SMT007 Magazine

SMT-July2018

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74 SMT007 MAGAZINE I JULY 2018 spokes provide isolation and can significant increase the probability of a good joint. Other considerations include component lead diameter to plated through hole diame- ter ratio mismatches. A plated through hole that is either too large or too small vs. the lead can equally result in insufficiencies. A recom- mended aspect ratio is typically 0.6 larger than the component lead will provide good results. Process Considerations Generally, this comes down to heat trans- fer or insufficient flux as either can have an equally significant impact on solder fill. Lack of flux penetration or presence due to profiles which are too hot are the most common root causes. Products such as Fluxometer, which use acid paper and specially designed PCBs with regu- lar spaced plated through holes, can be used to ensure the appropriate amount of flux and penetration (i.e., pressure) is applied for opti- mal use. Regular or monthly reviews including lev- checks or wave riders can also provide an indication of the wave levelness, tempera - ture profile and overall oven performance and is recommended to ensure process drift related to the equipment is not a contributor to defects. Solder Voids Solder voids or out-gassing (blow holes and pin holes) occurs when a solder joint has a small hole that penetrates the surface of the solder connection. This is typically due to moisture entrapment that during the soldering process out gasses from the joint. Process Considerations Like components, PCBs are also moisture sensitive, however, they are commonly not treated in the same manner as moisture sensi- tive components. As a general rule, all PCBs should be considered MSL 3 and be managed as any other moisture-sensitive device. Best practice is to ensure PCBs are sealed and only opened just prior to use. Extended periods between thermal cycle operations like surface mount reflow and wave should be considered when reviewing exposure time. If a board is not soldered within 72 hours after the previous thermal cycle oper- ation, it should be baked to remove exces- sive moisture in accordance with J-STD-033 or k ept in a dry cabinet with a relative humid- ity <5% to minimize the risk of such occur- rences. Solder Balls Solder balls and spatter defects are generally where a small sphere of solder adheres to the laminate, resist or conductor after wave solder- ing. There are typically three types, random, non-random and splash back, which are all typically process related. Process Considerations For random solder balls, these are the easi- est to address and are typically a result of an excessive flux prior to wave, uneven wave height. If you hear a "sizzle" while the board is going over the wave solder it is a good indi- cation that the pre-heat is either too low or the flux application is too high or the wave temper- ature is set too high. Non-random solder balls which appear in the same location or trailing pin are most commonly due to insufficient flux or pre-heats are too high. Figure 3: A Fluxometer can be used to ensure the appropriate amount of flux and penetration is applied for optimal use.

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