Minimizing Solder Spatter Impact

Estimated reading time: 9 minutes

Following reflow, memory-module gold connector fingers may exhibit solder spatter contamination, which could mean both a quality and a reliability issue for the product and manufacturing process problems.

Ross B. Berntson

David W. Sbiroli

Jeffery J. Anweiler

Solder spatter is only one type of surface contamination. Other types include watermark stains and glossy-flux spatter. These are of lesser concern, however - with solder spatter, solder has actually wetted to the gold surface (see Sidebar).

Effects of Solder Spatter

Although the concern that solder spatter may have a deleterious effect on connector interfaces is unproven, it remains an issue because slight spatter "bumps" create a disruption of the connector`s gold finger planar surface. These bumps are non-pliable, and the solder itself is less conductive than gold in addition to being subject to oxidation.

The first and easiest cause of solder spatter to eliminate is in the solder paste stencil printing process. If this process is established as the cause, control can be maintained with good housekeeping practices, including proper printer setup and operator training. If it is not the cause, other areas must be examined.

Watermark stains. The root cause of watermark stains is not completely understood, although it is likely that multiple sources are involved. Because it has been shown that clean, fresh, nonpasted and nonpopulated boards may exhibit watermark stains after reflow, a variety of causes, including printed circuit board (PCB) fabrication residues, condensation from ovens, dried flux spatter, residues from cleaning boards and heat-induced gold discoloration, may be among the sources.

Watermark stains are often difficult to detect, but they are unlikely to have an impact on connector interfaces. In fact, memory-module users are less concerned with this type of surface contamination, which is often referred to as gold discoloration.

Glossy-flux spatter is understood to be caused when flux droplets become airborne in the reflow oven and are dispersed and deposited over the entire board, including the gold fingers. Two theories attempt to account for glossy-flux spatter: the solvent-expulsion theory and the coalescence theory (cleanliness during the printing process, once again, has an impact but can be controlled).

- The solvent-expulsion theory recognizes that solvents used in flux pastes must volatilize during solder reflow. If excessive temperature is applied, the solvents could flash-boil to gas (similar to dripping water on a hot pan) and carry solids into the air to be randomly redeposited on the board as glossy-flux spatter.

To confirm or discount this theory, tests were performed during conductive heating of sample boards using a hot plate. Temperature set points used were 190°, 200° and 220°C, respectively. Paste flux (containing no solder powder) did not exhibit spattering under any conditions. However, solder paste (paste flux with powder) consistently exhibited spatter during solder melting and coalescing. The results are summarized in Tables 1 and 2.

It is reasoned that if boiling flux caused spattering, it should be observed when flux alone is heated. However, because spattering is observed during solder coalescence, the mechanism should be found here. The test illustrates that the solvent-expulsion theory does not explain glossy-flux spatter.

- The coalescence theory. As solder melts and coalesces, surface tension of the molten material - a powerful force - exerts pressure on the entrapped flux which, when high enough, is violently expelled. This theory is supported by research on solder voiding in ball grid array (BGA) assemblies, wherein the link between surface tension and flux exclusion is described (the flux exclusion rate model).1,2 Forcible ejection, therefore, is the most likely cause of glossy-flux spatter. Continued laboratory simulation of flux spatter illustrates the impact of coalescence even when solder paste is dried before reflow. However, complete drying reduces spattering significantly (Table 3).

Wetting Speed

Because the coalescence model appears promising, the wetting speed of various materials was investigated. Wetting speed is affected by alloy type, temperature, flux vehicle and reflow atmosphere. As illustrated in Figure 1, temperature has a dramatic impact on wetting speed; higher temperatures result in faster action.

Dr. Ning-Cheng Lee reports in his paper, "Optimizing Reflow Profiles via Defect Mechanism Analysis," that inerted atmospheres (nitrogen) will also increase wetting speed. And reports by SMT columnist Dr. Jennie Hwang and others indicate that wetting speeds of eutectic alloys tend to be faster than those of non-eutectic materials. Thus, Sn63/Pb37 generally has a faster wetting speed than Sn62/ Pb36/Ag2. Factors that affect wetting speed, and therefore coalescence and potential spattering, appear in Table 4.

