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Generation QR Code ISO/IEC18004 in Software Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.

Copyright 2008 by The McGraw-Hill Companies. Click here for terms of use.
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Introduction of Lead-Free Assembly The introduction of lead-free assembly as a direct consequence of the European Union as shown in Reduction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives, changes everything. While lead-free assembly is nothing new, it has existed as a function of design requirements and has been a niche market in a leaddominated one. Plating and assembly of these non lead-bearing platings and solders were learned and understood by the users in this unique market. However, with the RoHS and WEEE directives, this niche, highly-specialized market has become the norm overnight and the forgiving tin lead process is gone. Up and down the supply chain, it s a new game that the supplier base needs to learn and learn quickly if quality and costs are to be maintained. This new game can be learned quickly if real and useable data is available to the process engineers supporting the plating and assembly processes. This means the re-introduction of the incoming inspection departments once used for the testing of components and PCBs, but with more sophisticated tools than the tweezers and solder pots of old. Solderability testing, if performed as outlined and detailed in this chapter, will help with process development, aid with the choice of surface finish, distinguish marginal from excellent suppliers and if followed widely can short circuit the time to return to the process latitude that tin lead provided.
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To understand the issues of solderability it is first important to understand what is meant by the term, including appropriate definitions, the physical properties of the materials involved, and the various standards developed to guide the user to ensure that the solder process will be successful.
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Industry Standards Two common solderability specifications for Components and Printed Circuit Boards are ANSI-JSTD 002 and ANSI-J-STD 003 respectively. In each specification, there are detailed test protocols as well as accept and reject criteria. While there are different specific test methods, the general test method can best be described as dip and look with the responsibility of accepting or rejecting the parts being tested being placed firmly into the hands of an inspector that evaluates by eye. With this test, the inspector uses the percentage of area that is wet with solder: with 95 percent as a pass and 94.999 percent as a failure. It should be stated that these dip and look tests do not usually represent what actually occurs during the assembly process. Both specifications have process simulation tests, but the action of immersing a component or PCB coupon into a static solder pot using tweezers remains the most common test method in use. Table 42.1 details the test protocols found in these J-STD documents.
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TABLE 42.1 Solderability Test Methods with Accept/Reject Criteria Test document ANSI-JSTD-002 Test method Test A, Test B, Test C, and Test S Comments All basically dip & look methods that have little to no possibility of meeting GR&R requirements A vital test for Pb-free All basically dip and look methods that have little to no possibility of meeting GR&R requirements
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ANSI-JSTD-002 ANSI-JSTD-003
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Test D Test A, Test B, Test C, Test D, and Test W
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SOLDERABILITY: INCOMING INSPECTION AND WET BALANCE TECHNIQUE
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In both these specifications are test methods that are not approved for accept/reject criteria, but that produce data that is useful for process development, shows what really happens during the assembly process, and if tested correctly meets the most stringent of evaluations to modern measurement analysis.These are tests E, F, and G for the J-STD-002 and test F for the J-STD-003. The test protocol detailed in this chapter is designed to change the test status of the wetting balance to one capable of producing accept/reject data, and more importantly, to provide accurate data for process improvements. 42.2.2 Definitions of Terms For soldering to occur some obvious fundamentals must be met: 1. The part needs to be fluxed see following definition. 2. The part must be solderable see following definition. 3. There must be sufficient thermal energy to allow wetting to occur typically 35 to 50 C above the Liquidus point of the alloy being used. Soldering is defined as a metallurgical joining method using a filler metal (the solder) with a melting point below 842 F (450 C). Soldering relies on wetting for the bond formation, and is defined in the ANSI-JSTD-002 as the ability of a metal to be wetted by molten solder. Wetting is defined as the formation of a relatively uniform, smooth, unbroken, and adherent film of solder to a basis metal. 42.2.3 The Choice and Impact of Fluxes for Solderability Testing Flux is an integral part of soldering and solderability testing. A flux used for solderability testing should never produce false positives or negatives. It should demonstrate the true characteristic of the sample being evaluated. Some examples are whether it s old, composed of intermetallics, buried under a layer of oxides, contaminated with organics, hiding a poorly wettable basis metal, or showing a clean fresh readily solderable surface. 42.2.3.1 Flux Functions. A flux has three main functions:
1. To act as chemistry that reduces oxides on a wettable surface. 2. To provide the correct interfacial energy potential between the flux/solder interface and the solder substrate interface. 3. To prevent oxidation from forming during the soldering process and to a lesser extent in the case of liquid fluxes they may protect the surface on the PCB from the extreme temperatures. It may also act as a heat transfer medium. Fluxes are necessary for soldering. While flux-free soldering is possible, in the presence of a reducing gas atmosphere, it is very uncommon. When attempting to check for solderability characteristics without the use of a flux, at best the solder will stick to the sample but will not wet the surface to form a true metallurgical bond.At the other extreme end of the scale are people using fluxes so strong that nothing will ever fail a solderability test. These fluxes do not represent anything used in the assembly industry.A great example of this is the use of the Hot Air Solder Level (HASL) flux for outgoing solderability testing. This type of flux typically uses 5 percent HCl or 5 percent HBr as the active ingredient which, when applied to a surface finish, will reduce any oxide layer present and reactivate any potentially passivated nickel layer and thus always produce a pass result. This is not what is supposed to be used and it does not provide any information pertaining to the actual solderability of the sample being tested because it passes all the time. 42.2.3.2 Fluxes and Evaluation of Solderability. There has been a fundamental change in how solderability is evaluated. The need to demonstrate a year of shelf life has been
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