Issue link: https://iconnect007.uberflip.com/i/770652
14 The PCB Magazine • January 2017 FINAL SURFACE FINISH: HOW DO YOU CHOOSE? HASL—Hot Air Solder Leveling Let's start with HASL. Fifteen or 20 years ago, HASL was the universal go-to surface finish. Today, that is not at all the case. A couple of things greatly influenced this change. The first was RoHS and lead-free requirements. The second is miniaturization and the need for tight- pitch components. HASL is blown from the PCB surface to remove excess; this can create uneven coverage, which makes placement of these tight-pitch components difficult at assembly. This finish is used in aerospace, defense and high-performance electronics as well as lower- end consumer markets. Things to keep in mind: • The oldest surface finish • Tin-lead and lead-free versions are available • Tin-lead HASL currently in limited use due to RoHS and WEEE initiatives – Currently exempt: industrial vehicles, military, aerospace and defense, high-performance electronics • Leaded versions are harder to source • Long shelf life • Not suited for fine pitch OSP—Organic Solderability Preservative OSP is the highest volume surface finish worldwide, with applications spanning data/ telecom, automotive and both low-end and high-end consumer products. Older versions of this chemistry were not thermally resistant and were not able to resist more than one reflow cycle. Improvements have been made to allow higher temperatures and multiple reflows with- out degrading. This finish does well as a selec- tive finish. For example, when ENIG is applied as a surface finish and OSP is used selectively, it will not adhere to or stain any of the gold sur- faces, so there is no need to plasma clean. Things to keep in mind: • Highest volume surface finish worldwide • Applications range from low end to high-frequency server boards; also used in selective finishing • The latest versions are copper selective and more thermally resistant for high-temp, no-lead applications • OSP is applied through chemical absorption on the copper surface; there is no metal-to-metal displacement • Inexpensive surface finish • Limited shelf life Immersion Tin Applications for immersion tin are predomi- nantly in automotive, U.S. military and aero- space. One caution at the assembly level is the fact that pure tin thickness is lost to the copper intermetallic with time and temperature. Loss of pure tin will degrade solder performance. The first reflow exposure will dramatically reduce the pure tin thickness and deposit stress could result in tin whiskers. This is a naturally occur- ring characteristic of tin in direct contact with copper. Things to keep in mind: • Applications are predominately automotive, U.S. military and aerospace • Excellent for press-fit applications (i.e., large back panels) • All contain anti-whiskering additives, but tin whisker elimination is not guaranteed • Low-cost, flat and suited for fine-pitch use • Aggressive on soldermask Immersion Silver Immersion silver is well-suited for high-fre- quency applications. It has the greatest conduc- tivity of all the surface finishes and it is flat. The signal travels to the top of the circuit reducing signal loss. This finish is often used in the data/ telecom, automotive, high- and low-end con- sumer and medical markets. Things to keep in mind: • Greatest conductivity of all the surface finishes; well-suited for high-frequency applications • Applications range from low-end to high-reliability product • Topcoats have been formulated to overcome tarnish and corrosion issues in aggressive environments • Flat—suited for fine-pitch with excellent solderability