smt培訓資料05-全文預覽
【正文】 control over temperature profile – Unfavourable to low thermal mass part(solder wicking Dual vapour reflow system Batch type production Inline single vapor soldering system BoilingTe m p.C degre eD ensit y g/m lM fgr’sD esignati onM anufac tur er174215215230253260F C43F C53 11A P F –215LS/230F C71H S ./ 2603M3MA ir produ ct sG al den3MG al den Convection reflow soldering ? The process of heat transfer from a hot gas flowing over a cooler surface is called convection heating. ? If the gas circulation is caused by different densities of masses being heated then the term free convection is applied. ? If gas circulation is done by mechanical means such as a fan or pump then it is termed forced convection ? q = hc AS(T G TS) ? q = the heat transfer rate from the gas to the surface ? hc = thermal convection conductance based on fluid dynamics ? As = surface (the assembly) area ? TG = the gas temperature near the surface ? TS = the surface (assembly) temperature Convection oven classification ? SMEMA classifies the convection system s class III and the mixed convection and IR systems as class II Air flow in a convection dominant reflow system Comparison of heat flow transfer Nitrogen Reflow Soldering ? Reasons to use nitrogen in reflow soldering : – 1) More fine pitch package used – 2) the increasing in use of low activity fluxes ? Advantage : – can reduce solder ball – avoid the flux from hardening in the absence of oxygen
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