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1、Introduction Hand Soldering with Lead Free AlloysBy Simon HawkinsGlobal Business Development ManagerOK InternationalAs companies start to implement lead free soldering processes, hand soldering andassociated techniques have been identified as key functions in the manufacturingprocess requiring addit
2、ional research and development. Hand soldering tends to occur at the end of the process line where the circuit boardhas a high intrinsic value and so correct process control will have a significant affecton manufacturing costs and productivity.This paper discusses the fundamental aspects of the hand
3、 soldering process anddiscusses process adaptation requirements for successful lead free implementation.The Process Many manufacturing organizations control and define their hand soldering process byspecifying the soldering iron tip temperature. With the implementation of lead freealloys, with highe
4、r melting points than traditional tin/lead alloys, a morecomprehensive set of process parameters needs to be defined. The IPC defines the hand soldering process with a “rule of thumb” that talks in termsof reaching an optimum connection temperature for a fixed period. This places moreemphasis on hea
5、t transfer efficiency rather than absolute tip temperature. Factors suchas tip shape, tip condition, power output of the soldering iron and time on the jointwill all impact on heat transfer efficiency and should therefore be taken into accountwhen monitoring, controlling and defining the process. Th
6、e hand soldering process can therefore be defined by the following steps ;(a) The tip should be clean, well tinned and of the correct shape to maximize thecontact area with the lead/pad. The solder wire and the heated tip are appliedto the lead and pad.(b) The connection is brought to 40C above the
7、melting point of solder for 2 to 5seconds, during which time the flux starts to activate and the solder starts toflow.(c) The solder flows. It moves across the surface, wicks up the lead and fills thethrough-hole/covers the pad.(d) The heated tip is removed and the solder solidifies.Connection Tempe
8、rature As with any other soldering process, reaching the correct connection temperatureduring hand soldering is vitally important for the formation of good quality solder 第 1 页joints. Examination of the thickness and morphology of the intermetallic layer in thejoint can give a clear indication if th
9、e correct amount of thermal energy has beenapplied to the joint.The presence of an intermetallic layer is a good indication that there has been ametallurgical reaction between the solder and the termination and the solder andpad/land.Controlling the thickness of the intermetallic (rate of reaction)
10、is critical in theformation of a mechanically strong joint. The growth rate of the intermetallic layer istemperature and time dependant. Too much thermal energy will produce increasedvolumes of intermetallic, which are brittle. Too little intermetallic is an indication of insufficient thermal energy
11、, resulting in a dry joint during the soldering process.COLD SOLDER JOINTOPTIMUMEMBRITTLEMENT1.0 mCOPPER/TIN INTERMETALLICIntermetallic layer thickness can be an indication of joint qualityThe overall shape and surface finish of the solder joint fillet has traditionally been anindicator to solder jo
12、int quality. Unfortunately the surface finish and shape of leadfree solder joints are significantly different to those observed with tin/lead alloys. Pictures Courtesy of Cobar Lead free joints have a dull surface and higher wetting angles第 2 页Flux Considerations The application of the correct amoun
13、t of thermal energy will also affect fluxperformance. The typical constituents of electronics grade fluxes are shown below with therecorresponding boiling points.Acids. (Adipic, Glutaric)Alcohols. (Ethanol, Propanol)Water200C-260C B.Pt78C-180C B.Pt100C B.PtThe boiling points of the alcohols and some
14、 of the acids are well below standard handsoldering temperatures. It is important, therefore, not to apply too much heat, tooquickly during the hand soldering process, as this will cause the flux to evaporatebefore it has time to activate and promote the wetting of the solder.Flux selection is also
15、important for a good soldering process. With the higher processtemperatures, resulting in higher oxidation rates, and the weaker wetting forces thatlead free alloys have, “stronger” fluxes may need to be used. There is also likely to bean increase in the percentage volume of flux in solder wire, from about 1.0% typicallyused now to amounts in excess of 2%.
