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Solder Paste and the 5 Ball Rule



Solder Paste and the 5 Ball Rule
For either Type III or Type IV solder paste, should there be a minimum number of solder spheres deposited on each surface mount pad? The Assembly Brothers, Jim Hall and Phil Zarrow, answer this question.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil
Welcome to Board Talk. This is Phil Zarrow with Jim Hall and Joe Belmonte of ITM Consulting coming to you today as Board Talk from the Board Talk Cave high atop Mount Rialto in New Hampshire.

Today is solder paste day, isn't it?  And we have a solder paste question.

Jim
This comes from S.S. For either Type III or Type IV solder paste, should there be a minimum number of solder spheres deposited on each surface mount pad? 

Phil
Who's gonna sit and count 'em? All right, let's figure out what S.S. really meant here. Jim, take a stab at it, first?

Jim
Thinking of the number of spheres, maybe this questioner was getting into printing for ultra-small components, such as 01005 chips or maybe .3 or .4 mm BGAs or CSPs and where we are concerned about the deposit on each pad is so small. The surface area is large, relative to the volume and you tend to exhaust the flux and get defects such as graping.

There isn't any easy answer to that. Most people feel they try to put down as much paste as they can get away with without bridging. There are pastes designed to minimize those problems even when the volume of paste or, in this case, the number of spheres is very small. 

Joe
As Jim said, the key, especially for miniature components with such an incredibly small volume of solder paste, is to print as much paste as possible through the aperture. The way to do that is nothing new to most of the listening audience, is the area ratio calculation.

There's no question that many of the leading stencil suppliers are doing wonderful work to minimize the area ratio required to get the maximum release of paste so you certainly want to keep current on those developments. But as of right now in the industry, we still talk about a .6 area ratio to maximize the transfer of the paste from the aperture to the printed circuit board pattern. That's really the key to get repeatability.

One of the things we've talked to customers quite a bit about in our travels is the term "works." When we talk about something that works, we're talking about something on the production floor that works 24 hours a day, 7 days a week.

So some of these new technologies are wonderful. I'm certainly the last one to say you shouldn't look at them. You absolutely should, but you want to proceed with caution and do the proper experimentation and studies before you introduce any of these technologies to make sure they in fact work 24 hours a day, 7 days a week on the production floor.

Jim
The question mentions Type III and Type IV and we know that Type III has larger particle sizes than Type IV. The other question that might be implied here is the Five-Ball Rule for any given aperture. Five diameters of the largest particle in the paste should fit across the smallest dimension of the smallest aperture to ensure good fill.

We're not even worried about release, just to get good fill as the squeegee moves across the board. So the Five-Ball Rule is pretty well established for very small apertures and perhaps that's what S.S. is referring to.

Phil

Well I think we've interpreted this question every which way. So, whether using solder paste, bar solder, cored solder or any other kind of solder. Don't solder like my brother.

Jim

Don't solder like my brother.

Comments

A better focus than the "five ball rule (largest particle)," or the "five ball rule (average particle)," or choosing particle size distributions (PSD) by device pitch, or any other rule of thumb for PSD selection, is Area Ratio (A/R) of the stencil aperture. The old rules of thumb worked great as an empirically derived means to determine when T4 should be used over T3, but doesn't address when a PSD smaller than T4 is necessary.

In addition, the advent of advanced stencil technologies (nanocoatings, fine grain and laser-cut nickel), better solder paste inspection technology, and pastes designed with sub-0.6 A/Rs in mind allow for print (and reflow) performance that wasn't possible 10-15 years ago before the advent of ultra-miniature components in SMT applications.

It's always best to be careful about using old rules of thumb on new technologies that didn't exist when the rule of thumb was invented in the first place. The "five ball rule" is a 20th century guideline (I first heard about it in 1998) that doesn't necessarily reflect the current state of SMT material and process capability.
Jason Fullerton, Alpha Assembly Solutions
While it was circuitous (I enjoyed the journey), I think you nailed SS's question right at the end "Five-Ball Rule." We just learned about that ourselves here as we moved onto a new product with what I would call microscopic components! We've moved most of our product to the Type V and are seeing good results.
Lee Scanlon, Digital Monitoring Products

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