A colleague in California was on the phone last week: "We need a flux with greater activity!" He said. Choking down comments like "Maybe we can take it jogging next time you're in town?", I asked him to be more specific. "You know perfectly well what I mean!" He was right. I did. Sort-of... But I was also trying to force him to think what he was asking for. Too often people (even intelligent, well-educated people) ask for a flux with a higher/greater/stronger/better activity flux, without pausing to consider what effect they really want. If you're starting to lose the thread here, maybe it's best to begin at the beginning and consider what an activator does...
Activators are the chemicals that are added to solder fluxes to remove oxides from metal surfaces, and so allow them to join together to form a strong metallurgical bond. When I first started out as a solder paste formulation chemist, I thought like many people, that the more reactive the chemical was, the better activator it would be. Also, I thought that the key to this was the pKa, or dissociation constant: equating corrosivity with activity. For a chemical of formula AB that dissociates in solution:
AB <> A + B
Its dissociation constant, pKa, is therefore:
pKa = -log { [A].[B] / [AB] }
so the smaller the pKa, the greater the extent of dissociation. The data, however, did not make sense when I did the experiments. Low pKa chemicals often (but not always) did not work well; and in testing homologous series' of activators: some worked well, and many did not, but always without any apparent pattern. There were three things that quickly became evident:
1/ pKa is almost always given in aqueous solution
2/ I was not focusing on a single effect (we'll talk about what I mean by an "effect" in a minute)
3/ Certain chemicals can be added to activators to enhance their effect
With the exception of certain very corrosive/reactive fluxes, water is a poor choice for a solvent, so any analysis based on (for example) acids and bases is, at best, rather simplistic, so 1/ is inapplicable. Talking about 2/ and 3/... OK, so what is this "effect"? At the time, I was focused only on solder paste, and also only on the issue of solderballing; where individual solder particles partially coalesce, leaving some individual particles behind that do not join up with the main body of reflowed paste. When I took a solder paste that gave excellent solderballing data, and reflowed it into OSP-copper, the result was often very poor wetting onto the copper surface, so this was my fourth learning, and it is the key one in solder paste formulation:
4/ What effect are you looking for?
Activators that remove oxides from the surface of solder are, more often than not, not good at removing oxides from substrate surfaces, such as nickel or copper oxides and hydroxides. There was a fifth learning, too:
5/ Activators have to do two things (see figure), not one:
- Firstly: react with the surface metal oxides to form a metal-activator reaction product (MARP)
- Secondly: remove the MARP's from the metal surface
This leads me naturally to the activator koan: What good is done by replacing an insoluble metal oxide with an MARP, unless it moves away from the metal surface? Note that these MARP's can either be dissolved into the residue, or they can be made volatile: although this latter approach does raise the question of how toxic volatile organo-metallic compounds are, and I don't believe this interesting concept was ever reduced to practise. Maybe you know better?
The next time someone asks you to formulate a flux with "higher activity", smile cheerfully and send them a link to this blog posting (and tell 'em Andy Mackie sent you!) and finally , ask them "What is it you want the flux to DO exactly?", or to be more specific:
* Describe in detail the soldering defect the customer is seeing
* What solder alloy is used?
* Wetting onto what surface?
* Under what reflow conditions (profile / atmosphere)?
This way, you will be better able to guide the formulation chemist towards resolving the issue, rather than trying to best-guess their approach to the problem.
Cheers! Andy