Soldering process solder alloys

Tin-lead solder has been the basis for interconnections since the beginning of the use of printed circuits, and all associated materials and processes have been developed with the use of that material in mind. As a result, the abrupt shift to new solder alloys poses many questions about the related or affected processes and technologies that must now adapt and change to meet the lead-free requirements. Past experience or previous hterature cannot be rehed on to provide answers to these questions. The purpose of this book is to provide this information as specifically and in as much detail as possible, using industry standards, where they exist, or best practice that has a sound technical foundation and has been shown to work. 

In the United States and other countries, some legislative initiatives aimed at restricting the use of Pb have failed. This failure spurred research for solder replacement alloys. Several corporations and universities devised Pb-free solders, some of which were patented. As is the case with any patented item or process, the rights to invention reside with the inventor or sponsoring corporation in the short term, and several of these alloys may not be used freely. In some cases, only slight modifications in alloy composition separate corrunonly available solder alloys from patented solder compositions. The cost of patent licensing, whether by the end user or solder manufacturer, will be borne largely by the end user. 

As many PCB fabricators and assemblers transition to lead-free processing, they are faced with solder alloys that have an operating temperature up to 30°C (54°F) hotter than the traditional eutectic tin-lead solders. These increased soldering temperatures raise concerns about the ability of the solder mask (as well as all other materials) to resist embrittlement, discoloration, loss of adhesion, and cracking with repeated exposures to the higher temperatures. Some existing products may not be acceptable for lead-free processing. 

Selection of the flux type depends on the solder alloy, metal finishes on the board and components, condition of the surfaces to be joined, the type of soldering process selected, required solder-joint attributes, and the intended final use of the assembly. 

The most recent increase in N2-assisted soldering comes with the onset of large-scale lead-free soldering implementation around the world. Higher process temperature translates to higher oxidation rates during soldering. Also, as previously discussed, certain solder alloys are more prone to oxidation than conventional Sn-Pb solders. 

Solder requirements are the same as for any other process. There are no ahoy composition requirements specific to laser soldering, as this soldering method is compatible with leaded or lead-free solder alloys. Even high-temperature alloys can be soldered by this technique. When single-point laser reflow is apphed, the board quality and integrity are not compromised if parameters are chosen and adequately controlled. 

The diversity of materials, drive toward miniaturization, and globalization have significantly contributed to the corrosion of microelectronic devices [37]. However, the key point is that solder joints are often exposed to corrosive environments that can accelerate the corrosion process. Although corrosion resistance is an important parameter in choosing solder alloys, the corrosion behavior of Sn-Pb solder joints was rarely of interest because the oxide that forms on the tin-lead alloy is relatively stable. Mori et al. showed that both Pb-rich and Sn-rich phases dissolve when the Sn-Pb solder alloy is immersed in corrosive solution, and the corrosion rate is slower than that of the Sn-Ag solder [38, 39]. Compared to traditional Sn-Pb solders, Sn-Ag-Cu solders are easily corroded in corrosive environments due to their special structures (as shown in Fig. 3). The presence of AgsSn in Sn-Ag-Cu solders accelerates the dissolution of tin from the solder matrix into a corrosive medium... 

A process DoE was performed to determine a comparison of yield and reliability of solder alloys with various VOC flux types, laminate finishes, process operating windows, component types, and design styles. Effect on wave soldering equipment was also observed. To determine these characteristics and behavior of the solder, several modifications to the DoE were performed using different test designs. Due to concerns regarding time and cost effectiveness, a simplified DoE was performed. 

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