PCB 1b is a display board that accompanies PCB 1a, and measures approximately 3x3.5.”Īssembly Processes. PCBs 1a and 2 are both industrial controller boards, measuring approximately 6圆” and 8x8” respectively. The test vehicles used in these experiments are shown in figures 1a through 2. The capability of the SnCuNi solder alloy and flux to form quality interconnections on the low end of the reflow process window is tested by running it in a production process and comparing it with SAC305 in the same process, and with both alloys reflowed in a hotter process. Wetting speeds and intermetallic formation rates are slower at lower temperatures, introducing the potential to create “cold,” poorly wetted, or otherwise inferior and perhaps unreliable solder joints. ![]() Peaking reflow temperatures within 10 degrees of a solder’s liquidus point can prompt concerns about solder joint quality and integrity. Recommended peak temperatures in reflow processes are usually 12-30 degrees higher than the solder alloy’s melting temperature. Eutectic SnCuNi is completely liquid at 227☌, only 6-7 degrees higher than SAC 305. The non-eutectic SAC305 alloy has a solidus temperature of 217☌, a 3-4 degree pasty range, and liquidus temperature between 220 and 221☌. It is not as commonly used as an SMT alloy, however, due in part to its slightly higher melting temperature in comparison to SAC305. Its predominant application is in wave soldering, because it offers numerous advantages over SAC305, including much lower copper dissolution rates1, smoother and shinier solder joint surfaces without shrinkage cracks, and considerably lower cost due to the elimination of the silver content. Nickel-modified SnCu, also known as SN100C®, is a commonly used lead-free electronic solder alloy. Key words: Lead-free solder paste, silver-free solder paste, Sn100C, Ni-modified SnCu The performance of the solder pastes and joint formation in both cases are evaluated using numerous assembly metrics and reliability tests, including visual and automatic optical inspection, X-ray inspection, in-circuit and functional testing, optical microscopy, thermal aging, thermal cycling and shear testing. This important case is used to explore the lower limit of reflow capability and compare it against the baseline provided by the typical thermal process. In the other case, thermally sensitive components constrained the profiles to peaking near 235☌, only 8 degrees above the silver-free alloy’s 227☌ melting temperature. The major differentiator between the two cases is the reflow profile: In one case, the SAC profile was considered nearly perfect for the SnCuNi alloy, peaking at 245☌. This paper reviews two case studies that test Ni-modified SnCu solder paste on mixed technology circuit assemblies which currently use SAC305 in production. Many wave solder operations use silver-free alloys for through hole soldering, and SMT operations are also beginning to adopt the alloy for surface mount soldering. Low-silver and no-silver lead-free PCB solder alloys represent substantial cost savings over SAC alloys that contain 3% silver. Technol.By Karl Seelig, Vice President Technology, Tim O’Neill, Technical Marketing Manager, Kevin Pigeon and Mehran MaaleckianĪBSTRACT. Zhao, Methods for Phase Diagram Determination (Elsevier, Amsterdam, 2011) ![]() ![]() JinGang Gao, Wu YiPing, Han Ding, Solder. Morgan, Mechanics of Solder Alloy Interconnects (Springer, Berlin, 1994) When two joints are placed on the sample the temperature increase is close to 20 ☌. ![]() The maximum temperature increase of the joint is substantial: 25 mg of the solder paste increases the joint temperature by about 15 ☌. Analysis of the temperature profiles obtained during the reflow processes in a real furnace shows that latent heat influences the surrounding temperature not only during the phase transformation, but also during the whole following passage of the sample in the furnace. The released latent heat from one joint increased the temperature of the surroundings and delayed the solidification process of the second joint. While one peak was present during melting in DSC, two peaks were present during solidification of two joints connected by a resistor. Melting and solidification processes of two solder joints were investigated by both methods to determine the influence of latent heat on the surrounding temperature. The reflow process of SAC305 solder paste was investigated by differential scanning calorimetry (DSC) and measurement of the temperature profiles in a real continual convection reflow furnace.
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