TSOP Component Soldering Problems

Phil
And welcome to the Board Talk. This is Jim Hole and Phil Zarrow, the assembly brothers, here to answer your questions on SMT process situations, assembly, equipment, materials, and procedures. Jim, what question do we have today?

Jim
This question comes from DD. "I supervise three high-speed, no-clean, lead-free SMT lines. Unfortunately, these lines are in an area with no control over humidity. We are seeing a high fallout due to insufficient paste application on TSOP components. "

"We made changes to stencils, we clean the stencil routinely, and it helps, but we still have a fluctuating yield. Is there such a thing as a low-humidity, low-temp, lead-free, no-clean paste? What do you recommend?"

Phil, when I read, "Is there such a thing as a low-humidity, low-temp, lead-free, no-clean paste?" I think that's implying that temperature fluctuations are also important.

Phil
The real question here - is there a paste that's more compatible with your process and your environment? And the answer is, most likely yes.

So the question we beg to ask is, "Hey, DD, when was the last time you, or one of your colleagues, did a solder-paste evaluation and qualification?" I'll bet it's been a while. And therein lies the solution, and that is to evaluate different solder-paste offerings and find the one that best fits your process in terms of humidity, equipment, your oven capability.

This is something people neglect. They have the legacy syndrome, where they're using the same paste forever. Things change, and, most importantly, things improve.

Even the manufacturer you're buying your solder-paste from make improvements. You've been probably using their formulas for a while - chances are, with the competitiveness of the solder-paste market, they have an improved formulation.

Jim
Particularly lead-free, no-clean, because that's the highest-volume area where most of the paste manufacturers have been putting their primary research and development efforts.

Phil
So we recommend you do a paste evaluation benchmarking against your existing paste. When you go to the different solder paste companies, explain what your environmental is, that you are looking for a lead-free, no-clean paste relative to your normal humidity

Answering the demands of the various environments, a number of the solder paste companies are extending the windows or offering formulations for extended windows at either end of the humidity spectrum. So chances are, there's probably something new available for you there.

Jim
Taking a step back I want to compliment DD on being aware that the environment impacts your paste. It definitely does.

As part of your evaluations, you might want to look at the paste performance over the range of humidity and/or temperature conditions that you typically experience. You would do a design of experiments, in which you would vary the humidity and see, for any given paste, if it has an impact.

Ideally you would want a paste that has no impact, that is insensitive to your humidity and/or temperature changes, but, if it did, you could use that information from your design-of-experiments results, to optimize in the middle of your environmental ranges so that you should have a minimum amount of degradation as humidity in your environment varies to its extremes.

Phil
You stated that a symptom that you're seeing is insufficient deposits on your TSOPs, and you're doing stencil cleaning and all that. The other thing to make sure that you have proper stencil-aperture design, that your apertures are correct.

Jim
Phil, wait a minute! He said, "We have made changes to the stencil." I assume that implied they were optimizing the apertures, but that's a good point.

Phil
Maybe they've reduced the apertures so much that it's making even more problems. As Jim likes to say, the too tall, too narrow aperture syndrome.

Jim
That might be true, but based upon the infinite amount of superior wisdom expressed by this questioner, I would assume if they were having insufficients, they would not make the aperture smaller.

Phil
So this is Jim Hall and Phil Zarrow, the assembly brothers, pick in place, who go by day as ITM consulting, spooning out our wonderful wisdom and wit. Remember, whatever you do ...

Jim
Don't solder like my brother.

Phil
And don't solder like my brother.