We have recently implemented a selective solder process using no clean flux. We have experienced a problem with "micro solder balls" Some of them are not so "micro". The equipment manufacturer stated that this is a common problem with no clean selective solder and that we would see this with any selective equipment using no clean. Could anyone provide some feedback as to the validity of this statement. It seems to me that the process would not be so popular is this were a common problem.
We are using AIM 266-3. The topside preheat temp is approx 100 degrees C although we have played with that number both up and down. Raising the temp leaves a white residue, lowering the temp (below about 90 degrees) and the solder balls get worse. As far as the dwell time, it looks like it is about 3-4 seconds.
What's the board finish? HASL? What's the frequency of solder beading, like how many joints soldered exhibit these solder beads- percentage wise. Are the boards clean- nobody is touching them with greasy fingers? We need more clues.
The boards are HASL. The frequency is random. The solder balls are randomly deposited along the path of the select wave, not always along the component bodies. We are very careful regarding handling. There could have been contamination on some of the boards but we are seeing this on most of the selective soldered products.
OK, sounds like it may be more of an equipment problem: "The solder balls are randomly deposited along the path of the select wave,..." Does the equipment vendor have an applications engineering group to assist you? If not or are weak, try to see if you can get a list of other customers using your model machine to see if they've seen and/or solved this problem. Or post here by equipment mfg. and model to see if you can find any.
It looks like you might be close to the solution for your problem because you mentioned that you can make the solderballing worse with lowering the preheat temp. This means that the preheating has a significant impact. I would suggest to look for a no-clean flux that can function under higher preheat temperatures like 130C without leaving white residues.
That's what I thought originally. The equipment manufacturer stated that this was common among No Clean Selective solder applications. I was trying to get a feel for the validity of this statement. It seems to me that there would be a lot of ruckus regarding this subject if it were true.
I don't know what machine your using, but we're also playing with one at the moment and don't see any problems with micro solderballing as far as I'm aware (I'll double check tomorrow thou!) We're using Kester 977 with it and don't have a problem. Although Nozzle type would play a part I think it's down to flux. It doesn't really matter if your using your flux on a selective wave or a standard wave you still require to do the same things, preheat the card, dry the flux and evaoprate volitiles and so forth. if your getting solder ball's the most likely cause of it is wet flux especially if your getting it random, in fact more so if it's a selective machine. When it come to soldering connectors there are really only 2 types of solder ball's (excluding size of course - just don't get me started on the size!!) You've got random and non random. Non random can be surprise suprise repeated and predicted, it's where the solder breaks from the lead and a small amount of this break's off, if your card finish is a gloss then it can adheare well to that and you'll normally find it nicely behind the pin usually mid way (a matte resist will stop this) Non random balls are normally made 1 of 2 ways, the 1st is when a solder droplet fall's from the pin as the card moves through the wave, the surface tension on the main wave is enough to send the ball back up to the card where it impact's and stays (Soltec have a really cool video of this actually happening - you could do a cheap one with milk and coffee...)Anyhoo it's unlikely that you'r getting that on a selective wave so it must be the 2nd type which is due to flux spattering i.e. the fountain is hitting wet or partially dry flux and basically exploding. I'd heat it up a bit and see how it goes. Most flux manufacturers will spec a temperature in the region of 95 - 115 deg on the topside, they calculate for a typical 30 - 40deg C drop between top and bottom side so expect you flux on the solder side to be heated to between 125 - 160deg C.
Ah, yes...the solvents. I'll bet with SS machines vs. WS machines there is way less dwell time at preheat so you don't evap enough solvents. Do any of these SS machines use force-convection preheats or are they and IR type?
I can't think of any off hand other than the Ersa which is massive and from when I used it a pain in the A*SE to program and keep programmed, you always seem to have to baby sit it. I bet if you asked you could get one, the problem is that forced convection doesn't heat as quick as the IR does, but again I guess depending on the machine you have you might have the legnth, although I know one company that put some preheats on the front of a machine to see if it helped with soem planer cards, worked well but not soemthing you'd want to do in real production - health n safety and all that jazz. One question to all those flux manufacturers out there is are they doing any work in developing fluxes specifically for the selective soldering market?
Chris, could you tell me which brand of selective soldering machine you are using? We are looking at some machines at this time and I would like to which vendor has the best support. I have looked at RPS/TYCO and Vitronics.