Issue link: https://iconnect007.uberflip.com/i/565949
68 The PCB Magazine • September 2015 HASL—Hot Air Solder Leveling: • The oldest surface finish • Lead and lead-free versions are available • Leaded 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 HASL is blown from the PCB surface to re- move excess solder; this can create non-uniform coverage which makes component placement of tight pitch components difficult. The hot temperatures of lead-free HASL can cause warp- age and soldermask embrittlement. The plated through-hole may be plugged or reduced. OSP—Organic Solderability Preservative: • Highest volume surface finish, worldwide • Applications range from low-end to high-frequency server boards, also used in selective finishing • Latest versions are copper selective and more thermally resistant for high- temperature, no-lead applications • Applied through chemical absorption on the copper surface; no metal-to-metal displacement • Inexpensive surface finish • Limited shelf life OSP does have implications at the assembly level. Older versions of this finish are not ther- mally resistant and couldn't resist more than one reflow. The coating hardens with reflow exposure and becomes more difficult to solder. Material transfers onto the probe tip (during electrical test) can result in false readings and will require more frequent probe maintenance or a special probe style. Higher OSP thicknesses are detrimental to solder paste flow and hole fill. Immersion Tin: • 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 • Aggressive on soldermask Cautions at the assembly level include 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. Immersion Silver: • 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 • Easily scratched, sliding connector limitations The predominant issue seen at the OEM level is micro-voiding. Small voids occurring at the intermetallic layer of the solder joint could cause solder joint fracture. This defect manifests itself preferentially on solder mask defined pads which are more difficult to develop properly. ENIG—Electroless Nickel Immersion Gold: • Highest revenue surface finish • Applications associated with high reliability • Used often in the flex market • Aluminum wire-bondable • No degradation between reflow cycles, can be held mid-assembly for extended times • New deposit thickness specifications for gold are under review to address the high cost of gold and hyper corrosion/ black pad issues with extended dwell times for the gold FINAL SuRFACE FINISHES FOR AuTOMOTIVE: NO ONE-SIzE-FITS-ALL SOLuTION continues FlEx tAlk