Q2: What should we pay special attention to when changing process from Tin-Lead based solder to Lead-free solder?
A2: Many things have to be considered on beforehand but the most important is to:
- assure that the PCB and components can withstand the higher temperature impact during reflow soldering
- be aware that even though std 63Sn-37Pb can cause electromigration then the lead-free Sn 4.0Ag -0.5Cu significantly increase the probability and speed for migration so protection against liquid ingress must be thoroughly considered.
The process change is not without problems, but because a majority of companies already for years have become Lead-free it’s more just a job that have to be executed rather than pioneer experiments to be done.
When the soldering process itself is in place then you have to be aware of that all earlier environmental reliability testing results on your product is obsolete. You have to start from scratch with mechanical impact and environmental testing.
Because of the very high Tin content in lead-free solder it’s even more important than for 63Sn-37Pb to avoid soldering on ENIG because IMC formation will weaken the durability of the solder-joint. It’s strongly recommended to solder on PCB pads with OSP surface finish. This will give the best reliability.
Next challenge is to avoid sudden death of the device if it’s exposed to water ingress or just condensation. Experiments with Sn 4.0Ag -0.5Cu has demonstrated that electrochemical migration occurs between solder-joints in very short time. This means that leakage currents or even solid shorts that leads to malfunction of the device are created. Depending on solder-joint distance and voltage level it will only take between 1 and 15 minutes to create the first short circuit!!
If you make portable devices that is used both in- and out-door and if you don’t want a lot of customer complaints, then it’s very wise to underfill BGA-type components and conformal coat other components.