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Tin-Lead Solders Since these alloys have been the staple of PWB assembly and since Restriction of Hazardous Substances (RoHS) legislation exempts the alloy for certain applications until 2010, it is useful to understand this simple alloy system. A sound understanding of Sn-Pb will help in the intelligent selection of lead-free (Pb-free) solders and the mechanics of manufacturing reproducible and reliable solder joints. Until now, Sn-Pb solder was the alloy family of choice for electronics manufacturing with a long history dating back to the origins of circuitry. 45.2.1.1 The Sn-Pb Eutectic Alloy. Sn and Pb can be formulated into many alloys but its eutectic alloy is the most familiar to the electronics assembler. It is a ductile metal exhibiting a melting point below 200 C, a temperature range that has proven benign for most of the materials associated with or attached to a printed circuit board. Its fatigue resistance is sufficient for most commercial applications. Sn-Pb is easily fluxed by very weak organic acids and its shelflife in terms of solderability is long lasting and generally not an issue. There are many solder alloys to choose from, but few as compatible as eutectic tin-lead when it comes to production assembly soldering of PWBs. There are also several metallurgical properties that make this alloy attractive as a solder, which explains why it is so widely embraced as the paradigm of solders. That is not to say that other metal alloys cannot be substituted, but this composition has endured the test of time and many searches for alternatives. A eutectic alloy has several useful and interesting characteristics. Like all alloys, it has a melting point below that of its constituent metals (see Table 45.1)
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TABLE 45.1 Sn-Pb Constituent Melting Points Compared to Its Eutectic Melting Point Material Sn Pb 63wt% Sn:37wt% Pb*
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Melting point ( C) 232 327.4 183*
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Eutectic alloy composition.
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The eutectic composition has the lowest melting point of any other members of its alloy family. It has a discrete melting point rather than a melting range and its constituents are combined in a single, specific alloy composition. At the eutectic temperature, the eutectic metals coexist in a liquefied alloyed state as opposed to a state where there are some solids of one metal and liquid of the other. When non-eutectic Sn-Pb alloys melt, phases of Sn or Pb precipitate out of the liquid solution and give rise to a solid-liquid mixture with broad melting range. The further from the eutectic point, the greater the band of the melting range for the alloy. This region of broad melting temperature is known as the pasty or plastic range. Non-eutectic compositions do not solidify as quickly as eutectic alloys. Because eutectic solders nucleate and crystallize very rapidly at the solidus point, fine grain growth and high mechanical strength in the solidified solder joint result. Upon solidification, non-eutectic compositions result in internal stresses and coarse, dull solder joints with perturbed surfaces. These are all points in favor of using eutectic or near-eutectic solders for circuit board assembly. 45.2.1.2 High Pb-Content Solders. High Pb content Sn-Pb solders are known for producing compliant solder joints owing to elemental lead s high ductility. Due to high melting temperatures, these alloys are generally used to solder non-organic circuit board substrates such as ceramic. An organic laminate such as glass-epoxy is likely to incur degradation through a reflow cycle at such temperature extremes as required by alloys as 5Sn-95Pb (melting range = 301 314 C) and 10Sn-90Pb (melting range = 268 302 C). For these solders, the soldering
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process temperature is generally chosen to be about 20 to 40 C higher than the melting point or the upper bound of the melting range for reflow and even higher for wave soldering. Hightemperature solders are difficult to work with and are relegated to niche applications such as ceramic hybrid circuits or as a contact material for ceramic ball grid array (CBGA) and ceramic contact grid array (CCGA) packages and hand-soldering operations. Even in these applications, the solder ball or column is often soldered to the package using a lower melting-point formulation. 45.2.1.3 Tin-Lead Additives 45.2.1.3.1 Antiscavengers. Occasionally a small fraction of silver (Ag) is added (up to 2 wt percent) to improve solder joint appearance and to retard Ag scavenging. Scavenging occurs when there is the propensity of one metal to dissolve rapidly and thoroughly into another upon reaching liquidus. A good example would be silver traces on a thin-film ceramic device and the use of Sn-Pb solder to bond to it. During soldering, the Ag would dissolve very rapidly into the Sn-Pb. If too much Ag is dissolved, then the bonding pad on the thin-film ceramic device may be rendered unsolderable due to the lack of a wettable solder surface.The addition of Ag to Sn-Pb solder would slow the dissolution rate of Ag from the thin-film ceramic bond pad and retard scavenging. 45.2.1.3.2 Brighteners. Sometimes Ag is added to solder to improve wetting, making the solder joint smooth and shiny. When not used as a scavenger, the addition of Ag should be avoided as it is not usually needed for most electronic joining applications with Sn-Pb. 45.2.1.4 Sn-Pb Intermetallic Compound. In addition to lowering the melting point of pure Sn (232 C), Pb retards Sn-Cu intermetallic formation by piling up at the intermetallic boundary, frustrating tin-copper intermixing. Sn-Cu intermetallic compound (IMC) is crucial for solder-joint formation, but if the intermetallic layer is too thick, the resultant solder joint will be brittle and subject to failure during thermal cycling or mechanical shock. Although SnPb solder is known for its ability to wet well to a number of component lead platings and circuit board finishes, Pb actually inhibits wetting and keeps the solder localized to the targeted solder-joint area. Excessive solder spreading can be detrimental in three ways. First, if solder wicks away from the intended solder joint area, the resultant joint will be solder starved, and weaker than intended. Second, if the solder were too mobile, it could wick up connector leads and into the connector, decreasing inner contact flexibility, decreasing contact gap, and changing connector contact physics and resulting in a less reliable interconnect. Finally, if the solder wicks up too high on a gull-wing component lead, it inhibits the flexibility of the component lead and makes it more susceptible to mechanical failure.
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