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36 SMT Magazine • April 2014 fEATurE quate amount of gold has been deposited, as per fabrication specs. On an electroless-nickel immersion gold (ENIG) surface board, forensic analysis provides the assurances the nickel—which is the base plating before the gold is deposited— is performing according to the spec sheet. Also, it pinpoints in- finitesimally minute fractures within solder joints as well as cracks within a via hole barrel. These details are not visible at the 300–400x magnification level, but with SEM, those de- tails are magnified upwards to the 2,000x level. Plus, in some instances, black pads or the acute corrosion of a board's nickel-plating layer can be viewed very clearly. As mentioned above, in- termittent connections rep- resent a major issue associated with BGAs, micro-BGAs and PoP, which are widely used in today's PCBs. Forensic analysis locates these inter-me- tallic failures. It can determine the distance be- tween gold plating and the solder ball in the event BGA balls are detached after assembly. These are the occasions where BGA is not work- ing properly unless you press hard on the de- vice and then it starts working again. Forensics also gets a good view of solder thermal fatigue when testing for solder joint failure. Solder thermal fatigue occurs in the field when the product is used in excessive heat, es- pecially containing BGAs or flip chip devices. Those fatigue cracks begin slowly and over time grow to get detached from the surface of the board due to the excessive heat. This is espe- cially true if the BGA isn't designed properly or if it's an analog circuit and heat dissipation isn't incorporated at the board design level. In some cases, there can exist a coefficient of thermal ex- pansion (CTE) mismatch between packages that contributes to this failure. Sometimes pinpointing defects like this us- ing conventional AOI and X-ray may or may not identify all solder fatigue failures associated with a given package. For instance, the failure or failures could be due to organic, metallic or any other kind, like solder joint detachment and even tin whisker growth, which at times AOI and X-ray don't recognize, because it is de- veloped over time and may not be present at the time of initial assembly. Also, in some cases, the sol- der bridging of flip chip devic- es have leakage failures. Due to solder or flux, especially when the device uses under- fill, a flip chip device leaks current after thermal cycling. Therefore, current doesn't get to the proper ball of the BGA and a non-acceptable noise creeps into the design at cer- tain levels. Destructive anal- ysis or forensics via SEM can reveal the voids due to the fact that air gaps are created inside the solder due to poor plating, as shown in Figure 3. After thermal cycling, a high-powered optical microscope can be used to verify other issues, such as de- lamination or bulk solder extrusion. As a result of thermal cycling, a solder can leak out to such areas as vias, thereby reducing the amount of solder needed to optimize a BGA's solder joint. Summary OEMs are assured their next-generation designs can receive the utmost advanced in- spection now that conventional technologies partner with forensics analysis to perform the most comprehensive inspection possible. Even as the industry moves to smaller form factors and three to four packages are mounted on top of each other, OEM concern over maintaining high reliability for their systems and sub-sys- tems is considerably alleviated thanks to both conventional and forensics inspections. SMT FOrENSiCS UNCOvErS ElUSivE DEFECTS aND SavES pCB DESiGNS continues Zulki khan is the founder and president of nexlogic Technolo- gies, Inc. To read past columns, or to contact the author, click here. Solder thermal fatigue occurs in the field when the product is used in excessive heat, especially containing BGAs or flip chip devices. Those fatigue cracks begin slowly and over time grow to get detached from the surface of the board due to the excessive heat. " "