Today, advanced electronics products are shrinking PCB, component, and packaging sizes and in some instances as small as a postage stamp or even smaller. Therefore, caution has to be taken when products like these are subjected to high temperature reflow cycles.
It’s important to consider low-temperature profiling. It is more effective if you can reflow components and solder pastes at lower temperatures and with lower thermal profiles, if possible. There are several issues that can emerge when applying higher thermal profiles and reflow during the assembly of small PCBs, including a greater chance of board delamination, Fig. 1.
Therefore, it’s a good idea to revisit various reflow temperature ranges. Lead-free solder, like SAC305, is 235–245°C; leaded solder is about 180–190°C.
There is considerably more to know about low temperature solder for microelectronics assembly.
Check out our recent SMT007 article to learn more. In the meantime, here are some tips and hints that’ll get you on the way to knowing more about low temperature solder.
- Consider chemical mixtures in low-temp lead-free solder pastes. Most common are tin/bismuth and tin/bismuth/silver.
- Peak reflow temperature of between 165–195°C provides a Delta of 50° between SAC305 and low-temperature bismuth-based solder.
- Low temp bismuth-based solder paste can reduce peak reflow temperature requirements to as low as 165–180°C.
- Also, sizes of voids and head in pillow flaws on bottom terminated devices are reduced since there is a 50°C Delta between a low-temperature profile versus the SAC305 profile.
- There’s a possibility of solder cracking during cool down. To alleviate that problem, low-temperature solder (LTS) can be cooled down at a slower gradient.
- By applying low-temperature solder paste, the board sustains less of a thermal shock compared to SAC305, which is around 235–245°C versus 160–170°C.
The OEM, EMS company, or CM needs to ensure that although the composition of bismuth is more in the alloy for low-temperature profiles and low-temperature solder paste, the joint created and assembly level or the microelectronic level for chip on board are still as sturdy as lead-free or leaded assemblies. Product usage is not going to change because the solder paste or thermal profile is changed.
There are certain considerations that need to be investigated before a low-temperature thermal profile paste can be selected for both SMT and microelectronic assemblies. It is prudent that these considerations are properly addressed for optimal SMT and microelectronics assemblies.