07 January 2003
To facilitate the soldering of components to the copper features of printed circuit boards, it is usual for the board fabricator to apply a solderable finish.
The simplest and least expensive solderable finishes are the organic solderability protectives (OSP), thin transparent coatings which act to prevent tarnishing of the copper in storage and are displaced by flux during the soldering operation. Early versions tended to deteriorate when heated, but current formulations are suitable for multiple-stage reflow soldering, and account for about 10% of the market.
Hot-air solder levelling (HASL) is a method of applying a selective fused tin-lead finish, by immersing the pre-fluxed circuit board in molten solder then blowing off the excess. Because it subjects the bare board to soldering conditions, HASL is a "self-inspecting" process that will reveal potential solderability or bow-and-twist problems. HASL has fallen out of favour in fine-pitch surface-mount assembly because, being a fused finish, the surface is not flat and this can adversely affect solder-paste printing and component placement. However, HASL still represents about 60% market share. Lead-free HASL has been demonstrated, but is cosmetically inferior and can involve additional thermal stress from increased process temperatures.
Electroless nickel / immersion gold (ENIG) is the preferred flat finish for fine-pitch assembly, with about 15% market share. However, the deposition process, which involves long immersion times in aggressive chemistry at elevated temperatures, is expensive and difficult to control, and the soldered joint is formed not to copper, but to a relatively brittle nickel layer and there have been some reliability issues.
Immersion silver and immersion tin finishes are gaining popularity as alternative flat finishes, more cost-effective than ENIG and cosmetically more attractive than OSP.
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