# Does Solder Paste’s “Five Ball Rule” Remain Valid in SMT Today?

Folks,

My good friends, Phil Zarrow and Jim Hall, in their audio series “Board Talk,” were recently asked about the “Five Ball Rule”. In the comments section for this session, one listener asked if this rule, created in the 1990s, was still valid. After all, the 1990s was the era of 0603 and 0402 passives; 01005 and even 008004 passives have arrived.

First, let’s consider what a “rule” is verses a “law.” As an example of a law, consider Newton’s Laws of Motion. At everyday speeds, these laws are shown to be accurate to within our capability to measure. As we will recall from Physics 101, these laws were superseded by Einstein’s Theory of Relativity, at speeds close to those of the speed of light. However, in our everyday world, Newton’s Laws are well … laws. They are, for practical purposes, exact.

What is a “rule” then? A rule is an expression that approximately fits some empirical data or the experience of experts. Moore’s Law is actually a rule, as it is not precise. The doubling of transistor density has varied from every 18 months to every two years. That’s why I call it a rule, a very useful rule indeed!

The “Five Ball Rule” is clearly a rule. It was likely developed a generation ago by some of the first SMT pioneers. It may be backed up by experiment, but I think it was likely more a consensus of SMT industry authorities from the 1980s and 1990s.

What is the “Five Ball Rule?” It states that the solder paste’s largest solder particle diameter should be such that at least five of these particle diameters would span the width of a rectangular stencil aperture (Figure 1).

Figure 1. The Five Ball Rule

When this rule was developed, stencil apertures were much coarser than today, and the finest solder powder was a Type 3, with Type 4 on the horizon. While it is true that stencil aperture widths are much finer today, solder pastes of Type 4.5, 5, and even 6 are now in use.

The particle sizes of different “Type” solder pastes are shown in Figure 2. Note that, for Type 4 powder, 80% by weight of the particle diameters are between 20 and 38 microns. 38 microns is considered the “largest particle.” So, from Figure 2, for Type 5 powder, the “largest particle” is 25 microns. For the sake of the Five Ball Rule, the “largest particle,” for each powder type are those shown in Figure 2.

Figure 2. Solder Powder Sizes.

So, is the Five Ball Rule still valid? It would be hard to argue that it is not. Hundreds of experiments have been performed using the Five Ball Rule, combined with the aperture ratio being >1.5 for rectangular apertures or the area ratio being > 0.66 for square or circular apertures, with successful results.

StencilCoach software now includes the newer (finer) solder powder sizes to 1) tell the user the fineness of solder paste powder for the Five Ball Rule, as well as 2) help with calculating aperture or area ratio. By the way, some have suggested that, for a square or circular aperture, an “Eight Ball Rule” is more appropriate. So, StencilCoach uses the Eight Ball Rule for such apertures.

Cheers,

Dr. Ron

# Phil Zarrow Weighs in on Productivity

Folks,

I ran into good friend Phil Zarrow the other day. Phil, Jim Hall, and I developed the SMTA Certification Program. We ended up chatting a bit about productivity, one of my favorite topics.

Ron: Phil, you have likely visited more assembly factories than anyone I know, hundreds for sure. What are some of your observations on how folks address or don’t address productivity?

Phil: Ron, there are so many bad practices that result in low productivity. More often than not, when we enter the manufacturing floor (for a process audit or other reason) we see a sea of red and/or orange light towers – rather than PCBAs in process. Most managers have no concept of the capacity they are operating at and usually feel that adding another line (with faster equipment) will increase capacity. However, there are three top “sins” that should be addressed – immediately!

The first is setup time. Unless you’re an OEM building the same PCBA day in and day out, this is something you have to master. And the higher the product mix, the more line changeovers prevail, and the more this impacts throughput. There are a number of things that can be done to “expedite” setup and they all add up. Any facility with more than one active line can benefit from a systematic approach toward setup. I tend to favor (and have had excellent luck with) the “Pit-Crew” approach. Note that the operators and setup crew are working together. Sequential changeover goes a long way: as soon as the last PCBA in a run passes through a machine center the crew commences changing over that machine (stencil, feeders, programs, etc.) rather than waiting for that last PCBA to clear the reflow oven.

Usually, hand-in-hand with this situation is a lack of adequate feeders for the different components that need to be changed over. Having a feeder already loaded with the component and “popping” it in rather than having to remove a reel and replace the component reel goes a long way. Feeder carts go even further. But this costs money and management usually doesn’t “get it.” In fact, we’ve encountered situations where there is such a shortage of extra feeders that, when the tech or engineer discovers that a feeder is malfunctioning, they don’t have a “spare” and are forced to continue using it, continuing to produce defects that have to be attended to (more time, expense, etc.).

Ron: Phil, I have observed similar practices as, noted in my book “The Adventures of Patty and the Professor.” What is the second sin?

Phil: Another common situation is a lack of balance in the line. Particularly predominant in the placement machines, if one machine is waiting a disproportionate time for another machine, the line is unbalanced. Components can and should be shifted from one machine to the other. While most of the placement machines come with software for calculating this, it is very simple math – single variable algebra (like we learned in 8th grade). But the “math phobia” we seem to suffer from is a subject for a different day….

Ron: I agree. The engineers will tell me that the line is balanced, but when I go out to the shop floor and check with my watch, the lines are almost never balanced, even though, in theory, the placement machines will easily handle it.

Now, we are holding our breath, what is number 3?

Phil: I’d finally like to comment on, to use a term you originated, “floundering time.” This is where the operator or tech comes across a problem or situation and has no idea what to do. She is not sure of the reporting system or “who to call.” It could be a machine problem, a tooling problem, a component outage, or a variety of other things. But, they all result in unscheduled downtime and severely impact productivity.

That’s just the tip of the iceberg, Ron. But just addressing these areas can improve productivity and cost a lot less than adding another line.

By the way Ron, I know you have thoughts on how materials can affect productivity. What’s a top example?

Ron: Obviously the main consideration for materials is that they perform their material function well. As an example, you would want your solder paste to form a reliable solder joint. However, solder pastes can affect productivity. I have seen cases where the poor response to pause of a solder paste was so bad that, if the line was idle for more than 20 minutes, the paste would stiffen up and have to be wiped off the stencil and replaced with fresh paste. These types of issues are discussed in “The Adventures of Patty and the Professor” in Chapters 9, 10 and 21 and can affect productivity and profitability more than you might expect.

Phil, thanks for the nice chat!

Cheers,
Dr. Ron

# Chatting It Up

Fresh off the success of our premiere PCB Chat, we have quite a few more planned.

Tomorrow (Feb. 7), SMT process consultant Phil Zarrow will take your questions. Designers may remember Phil from some of the past PCB West and PCB East conferences, where he spoke on DfM/DfA.

Next week we will host the IPC-2581 Consortium, taking questions on the new data transfer standard.

On March 2, yours truly will discuss the IPC Apex Expo trade show.

We begin accepting questions for the chats a few days early, so don’t worry if you can’t make the live event. Transcripts are available in real-time and after, too.

# Chatting Away

We had a great premiere of PCB Chat last week. Eric Bogatin, the signal integrity guru, hosted the nearly two hour session, answering more than 20 questions.

The transcript can be seen here (you must be signed in to Printed Circuit University to view it; registration is free).

The next chat will be Feb. 7 with SMT process consultant Phil Zarrow. Note that you don’t need to make the live session in order to ask a question: questions may be submitted in advance.

If you have recommendations for future moderators, drop me a line or post in the comments. Thanks!