May, 2018
www.us-tech.com
Voiding in SMT Assembly: Causes and Solutions
By Terry Buck, President, Amerway
larger SMT pads where solder vol- ume is higher than typical joints, or
S
older voiding is a maddening problem. It often occurs within the solder joints of BGAs and
troublesome as too much. Some prob- lems, like a profile that is too cool, can be diagnosed visually. Normal solder joints are bright and shiny, a cold solder joint is dull, non-reflective and has a rough surface. A correct profile and proper
thermal distribution are essential to minimizing voids and optimizing the soldering process. Optimizing the reflow profile to remove any volatiles by extending preheat times and increasing time above liquidus will help to reduce voiding.
Stencil aperture designs can
also affect the overall level of voiding. An overabundance of solder paste can contribute to voiding. When printing onto large pads, use small opening grids instead of large aper- tures to reduce overall volume. Paste oxidation is another fac-
tor, especially when using expired solder paste. Consider changing to a new type of solder paste for a new product run where voiding is an issue, and always consider using a new solder paste for BGA assembly.
Acceptable Voiding? Some solder voiding is present
in the majority of solder joints and is generally accepted when the voids are small and the total void content is minimal. X-ray inspection is the primary method of solder void detec- tion and analysis. However, interpre- tation can be subjective and ham- pered by the two-dimensional char- acteristics of some X-ray images and software limitations.
Continued on page 76
Page 73
X-ray imaging is effective for discovering voids in SMT and BGA connections (photo credit: YXLON International).
where the solder is sandwiched between structures. Voids are often caused by flux trapped within the joint, as well as other factors, such as paste oxidation. A high number of voids will
make the solder joint porous, reduc- ing its physical reliability and elec- trical conductivity. Voiding is particularly challeng-
ing to eliminate as there are many possible root causes. To be sure that the problem is solved, the process engineer has to address every single possibility. However, addressing the root causes and applying a few simple solutions can reduce the problem to a manageable level. Then, modifying the flux, solder choice and formulation may solve the problem completely. One of the most common causes
of voiding is too great a percentage of flux in the solder paste. This excess flux is not given enough time to outgas before the solder is solidified. This can be remedied by changing the paste for- mulation to contain less flux, as long as it does not compromise the process. Another option is to use less solder paste per joint, if it can be done with- out “starving” the joint.
Proper Profiling Another major contributor to
voiding is using an incorrect or non- optimized reflow profile. Perhaps the preheat temperature is too low and solvent in the flux does not outgas completely or the soak zone is too brief. Another cause might be the type of solder alloy used. Lead-free solder typically has a four percent shrinkage in volume when it is cooled to solidify, thus it is possible for voids to develop when large pads are cooled down unevenly. Due to an improper thermal
profile, the preheat time can be too long and the temperature too high, preventing the flux from activating. With a non-optimized profile, the solder may not absorb enough heat. It is important to ensure that the peak temperature is at least 27°F (15°C) higher than the solder’s melt- ing point for more than 45 seconds. In other cases, the cooling speed is too slow. Too little heat can be just as
See at NEPCON China, Booth 1F25 and at SMT Hybrid Pkg., Booth 4-551
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