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A Defense of Hot Air Solder Leveling: Why it is Still a Viable Solution

by Dave Corey

Hot Air Leveling Viewed From a Different Angle

Hot Air Solder Leveling (HASL) is, in fact, an art. The ability and knowledge of the operator is very important to the quality of the finished product. In addition, a thorough understanding of each process parameter, and periodic routine maintenance, will also contribute to a desirable finish.

 Today’s hot air leveling systems can handle the entire process automatically. By accurately controlling the various process parameters, and mechanical functions, these systems can achieve a high degree of repeatability. A complete in-line system will automatically pre clean, flux, solder, cool and post clean the panels. In general, these systems can achieve production rates of 150 – 200 panels per hour. Semi-automatic systems are also available, automatically performing only the solder leveling process. The pre-clean, fluxing, and post clean process can be accomplished either manually, or in separate off-line units. These semi-automatic systems can achieve production rates of 60 – 120 panels per hour.

 Operator ease and accessibility are critical with hot air leveling systems. The operator must spend time cleaning and calibrating the machine in order to keep it operating efficiently. Solder and flux residues can accumulate very rapidly and impair the machine’s performance. Ease of accessibility aids the operator in maintaining the machine at its optimum performance level. Accessibility is also important, because if the system is not readily accessible, an operator can become discouraged with performing routine maintenance tasks. Serviceability with respect to repairs and maintenance can be summed up by saying, “The simpler the better.”

 The most common method of fixturing a panel in vertical HASL system is through the use of a pneumatically operated clamp. To prevent solder from adhering to the clamp, they are usually manufactured out of stainless steel or titanium. Traditional clamping systems only clamp the panel at the top, and passive side guides are used to further assist the fixture of panels. These guides help maintain the structural integrity of panels of various sizes. Other systems offer the ability to fixture the panel at a 45 angle. This 45 fixture supports the top and bottom of panels, greatly enhancing the structural integrity of the panel. The 45 fixturing method has a tremendous advantage to that of traditional clamping systems.

 Key advantages of the 45-fixturing method:

             Eliminates solder short—20 mil quad packs and below are achievable.

           Decreases the variation of solder thickness on Quad Packs—coplanarity is greatly improved.

           Eliminates bowing of panel, giving a more uniform front to back finish, bottom panel support breaks the surface tension of the solder pot.

 The uniformity and wet-ability of copper surface depends mainly on the following factors:

A. The cleanliness of copper surface

B. The viscosity, surface tension and activity of the flux

C. Pad Orientation

D. Copper Pad Width

E. Air Pressure

F. Clamping, 45 vs. traditional 90

G. Solder

 PAD ORIENTATION

 Pad orientation has a large effect on solder thickness variation. The greater the variation in pad orientation, the greater the variation in solder thickness across the panel. In general, HASL systems have a great leveling effect on vertical pads. Solder will tend to flow down on vertical pads, creating a puddling area on vertical pads. Horizontal positioned pads hold more solder than vertical pads. The solder thickness of horizontal pads is approximately equal to the solder thickness of puddled areas on vertical pads.

The greater the pad orientation varies between horizontal and vertical, the greater the likelihood of variations in solder thickness. One of the most prominent examples of pad orientation variation is the surface mount Quad Packs. Thus, some compromises in solder thickness may be required in order to solder level surface mount panels. In addition to pad orientation, the individual pad dimensions are also a factor. The surface mount pad width has a large effect on the solder thickness. Narrow pads have a greater tendency to retain solder, than do wide pads. Larger pads with their greater surface area tend to allow an increased solder flow, leading to a decrease in solder thickness. See Figure 1


 

 

 

 

 

 

 

 

 

Figure 1

From the above graph, you can see the clear relationship that exists between narrow pads (thicker coating) and wider pads (thinner coating). This is a universal relationship among all liquids. The tighter the area the liquid is confined to, the more height you will get to the drop, or bead, of liquid applied. The issue of prime importance here, however, is not the solder level within a given pad width—but rather the delta between the vertical and horizontal pads of similar width. In the graph above, you will notice that this delta (which we call “variations”) starts out low, in the 8 Mil pad widths, and rises as you approach the 24 Mil pads. This is also normal in solder leveling, and occurs whether you use horizontal or vertical leveling systems. Once again, the delta – or variation – between the horizontal and vertical pad thickness is the main issue here.

