RoHS Manual Soldering Hints

For several years F.O.M. Systems has been working with various lead free (RoHS compliant) soldering alloys for printed circuit board assembly. Our experiences with these materials has ranged from the “hardly any difference” to “virtually unusable”. Depending on the alloy used you may have a similar range of experiences. Besides the supposed environmental advantages of lead free solders the main practical advantage associated with these materials is that they create solder joints of much higher strength than older tin/lead solders. But that’s pretty much where the advantages leave off.

Once the pcb reflow process has been tweaked and a suitable solder paste has been selected process yields are equivalent to older tin/lead processes. Since the reflow temps can be about 20oC higher than tin/lead processes it’s even more important that components are kept appropriately moisture-free prior to assembly. Also of note but probably not wholly related to RoHS processes is that we get better process yield with BGA footprints of any size over TQFP and TSSOP components. But you’d better get it right the first time since rework can be considerably more difficult with RoHS soldering processes.

Working with RoHS solder

If you are used to working with a very small tip on your soldering iron (such as a Weller NT1 tip) you should switch to working with a larger tip such as an equivalent to the Weller NT4 or NTAX. Smaller tips simply don’t have the thermal mass needed to work properly with RoHS solders. Don’t turn up the iron temperature too far (I use 660 to 680F/349 to 360C) since this will cause the flux to burn and make adequate heat transfer even more problematic. Speaking of flux, a more aggressive flux will be needed, and lots of it, when doing hand soldering of RoHS materials. Something like Kester #2331-ZX works well. Having an iron tip cleaner handy will be helpful as well as keeping lead free tips clean is a lot more of a chore than with leaded solders.

Note that it is unwise to mix leaded and lead-free solders, not just from a legal or environmental standpoint. As little as 0.5% of lead contamination of a lead free joint will greatly weaken the joint and lead to premature failure. Metalurgically it’s OK to use leaded solder to affix lead free components since enough lead will be present in the alloy to prevent attachment problems, but do NOT use RoHS solder to affix components with tin/lead plating on their leads as this will result in weakened solder joints.

RoHS solder joints look different. The material doesn’t wet the joints as readily as tin/lead solder, and when cooled has a dull “frosty” appearance. This is what makes using plenty of flux so important as it’s much more difficult to distinguish a “cold” solder joint when using RoHS solders. Also the transition zone of RoHS alloys is much narrower than tin/lead. One moment it’s solid, the next it runs like water. There’s no in-between semi-molten state like there is with tin/lead solders. If you’ve done any soldering on copper pipe used for potable water supply in the last 15 years using tin/antimony solder you know what I mean.

The easiest alloy to use for hand soldering seems to be SAC3. This is an alloy of 0.6% copper, 2.5 to 3% sliver, the rest tin. Of course it’s the most expensive due to the high silver content. Kester 331/66 is a good choice, as is Warton Metals S.A.C.3 with a 2% core of Future HF Fast Flow flux. Remember to use plenty of flux both on the board and component leads, and keep those tips clean!