Variations in SAC 305 Solder

Phil
And welcome to Board Talk with Phil Zarrow and Jim Hall, The Assembly Brothers, who are consultants by day and by night we do Board Talk and answer your questions. Jim, today’s question is from M.K. He or she asks, we have been using SAC 305 bar solder from one supplier. We now want to source it from a different supplier. Should we be concerned if we mix the new bar solder in with the current solder in our wave soldering systems? Are there any reliability concerns?

It is an interesting question. It sounds kind of flaky but, bar solder is bar solder and SAC 305 is SAC 305. I think one of the things that you may be worried about is the purity levels. There is a specification as far back as the 70s that was derived from a mill spec called QQS571 and that is a specification that the bar solder purity of the alloy, in terms of how much of it is tin silver copper versus other elements. So, it is a purity level.

It has to meet that to be QQS571 compatible. That spec is still in place and generally it is stamped right on the bar itself. I would say probably shouldn’t make any difference. Jim, do you have any concerns about that?

Jim
A purity spec generally involves listing a set of potential contaminants and saying the level in this bar of solder has to be below .0001% of something like that. Your solder supplier may have the information available. If it is, you can compare those two sheets, their actual measurements from purity of specific contaminants and see if there is any difference between what they are guaranteeing. But I agree SAC 305 is 96.5% tin, 3% silver and 0.5% copper. That should be the same.

Phil
A good friendly reminder to analyze your solder pot now and then.

Jim
Your contaminants, the contaminants that you are putting into it.

Phil
That’s right, starting off with nice pure solder. But it doesn’t always stay that way. So, please do that. Well, you just wasted some valuable time of yours listening to us at Board Talk. We hope we imparted some wisdom on you or something. In the meantime, regardless of what you are chucking into your solder pot please don’t solder like my brother.

Jim
And don’t solder like my brother.

Curious, after dross recovery what phosphorus, at such a low % level would affect a selective solder system based on your comment of "equipment cant tolerate phosphorus", not to mention, what about a wave solder? What's driving that?

Remember, You get what you pay for. If just seeking low or lowest cost, the result has good potential to be inferior solder, which often enough shows up as dull surface appearance after soldering along with issues of 'wetting' improperly. As already mentioned, you should have a sample pulled from the solder bath and sent out for analysis.

If the solder is not replenished properly then you have build-up of many alloys during each soldering cycle and the over-all content can rapidly have high Cu and Ni content along with many other metals that wash off the various components, including Pb which often is found on component leads that supposedly are ROHS, Pb free, but are not. So, at least once per year have the solder bath tested for contaminants. In high volume production this test should be done about once per month.

I agree with everything Phil and Jim say but there are some additional considerations. 1 ) J-Std-006 3.1 states "The use of recovered or recycled materials is encouraged." While reclaimed alloys can pass impurity level requirements, the addition of dross reducing agents can result in performance variation. Also, selective soldering equipment cannot tolerate the presence of phosphorus which is not monitored per IPC. Finally, there may be an "acclimatization"period while the new vendor an old vendor products are commingled that results in performance variation. It may not be cost effective to "vendor hop" for a minor cost savings with all of these considerations. Asking your existing vendor for cost reduction strategies may be the least disruptive approach.

Hi Guy, always enjoy the chat Phil and Jim.Just pointing out a typo SAC 305 has 0.5% Cu. It is stated as 5%.

But I agree SAC 305 is 96.5% tin, 3% silver and 0.5% copper.

Ersa recommends to use non-phosphorus alloys. Please check phosphorus level, and ask yourself do I need it.