I recently posted that circular apertures deliver much less solder paste than square apertures. One of the obvious reasons is that a circle of diameter D has only 78.5% of the area of a square of side D. However, in addition, the circular aperture has poorer release than a square aperture. In the aforementioned post, I theorized that the reason for the poorer release is that the curved surface of the circular aperture adheres to the solder paste solder balls more effectively.

I recently thought of the above situation in light of the “Five Ball Rule.” This rule 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.

See Figure 1 for the Five Ball Rule applied to circular and square apertures. Note that the ratio of solder balls is 19/25 = 76%, almost the theoretical maximum ratio. However, for square and circular apertures, the ‘Eight Ball Rule” is suggested. But, in some configurations the Eight Ball Rule may result in less solder paste — 40/60 = 62.5% (Figure 2). It should be remembered that this is just a surface area argument, not a volume argument. Solder paste is printed in volume and in this discussion we are just looking at one layer of paste.

However, the bottom line is that square apertures should be preferred over circular apertures.

Dr.
Ron, when if comes to SMT printing of solder paste, why do some people use the
five-ball rule for rectangular apertures and the eight-ball rule for circular
apertures?

Michel:

The
“Five Ball Rule” is another metric that SMT assembly industry leaders believe,
but it is difficult to find its origin. It states that when selecting a solder
paste, five of the largest solder balls should be able to fit across the width
of the smallest rectangular stencil aperture. See Figure 1a for a 0.2mm wide
rectangular aperture.

Typically,
the largest solder ball diameter is assumed at the 90^{th}
percentile. See Figure 2. So, in this example, a type 4 solder paste would fit
the five ball rule as the largest solder ball is 0.038mm. Five times 0.038 is 0.190mm,
just a little less than the aperture width of 0.2mm. It should be remembered
that this is a “rule.” not a “law.” So let’s say you had
4.5 balls across the aperture with instead of 5, it would most likely be
OK.

Figure 1.A
comparison of the Five and Eight Ball Rules

Figure 2.Solder
Powder Sizes

A
generation ago, the advent of circular apertures to support BGA and CSP
packages necessitated a new “rule.” Figure 1b shows why the five-ball rule is
inadequate for circular apertures. Although five type 3 solder balls fit along
the 0.275 diameter, off the diameter, there is not enough room for many
solder balls. Hence, an insufficient amount of solder paste would be
printed.

For
the same aperture, if a type 4 paste is used, 7 or 8 solder balls span the
diameter and the amount of paste printed would be much closer to the volume of
the aperture.

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.