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62 SMT007 MAGAZINE I JULY 2019 tion, or high temperatures, such as TVs and some appliances. These alloys are more brit- tle than SAC-305 and more creep resistant. This makes them well-suited for low strain rate stresses (temperature changes) but not well-suited for high strain rate stresses (shock, drop, vibration, bend, etc.). Some solder material suppliers have been working for several years to develop a version of this (low-temperature) alloy, which is com- parable to SAC-305 in terms of higher strain rate reliability. Historically, they had come close, but only recently have they hit the mark using "secret" recipes that include trace amounts of "dopant" materials to make the alloy more ductile and crack resistant. Other papers [1 & 2] discuss the dopant effects on lattice and grain structures by adding trace amounts of elements, such as copper, nickel, manganese, and antimony. We will not go into that detail here, but the main point is that some suppliers seem to have found an appropriate recipe that maintains the bulk alloy (and hence low melting point) but alters its properties in a sufficiently beneficial way. Another key factor in our consideration was that there had to be more than just one sup- plier of these types of materials before we per- formed any serious evaluations. We tended to avoid embracing proprietary processes or materials, which are only available from a sin- gle source. Advantages Some of the possible advantages of LTS include: • Reduced board and component warp • Reduced head-in-pillow • Lower residual stress • Reduced pad cratering • Reduced CTE geometrical effects (scale factor) – Allows for smaller land pads/footprints • Less expensive materials – PCBs – Components • Less thermal exposure – Thermally sensitive components – Semiconductors • Lower energy and maintenance costs – Estimated 20–25% lower energy cost depending on the profile – Lower maintenance cost on oven moving parts • Possible hybrid assembly – SAC-305 on one side and low temp on the other • Reduced voiding – ~50% void reductions observed Disadvantages As with any new material, there are unknown risks that may not come to light until the mate- rial is in use for some time. Some of the known risks include: • Package warp—hot tearing – Large BGA packages that were previously designed to flatten out at over 210°C can cause "hot tearing" in the solder joint because the package never completely flattens and remains warped throughout the reflow cycle. One remedy for this is to use more solder paste to make up for the gap difference. The lower-tempera- ture alloys did not tend to wet as aggres- sively (discussed in more detail later) and therefore can allow more solder per unit area without bridging. If the indus- try moves towards accepting these alloys on a large scale, it is likely the packaging vendors will adjust accordingly and may offer "LTS" versions of these packages • Rework – Solder wire: Very limited (only one known source and it is not commercially available yet) and it is brittle – Cracked solder pots: Bismuth expands when it cools, so it can cause some sol- der pots (wave solder) to crack • Wide liquidus/solidus range – We observed that a very rapid cool- ing was necessary to guarantee that the assembly was below 138°C exiting the conveyor reflow oven • Flux residue

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