barcode scanner programming Solder alloy Melting temp. SMT peak temp. Wave solder pot temp. in Software

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Solder alloy Melting temp. SMT peak temp. Wave solder pot temp.
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Solder reliability Process compatibility
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FIGURE 45.3 SAC solder requires higher process temperatures than Sn-Pb.
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Figure 45.3 demonstrates the process temperature differences for Sn-Pb soldering compared to SAC. A condition known as sporadic brittle fracture has been reported and may be related to the use of SAC alloy soldered to electroplate gold over electroplate nickel. It is most often associated with BGA solder ball-to-BGA package attach, but may also occur in board-level solder joints at the BGA ball-PWB interface. The root cause of failure is still unknown. Sn-0.7Cu Alloy. Eutectic Sn-Cu (m.p., 227 C) is being used as a wave solder charge material. It is compatible with most board surface finishes and component lead-frame finishes. Its relatively low cost (about 50 percent more than Sn-Pb solder) makes it an attractive Pb-free alternative. It also has been demonstrated to be an effective alloy for Pb-free hotair solder-leveled (HASL) surfaces on PWBs. One problem with Sn-Cu alloy, especially at wave soldering or fountain soldering, is that small amounts of dissolved copper will cause a dramatic rise in the melting point of the alloy. The high melting temperature of this alloy causes rapid material dissolution from PWB surfaces and component lead platings. Care must be taken to top off the wave solder pot with an appropriate amount of Sn or Sn-Cu to maintain proper Sn-Cu alloy proportions. Indium Alloys. This metal and its effect on the solder joint is very much the same as for bismuth. Indium (In) is very soft and ductile in its elemental form and melts at about 157 C. In-based soldering alloys are much more expensive about 25 times that of Sn-Pb due to indium s scarcity, thus it is not suited as a dominant metal in solder. Price and supply make it unattractive even as a trace agent or a minor solder alloy constituent.10 B. Allenby et al.11 concluded that the world s indium supply would be depleted in about one year if indium were consumed at the same rate as Pb. An In-Sn eutectic occurs at 58 wt percent In with a eutectic point temperature of 120 C. It forms intermetallic compounds with copper from the circuit board s bonding pad or copperbased component leads. Copper is highly soluble in indium.12 Indium has been demonstrated to be of value in reducing the leaching of gold into solder. F. Yost13 reported a lower Au dissolution rate for indium-based solders as compared to Sn solders. The use of indium allows soldering to gold films without thoroughly consuming the gold. The resulting solder joints are not brittle, as is the case of Sn-based solder joints exposed to too much gold.
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Indium and some of its alloys can wet to glass, quartz, and other normally unsolderable inorganic substrates with the aid of an ultrasonic soldering tool. Ultrasonic soldering will be discussed in the chapter on soldering techniques. Zinc Solders. Zinc (Zn) alloys, favored by some Japanese companies, oxidize rapidly. Solder paste shelf-life has been an issue even when these alloys are refrigerated at very low temperatures. Dross formation in wave soldering has also been problematic. Zn alloys are also known to have some corrosion problems.
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Most every circuit board consists of a stack-up of insulator material such as fiberglass and conductive layers composed of copper. Copper oxidizes quickly and its oxides are somewhat chemical-resistant, making it tough to solder without aggressive fluxing. Copper also corrodes in the long term from left-over process residues, fingerprints (body salts), or airborne contaminants. It is for these reasons that copper traces and solder lands on circuit boards are protected by solder mask, plating, or other surface treatments. The following section covers the most common board finishes. The process engineer needs to bear in mind that in addition to solderability, other surface finish related issues may affect manufacturing or cost, including the following:
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Probe-ability This is the ease with which conventional in-circuit test probes are able to penetrate surface oxides or solder flux residue. Tribology The frictional properties of a material become important for press-fit connections. Cost Some surface finishes are significantly more expensive than others. Shelf-life This is the ability of a surface finish to resist oxidation or other surface conversion or corrosion and remain solderable. Reliability This refers to the resultant solder joint strength as it relates to long-term use and resistance to shock, vibration, and other environmental factors. Corrosion resistance Some surface finishes are more prone to corrosion than others. Immersion silver and copper are two such finishes. Other surface finish related defects Immersion silver (Imm-Ag) sometimes exhibits linear arrays of microvoids at the intermetallic interface which detracts from solder joint strength. Electroless nickel/immersion gold (ENIG) sometimes results in brittle fracture if the plating chemistries are not maintained properly.
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Hot Air Solder Leveled Surfaces HASL coated boards have been the industry mainstay and the most common board surface finish worldwide. HASL is generally eutectic or near-eutectic Sn-Pb flowed onto exposed surface pads and traces on a finished PWB at the board shop prior to electronic assembly. It is inexpensive, eminently solderable, and easy to apply, and has excellent shelf-life/aging properties.There is the old adage in the industry that nothing solders like solder, which has made HASL a safe and preferred finish for many years. Previously it was thought that the HASL process could not deliver flat enough pads for fine-pitch surface-mount. Now it is a wellproven surface finish for component lead pitches down to at least 0.5 mm. Reputable board shops no longer consider HASL difficult to apply. RoHS for the most part marks the impending demise of the Sn-Pb HASL surface finish. There are efforts to find other metallurgies to extend the useful life of HASL equipment.
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