A low-temperature solder like DurafuseTM LT can be essential for many applications where keeping the peak temperature below a certain point is critical (LEDs, flex substrates, opto-electronics, shield attach...). BUT, that's not the only reason we control temperature. Ask any chemist and they will tell you that heat makes things happen - so with a bit of temperature process control, we keep our reliability high on the solder, the component, and the final product.
I've mentioned in the past how time above the liquidus temperature is a common process control parameter in solder paste reflow. For any process, we need to know which parameters will really impact the end result - and how much of a window we have (a larger process window being better of course!). Using Durafuse LT (because the relationship with liquidus is different), we control time at the peak temperature since that's where the energy coming into the system really counts.
So... someone interested in having a low temperature solder process with good drop shock reliability might ask: "How do we control the process window with Durafuse LT?" Happily, we have an answer:
- What is the peak temperature experienced by the coldest solder joint on the board?
- How long does that single solder joint stay at its peak temperature?
That's it! For Durafuse LT, the coldest solder joint on the board needs to reach a minimum of 200℃. That's the joint we need to ensure gets enough thermal energy (time + temperature) in order to give us that drop shock reliability. If the peak temperature is 200℃, then the joint needs to stay at that temperature for at least 120s. Every other solder joint should be getting at least that much energy, and since Durafuse LT is very robust to higher temperature reflow (we'll need to cover that in another blog), we don't have to worry too much about those.
'Till next time!