed condition, if opened PCB keep in20%RH till use to avoid board warpage.
Supplier Quality Leader
In order to mitigate the effects of warp and twist of theboard and BGA causing Opens or Head In Pillow defects ample past needs to bedeposited.For the more difficult air atmosphere reflow additionalpaste volume is needed than in a nitrogen atmosphere, no less than 1:1 paste topad or over print may solve the problem. Nitrogen atmosphere reflow is moreforgiving and if employed it may also solve the problem providing ample pastevolume is present, generally 1:1 will provide the required volume in this atmosphere.RL Brown, Consultant, USA
A head-in-pillow open solder joint is often caused by a "perfect storm" of three contributing factors. The primary cause is, of course, warping of either board or component near the high end of the profile. However in many cases, as the part cools, the tension relaxes, bringing the sphere and paste into contact while still molten. So long as there is sufficient flux activity remaining, and sufficient volume of solder to make contact, the solder will still wet and a good joint will form. Solutions to head-in-pillow opens, then, usually take three approaches: 1) attempting to minimize warping of component (or board, in some cases); 2) increasing the volume of solder in the problem area(s); or 3) increasing flux activity at the end of the profile. Each of these process tweaks comes with some cost, risk, or trade-off.Alan Couchman, Process Sciences, Inc
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