PCB manufacturers use surface finishing technologies to coat the exposed copper connection surfaces of PCBs. Surface finishes serve two primary purposes: they protect the PCB surface from contamination, which is often in the form of copper oxidation, and they provide a solderable surface for the application of both conventional and surface-mount components.
Although hot air solder leveling (HASL) is currently used on over 70 percent of the PCBs manufactured in the United States, alternative finishes are gaining wider acceptance. Due in part to government environmental encouragement, PCB manufacturers are investigating lead-free alternatives to HASL while continuing their search for improved cost and process control. Lead-free alternatives include:
As the lead-free debate intensifies, PCB manufacturers are being forced to reassess their expectations for optimal surface finishing technologies. In short, what are the advantages and disadvantages of the different finishing options? To maintain board integrity and optimize final reliability advantages, PCB manufacturers should evaluate how the following considerations affect overall process control and cost.
Solderability. The surface finish should maintain the solderability of copper pads and through-holes. Holes, fillets, and the bond lead to the surface mount technology (SMT) pads should be evenly coated for optimal connectivity. The surface finish should be deposited consistently to prevent voids, pin holes, and pits, and to produce consistent and reliable solder joints.
Shelf life. If properly applied and packaged, the PCB's surface finish should have a shelf life of at least one year. Storage conditions greatly affect the shelf life of the surface finish. Gold currently has the longest shelf life. HASL is a close second. However, intermetallic layer growth during storage can reduce solderability. Without proper temperature and humidity controls, no storage estimates can be made. For example, the solderability of OSPs degrades from prolonged exposure to high humidity, but can last up to a year in optimal storage conditions.
Handling. From a cost perspective, surface finishes with critical handling requirements demand greater process control and are not as resilient throughout the assembly process as finishing alternatives with normal handling standards. White tin handling requirements prevent boards from being stacked; any human contact with the surface produces defects that prevent solderability.
Surface Insulation Resistance (SIR). Residual materials deposited by surface finishes and chemical substances used in the finishing process can affect SIR readings. Manufacturers should incorporate the proper rinse stages and chemical removal stages into the finish process. The flux stage in the HASL process maximizes the wetting of copper surfaces, but the subsequent pressure rinse and cleaning stages are essential for removing the flux, which can embed in the porous surface of the solder mask.
Waste Treatment and Safety. As a result of the expanding lead-free movement, manufacturers are becoming more aware of the cost and health implications related to surface finishing technologies. HASL alternatives are being reviewed for environmental considerations and health benefits.
Thermal Cycles. As the number of thermal cycles increases, the solderability decreases. The HASL process has an extra thermal cycle and is subject to accelerated intermetallic growth if the coating is too thin. First pass yield capabilities should also be considered with OSPs, which decrease solderability with reworking. In general, OSPs do not perform well in double-side reflow processes.
Compatibility. Surface finishes should also be evaluated for compatibility with alloys, fluxes, and contact applications. The mechanical properties of the surface determine whether fine-pitch technology (FPT) components such as surface mount technology (SMT), ball grid array (BGA), flip chips, and wire bonding, can be applied.
Future articles in the dsi Quarterly Tech Review will present further details for comparative risk and evaluative data on surface finishing technologies currently used or newly emerging in the PCB manufacturing industry.
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