Hello everyone,
Several years after the advent of Lead-Free, many customers still ask and/or have doubts about what happens if we have a Lead-Free component and process it in a Tin-Lead process. We conducted this test back in 2008 and, thinking it might be helpful, we would like to share the results of this experience with all of you. We apologize in advance if the discussion may seem… “a bit” technical. You have surely wondered, especially at the beginning of the Lead-Free transition (2006), what might happen to a Pb-free component if it were soldered with a Tin-Lead process. We tried this with a board that also had two Lead-free Ball Grid Arrays (BGA), and we treated it with a Tin-Lead process!… …….. this is our curious experience! So… have you ever wondered what happens to a Pb-free Ball Grid Array (BGA) when it’s treated and soldered with a Tin-Lead process? What happens is that, thanks to tin’s (Sn) solubility and its ability to dissolve materials that melt at higher temperatures, you get surprising results! The test was performed with KOKI Sn63-Pb37 solder paste (model intentionally not specified) with a temperature on the printed circuit board, during reflow, of about 215°C. The purpose of this analysis was to demonstrate that mounting the BGA (Lead-free) can be performed at lower temperatures compared to the Pb-free alloy reflow standard (about 240-245°C), obtaining “acceptable” results. But why don’t we always solder at lower temperatures? A fundamental… “particularity”: It’s essential to highlight a characteristic of the material, namely the incredible solubility of tin (Sn) and its ability to dissolve materials that have a higher melting point. Thanks to this, it’s possible to use tin-based alloys to still obtain a good intermetallic. This also applies to “harder” materials, like Nickel and Aluminum. Therefore, thanks to this particular characteristic of Tin and skilled “management of the soldering profile,” it’s possible to obtain a result that we can define as “acceptable.” This test certainly doesn’t aim to demonstrate that Lead-free soldering on BGA can be done at 210-220°C. We have “pushed the process to the extreme” where there are a series of “borderline” situations that depend too heavily on individual experience and subjectivity! In reality, it would be much easier to use a standard Pb-free reflow profile with Sn-Pb solder paste to ensure a good intermetallic, though there would be other issues that aren’t addressed here. The objective of this test was only to demonstrate that it’s NOT necessary (thanks also to some intrinsic peculiarities of Tin) to go up to 245°C to obtain appreciable results. In essence, “the key to everything” is in the soldering profile!!! In fact, a reflow profile for Sn/Pb alloy can be used to obtain acceptable soldering results using a mixed method (In this case Lead-Free BGA component, Sn/Pb board process) with appropriate adjustments in the soldering profile. Caution when performing the opposite test; Sn/Pb Component on Lead-free Process, you operate in the component’s temperature limit zone, seriously compromising its reliability/functionality over time. We hope that the above has been interesting and helpful!
