PCB Tech - SMT lead-free assembly process surface treatment summary

SMT lead-free assembly process surface treatment summary
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Due to the increasing use of fine-pitch component boxes BGA devices, the electronics industry began to use some lead-free surface treatment processes long before the lead-free boom arrived. In addition to HASL, all surface treatment processes are suitable for tin-lead and lead-free assembly, such as organic solderability protective film (OSP), chemical nickel gold (ENIG), immersion silver and immersion tin. Because what is needed is a flat surface coating, they can be used to replace HASL.
Due to the increasing use of fine-pitch component boxes BGA devices, the electronics industry began to use some lead-free surface treatment processes long before the lead-free boom arrived. In addition to HASL, all surface treatment processes are suitable for tin-lead and lead-free assembly, such as organic solderability protective film (OSP), chemical nickel gold (ENIG), immersion silver and immersion tin.

Because what is needed is a flat surface coating, they can be used to replace HASL. Each of these surface treatment processes has its own advantages and disadvantages, and none of them is perfect. HASL has the characteristics of uneven surface: some places are not thick enough, and some places are thicker, so the tin/lead process is also reducing its use. People choose OSP to replace HASL at first, but OSP is very fragile and needs special treatment. There may be problems when filling holes and testing on circuit boards, especially in the two processes of reflow soldering and wave soldering. Case. Electroless nickel gold ENIG nickel can effectively prevent the transfer of copper from the PCB pad to the solder ball package interface: prevent the formation of brittle metal compounds. The flat micro-holes or champagne-like bubbles in the immersion silver, as well as the black pads in ENIG, are the focus of attention. The generation mechanism of these defects is different, but one thing is the same: they are all related to the process control of the final manufacturer of the printed circuit board, and are related to the chemical material of the electroplating.

Which surface coating is most suitable for lead-free? There is no conclusion for the time being, it must be selected according to actual product needs and depends on the printed circuit board supplier. In some cases, two surface coatings may be used. For example, single-sided SMT uses OSP, and double-sided circuit boards and mixed (SMT and SMT) circuit boards use immersion silver. If you are using OSP, you can use nitrogen or a very corrosive flux during re-soldering and wave soldering, which can be flexibly controlled according to the product. If you use ENIG, you do not need to use nitrogen. When choosing a surface coating, the use of nitrogen, the type of flux and the sensitivity to cost are all important factors.

Compared with the excellent performance of HASL SnPb, lead-free alternative surface treatments have certain problems. OSP's tin penetration performance, ICT test compatibility, chemical Sn tin whiskers, and environmental issues have not been properly resolved, and chemical Ag, which has been favored by many companies, has also begun to be troubled by the Planar Void problem. In 2005, Intel reported the Planar Void phenomenon of chemical silver, which caused the early failure of solder joints, and the cracks extended along the cavity and penetrated: further research showed that the cause was the Cu cavity caused by the variation of the chemical silver process itself. Some potion suppliers claim to have resolved, but the endless market accidents show that there is currently no good control or prediction method for this variation.
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