Transcript
Phil
Welcome to Board Talk. We're the Assembly Brothers, Phil Zarrow and Jim Hall of ITM Consulting, and we're here to answer your questions on SMT process, equipment, materials, procedures, or anything else you have on your mind. We're here today with a question that was sent to us by Circuitnet and this is from Ray W. in Andover, Massachusetts.
Ray's wondering, "What are the determining factors in switching from a Type 3 solder paste to a Type 4 solder paste for the printing operation?" Well, Ray, that's a good one. There's some real drivers there and perhaps you're seeing finer pitch components and some of the smaller passives and therefore, some of your apertures are getting smaller.
Jim
That's exactly correct, Phil. The only reason to move to a Type 4 paste - and what that means is that it has a Type 4 powder. It's a solder powder within the paste has smaller particle size. Type 4 paste particles are smaller than Type 3. Another one of those wonderful inverse scales that we have to deal with, such as wire gauge and golf scores.
With smaller particles, it's simply easier physically for the paste to flow into the smaller apertures. And the rule of thumb that we typically use is called the five-ball rule.
The largest particle in a Type 3 paste is 1.7 mils. The five-ball rule says that the smallest aperture I should use with that paste is at least as big as five times the diameter of the largest particle, which means for a Type 3 paste, that would be an 8.5 mil aperture. Anything smaller than that, five-ball, the largest particles would not span across that aperture and that's an indicator of the ability of the paste to easily flow in and out of the aperture. Type 4 has a smaller particle size. Its largest particle is 1.5 mils, so it can go down to an aperture of 7.5 mils.
In practical sense for leaded components, this transition is usually seen between 20 mil pitch, .5 millimeter, and 16 mil pitch, .4 millimeter. In other words, most people feel no problem doing 20 mil pitch with a Type 3 paste but if you get down to 16 mil pitch, that's when you usually run into these limits of this 8.5 mil aperture.
Phil
And that isn't to say you can't use a Type 3 paste for going down to a 12 mil. We've done it but it gets pretty tricky. You start fooling around with not only your aperture reduction but also the shape of the apertures. So you get to the point, why make life difficult for yourself?
And, of course, we're also talking about what we call sub-miniature passives these days, 0201's, 01005's, very small apertures.
Jim
These are driving a lot more people to look at the application of a Type 4 paste. In fact, for 01005's, people are actually looking, at least on a lab theoretical experiment basis, at Type 5 powders to print these very small apertures for these small chip components.
Phil
We'll talk about some of the limitations in the future, but there are some limitations using Type 4, but you can use a Type 4 across the board, even with large apertures, but there's a few reasons why you wouldn't want to and the first one is it costs more. One thing that should be noted is that more and more people, because of the mitigating factors we mentioned earlier, are starting to use Type 4 and so the cost is going down as demand increases, but there's still a pretty good size differential.
And there are some other problems of using Type 4 because we're talking about more particles and therefore, more aggregate surface area. The solder paste companies, with increased demand, have been working on trying to reduce these effects.
Jim
Let's review. With smaller particles, you still have the same volume or weight of metal in your joint, but it's divided up into smaller particles which inherently have more surface area, therefore more potential oxide or more potential oxidation during the process. So concerns include increased solder balling, poor wetting, and perhaps a greater probability to have to use nitrogen or another inert atmosphere during your soldering to prevent oxidation.
Phil
Another addition to process cost.
Jim
The other thing is that in addition to smaller particles, a Type 4 has a narrower range of particles so they tend to pack together more tightly. This can give you increased viscosity, which has been seen to make printing more difficult. And because the particles are packed tighter together, it's possible to do more entrapment of the flux gasses, resulting in higher levels of voiding.
These are all known factors that have been around and with the greater increased interest and use of Type 4 paste, the solder paste manufacturers have been working very hard to overcome and mitigate these concerns. So we would say to you what we say with any new material or new piece of equipment, check it out. Do a proper solder paste evaluation of your Type 4 powder in your process with your stencils to make sure that it gives you the right combination of improved printing versus solder defects.
Phil
Remember, all solder paste manufacturers are not created equal and neither are their products.
This has been Board Talk, Phil and Jim, the Assembly Brothers, and remember, whatever you do ...
Jim
Don't solder like my brother.
Phil
And don't solder like my brother and keep those kids from sniffing the solder paste jar.
|
