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May, 2018 Voiding in SMT Assembly: Causes and Solutions Continued from page 73 Voids in BGA/CSP package-to-substrate con-
nections can cause issues with thermal manage- ment, drop shock resistance and signal interfer- ence. No voiding is the ideal result, but this is not easy to achieve. Solder paste printing, reflow, alloy, and flux chemistry all have a significant effect on the amount of voiding. Solder joint geometries can also contribute to
voiding. Leadless chip carriers or large, flat compo- nent surfaces, perpendicular to the board, can inhibit outgassing during soldering, increasing the chance of voids. Flux byproducts, both liquids and gases must slowly make their way upward. If they are impeded by component geometries that prevent the proper upward flow, voids are usually the result.
Strength and Resistance A solder joint is expected to provide mechani-
cal strength, as well as electrical conductivity. The resistance of the solder joint will increase with the occurrence of large or many voids, as the cross-sec- tional area of the solder joint is reduced. The same voiding will also reduce the joint’s physical strength, due to it becoming porous, and the joint is continuously affected by the stress and strain of CTE (coefficient of thermal expansion) mismatch. Eventually, the weak joint can fail after mul-
tiple stress and strain cycles. In addition to phys- ical joint strength and electrical conductivity ver- sus resistance, thermal conductivity is affected by voiding. An active component can produce a signifi-
cant amount of heat during operation. This energy must be dissipated to prevent overheating or com- ponent breakdown. Too much voiding in solder joints can inhibit the necessary thermal transfer from the component to the PCB. An aggravating factor is that voids in solder joints tend to accumu- late toward the top of the joint at the interface between it and the package. Void location and size have the most influence on the performance and reliability of solder joints.
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Void size negatively impacts the joint by allowing entrapment, raising mechanical stress, creating thermal barriers, and by restricting the path of current flow. Larger voids reduce the physical strength of the solder joint more than small voids, because the mate-
Developing a robust lead-free soldering
process that minimizes voids will, in addition to the mitigating actions for void reduction, require strict attention to many parameters. These include sol- der paste chemistry, a tightly controlled thermal profile, solder surface tension effects, oxidation of the outer surface of solder joints, metallization of finishes for boards and components, and compo- nent and board outgassing during reflow. It is critical to select a solder paste that has a
flux chemistry designed for higher preheat and peak temperatures. Choosing a solder paste that does not contain resins and activators that might decompose at higher temperatures is a primary factor in void reduction. Leading solder paste manufacturers have
designed flux systems for lead-free alloys, reducing the possibility of voiding. This information is often available during the paste selection process. PCBs that are free of moisture and plating contaminants will also help to reduce voids.
Trusted Supplier Amerway manufactures its solder alloy
BGA with voids (photo credit: YXLON International).
rial thickness between the void and ball exterior is decreased. In some cases, small voids can actually increase reliability by changing the crack pattern. Studies have shown that there is no reduction
in reliability when voids take up 25 percent or less of the volume of the solder joint. Small voids can act as stress relievers, due in part to the compres- sive nature of the air pockets within.
Lead-Free Solder and Voids Lead-free solders generally have tighter
process window and process requirements than leaded solders, including higher peak tempera- tures and ease of wetting issues. It is safe to say that any voiding in a leaded process is going to be magnified or worsened when switching to lead-free.
blends from pure, virgin metals. This means that voiding issues are a result of a combination of many factors in the customer’s production process, and not the alloy. Amerway’s staff visits customer facilities and analyzes the flux, percentage of metal, solvent chemistry, the board and component finishes and metallizations, and the thermal pro- file. It is often wise to switch to a different solder formulation when changing products, for example, when moving from an ordinary assembly to one with a number of BGAs. Since every customer is different, Amerway
creates a unique soldering solution for each one. The company does not sell these individualized solutions to any other customer. Through careful tailoring of its customers’ solder materials and process, Amerway helps to reduce voids to an absolute minimum. Contact: Amerway, Inc., 3701 Beale Avenue,
Altoona, PA 16601 % 800-829-6337 E-mail:
tbuck@amerway.com Web:
www.amerway.com r
Award-winning solutions. Call: (618) 205 5007
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