 In today’s market, the whole issue of horizontal to vertical variation has been reduced to the SMT user’s need for solder coplanarity, on all sides of a quad pack. Since two of these sides are vertical, and two horizontal, the need for minimal height differences is important.

 AIR PRESSURE

Historical data gives us the ability to see what happens to the solder thickness characteristics, utilizing different air knife pressure settings. Using pads in the 16 to 18 Mil width range, we sorted by the amount of PSI used by the air knife. Keeping all the normal parameters constant, the results of this study are shown in Figure 2.


 

 

 

 

 

 

 

Figure 2

As might be expected, the higher the PSI used in the air knives, the lower the solder thickness and an increase in thickness variation. While the above is a representation of nominal values, we caution against using high PSI to solve thickness problems. There are aberrations that occur once you go above 100 PSI, and you will find that the thickness above this range is too erratic for normal processing purposes.

 45 FIXTURING

As mentioned previously, 45 clamping is superior to traditional 90 clamping. The premise was, that the traditional 90 clamping of panels influenced the horizontal and vertical pads is such a major contributor to solder level differential, than the 45 clamping had to be an improvement.

 
 Tests were performed on a population of panels divided into three groups. The panels were all identical in terms of circuitry, and the focus of attention was on the leveling performance of the 20-mil pitch Quad Packs. One group ran at 75 PSI, the second at 80 PSI, and the third at 85 PSI. For each of these settings, the nominal solder thickness values for horizontal and vertical pads are charted in Figure 3. For the test, normal parameters were kept constant, with the exception of the PSI settings. In addition, the front to back PSI differential between air knives was kept to a 2 to 3 PSI minimum.

 

 

 

 

 

 

 

 

 

Figure 3

 As Figure 3 illustrates, by clamping the panel at a 45 angle, the Quad Pack horizontal and vertical pad variation was greatly reduced. The results at 75 PSI were improved over the traditional clamping method at 75 PSI as shown on the previous chart. This is worth noting as it correlates with the improvements seen at 80 and 85 PSI. At 80 PSI we are starting to make progress. A 300 micro inch variation has been reduced to roughly 180 micro inches. At 85 PSI, the solder thickness of 287 micro inches on the horizontal pads, and 272 micro inches on the vertical pads show a reduction in variation of 15 micro inches on the 20 mil Quad Pack. Below are the results of four individual tests that were performed, confirming the consistency of the 45 clamping process.

 

Horizontal Pads

Vertical Pads

Variation

Test A:

304

308

4

Test B:

293

262

28

Test C:

278

246

32

Test D:

349

291

57

SOLDER

 The selection of proper soldering material must be taken seriously. Saving .10 per pound will only degrade the ability of the HASL system to peak at its optimum level. Not all solders are the same, nor do they offer the same flow characteristics. High purity 63/37 (eutectic) extruded virgin solder should be the choice material. A regular replenishment of the same material will contribute to the maintenance of the satisfactory impurity balance in the solder pot. In addition, the solder pot should be analyzed on a regular basis depending on workload.

CONCLUSION

The Hot Air Leveling process has come a long way over the years . . . a process that is in itself, an art, not a science . . . a process that has been threatened by termination year after year, due to the limitation of past equipment capabilities. The HASL process existence has been extended with the advancements of 45 technology, to a process that has stood the test of time and still remains the preferred means of preserving solderability. Kind of reminds one of that little bunny, “STILL GOING”, going, going,  going.

 Dave Corey is co-founder of ACS Technologies, Corp. and has been involved with the Hot Air Leveling process for over twenty years. He can be reached at corey@acs-tech.com 

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