20140225热风平整的优缺点.docx

The Pros and Cons of PCB Surface Finishes（中文译本见4th页起）by Al Wright EPE CenGineeredtecHnoloGiesAnyone involved in the PCB industry understands that PCBs have copper finishes on their surface and, if left unprotected, the copper will oxidize and deteriorate, making the circuitboard unusable. The surface finish forms a critical interface between the component and the PCB. The finish has two essential functions: to protect the exposed copper circuitry and to provide a solderable surface when assembling (soldering) the components to the PCB.Hot air solder leveling (HASL) was once thetried and true method of deliver consistent assembly results. However, ever-increasing circuit complexity and component density has stretched the capabilities of even horizontal solder levelling systems to their limits.As component pitches became finer and the need for a thin coating became more critical, HASL represented a process limitation for PCB manufacturers. As an alternative to HASL, alternative coatings have been around for several years now, both electrolytic and immersion processes. Here are some of the more common surface finishes used in PCB manufacturing, along with key advantages and disadvantages for each.HASL/Lead-free HASLHASL is the predominant surface finish used in the PCB industry. The process consists of immersing circuit boards in a molten pot of a tin/lead alloy and then removing the excess solder by using ‘air knives’ that blow hot air across the surface of the board.One of the unintended benefits of the HASL process is that it exposes the PCB totemperatures up to 265°C,which will identify any potential delamination issues well before any expensive components are attached to the board.图：1Advantages:? Low cost? Widely available? Re-workable? Excellent shelf lifeDisadvantages:? Uneven surfaces? Not good for fine pitch? Contains lead (HASL)? Thermal shock? Solder bridging? Plugged or reduced PTHs (plated through holes)Immersion TinAccording to IPC, immersion tin (