SN100CL, the HASL version of SN100C, is being adopted in the PC FAB industry as the de-facto standard for LF HASL. SN100CL is a solder alloy with the following composition: 99.3% tin, 0.7% copper, .05% nickel and a nominal of 60ppm germanium. Benefits of SN100CL are high fluidity close to its melting point, a property that is useful for the lead free HASL finish process and a smooth bright shiny surface comparable to the surface seen with tin-lead.
Hot air leveling is a process that involves dipping a bare board into a solder bath. The excess solder is then removed by a stream of air on both sides of the board. Hot air leveling can be performed both in a vertical and horizontal mode.
Choosing the proper lead free alloy is critical in LF HASL and SN100CL has been chosen as the standard in the LF HASL due to the following reasons:
- SN100 does not attack stainless solder pots
- Dross is quite low compared to SAC
- Raw material cost is low
- SN100 dissolution of copper is more controlled than SAC alloys
One of the main criteria for evaluating a solder finish is of course solderability. But, it is important to discuss the actual reaction that takes place in hot air solder leveling. The circuit board with exposed copper is immersed into the solder pot. An exchange takes place whereby tin and to a certain extent the nickel, exchange with the copper on the surface. The exchange at the surface of the copper results in the formation of an intermetallic copper, otherwise known as IMC layer. Basically two IMC layers; Cu6Sn5 and Cu3Sn. These two intermetallic layers do not exist independently in the SN100CL coating. Instead, they are combined to form into a single IMC layer.
Solderability of the SN100CL coating was studied in comparison to other surface finishes. It was done as a function of thermal aging with reflow cycles and 4 hour heat aging. As expected, solderability decreases with multiple reflow and heat aging steps. SN100CL showed a reliable surface finish that performs well after multiple reflow and heat cycles.
Solder layer thickness varies as a function not only of air pressure, but pad size and board orientation. Different types of solders give different thickness distributions. HASL gives an overall lower thickness, while vertical HASL with lead solder gives a higher thickness.
The range and standard deviation was similar for both SnPb and SN100CL solders. A solder thickness of 1.5 to 2.5 microns was easily obtained using the HASL process with SN100CL lead free solder.
Measurement and control of thickness is a key to ensuring good solderability. There are different methods for measuring thickness. XRF, which quickly reports a total solder thickness is a non-destructive method with widespread use. Cross sectioning is another method, but a microscope with a magnification power 1000 or greater is required and it is time consuming. SERA, another method and can distinguish between pure solder and the IMC layer, similar to cross sectioning. It is a destructive like cross sectioning, but faster.
SN100CL lead free HASL is a good alternative to leaded HASL and other surface finishes. The alloy has been used successfully for may years in HASL applications and can be easily controlled through analysis, drossing, and additions. It is a reliable finish that contains a barrier to IMC growth. The HASL method applies an appropriate solder thickness to ensure good solderability after multiple cycles. SN100CL is compatible with both leaded and lead free assembly operations.
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