Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, who, by day, go as ITM consulting, here to answer your process and other SMT related questions. Jim, what's today's question?

Jim
It's about defective solder joints on a HASL boards. HASL, of course, is hot air solder leveled. The question goes, "What is the possible cause of incomplete soldering after surface mount reflow? The bare circuit boards in question are hot air solder leveled. At the problem areas, we noticed a yellow stain on the hot air leveled surface on the PCV."

Phil
Before we get into our usual diatribe, the straight answer to your question is you are suffering from a case of contaminating residue and pollutants, or CRAP, in your HASL process.

So it's obviously something your fabrication shop is throwing into the works.

Jim
The one thing that HASL has always had going for it is it solders well, even after it's been stored or has a long shelf life. You've got to remember that solder leveling is a very rigorous process. It starts with a bare circuit board, bare copper, scrupulously cleaned, washed and dried. Then it's coated with a water soluble flux. Then it's preheated. Then dipped into a solder pot. It's pulled out. It's blasted with the air nozzles to blow excess solder off the surface pads and out of the plted through holes.

At that point, it's completely covered with flux residue and whatever else has been blowing around in the air knife chamber. It has to be cleaned very scrupulously. Likewise on the front end of the process, that water soluble flux has to be properly preheated before it goes into the solder pot.

So it's a complex process, but it should give you a really good solderable surface. If you're seeing stains, something was amiss in the way they operated the HASL process, most likely in the final cleaning, but maybe some combination of preheat and airflow or something else, just general contamination.

Phil
We're not big fans of using HASL for many surface mount applications. It's kind of an archaic process. It was developed originally for through hole. One of the things that trips people up is the unevenness of the surface you get with HASL.

When you start getting below 25 mil pitch parts the topography of the board surface can really throw you for a loop. Another reason you might want to look towards surface finishes other than HASL.

Jim
But to be fair, a HASL process, if done properly, should give you a very solderable surface and you should have no problems getting good wetting. The bare metallic bond has already been formed. All you're doing is melting the solder on the surface of the pad with the solder and the paste. So it's really a much easier task in terms of soldering if the HASL was done right.

Phil
That's should take care of this question for today. This is Jim Hall and Phil Zarrow, the Assembly Brothers. In the meantime, whatever you do –

Jim
Don't solder like my brother.

Phil
– and don't solder like my brother. Stay away from the CRAP.