Solutions to Solder Spattering

Prevention. One method of preventing deposition of glossy-flux spatter is to coat the gold fingers with a peelable soldermask, applied after the solder paste is printed and removed after reflow. This process is unproven and would be expensive, as it involves manual application and removal, and is also difficult because it would have to be applied to a selective area of the board, interrupting production flow. The application of temporary tape to the fingers is another option. However, it suffers from the same drawbacks as peelable soldermask, with the additional disadvantage of causing poor gasketing of stencil to board if applied before solder paste printing. For these reasons, it makes sense to look at minimization as a solution to the problem.

Minimization. Optimization of the flux vehicle chemistry and reflow profile will produce a minimum of solder spatter. To prove this, standard solder paste systems were evaluated with the support of a memory-module manufacturer to assess the impact of material- and reflow-profile optimization.* They gave clear indication that activator, solvent, alloy and reflow profile significantly affect the extent of spattering. Consequently, the problem was approached with confidence that proper adjustment of those parameters could produce a means to minimize spattering to the point of virtual elimination.

Nonstandard materials such as polymerizing flux systems were not included in the study because of their higher cost, shorter shelf and stencil lives, a tight process window, and difficulty of rework. However, polymerizing fluxes promise to eventually offer a possible minimization solution because potential spattering materials are entrapped during the thermally activated polymerization process. In effect, no liquid flux remains to spatter.

The test vehicle is a six-up array memory module upon which no components were placed. (Components have been found to decrease the impact of spatters, presumably by blocking expelled flux from the gold fingers). Current production materials and profiles are used as the baseline performance (Table 5). Spatter levels on production boards are roughly one spatter per 100 panels. Two engineers inspect all boards with a 20X microscope to evaluate the extent of spattering.

Standard solder paste materials with varying characteristics are used for the line study. These materials are selected because of their different wetting speeds and solvent characteristics. To reduce the variable in the study, the same alloy is used for all solder pastes: Sn63/Pb37 at -325/+500 mesh.

Minimization Testing Results

Reflow profile selection. During the testing, it became evident that both the reflow profile and material type would have to be adjusted to minimize spattering (the memory-module testing added credibility to this theory). The two main profiles tested differed in their soak-zone characteristics. A linear ramp profile with no plateau soak zone (Figure 2) resulted in some spattering for all materials and increased spattering for the baseline production material. Accordingly, this basic profile shape was not investigated further. Based on the hypothesis for a spattering mechanism, this linear profile did not dry the flux sufficiently.

A more promising basic profile shape included a high-temperature soak (dry-out) at 160°C to evaporate all solvents (Figure 3). This solvent loss increases the viscosity of the flux remnant and reduces the volatile content, thereby minimizing spatter. Potential problems with such a dry-out, however, include poor wetting and voiding. The use of an inert (nitrogen) processing atmosphere can help improve wetting and reduce voiding but appears to have no effect on spattering. This profile was also a "long" profile, eliminating the need for excessively fast ramp rates (maximum is 175°C per second).

The results of all the profile studies are summarized in Figure 4 and Table 6. The level of spatter measured on the bare boards is diminished significantly on populated production boards. Estimates indicate that less than 10 to 20 spatters on an unpopulated board will result in no spatters on a populated board. Thus, flux types D, E and F (Table 5) all provide viable solutions to spattering. The D-type flux vehicle has the added advantage of a wide process window and air-reflow capability. All three materials feature slow wetting speeds but different solvent types, indicating that all solvents can be dried effectively and wetting speed is a key factor in flux spatter.

Detection and Cleaning

Detection and subsequent cleaning, while resulting in clean connectors, are expensive and time-consuming corrections to solder spatter. Through formulation changes in solder paste residues, however, detection can be simplified through dyes and fluorescing chemicals. Cleaning can also be improved with proper residue design. Unfortunately, as with prevention methods, cost and time requirements make detection and cleaning undesirable options.

Conclusion

Solder pastes combined with the correct profile can result in virtual elimination of solder and flux spattering. Pastes that have high contents of relatively volatile solvents and slow wetting speeds give the best results. Masking connector fingers and detection and cleaning may provide interim fixes, but do not address the root cause of solder spattering. SMT

* Based on studies conducted at Indium Corp. of America.

REFERENCES

1 Dr. Ning-Cheng Lee, "Voiding in BGA," Indium Corp. of America.

2 William Casey, "Voiding in MicroBGA," SMI 1998 Proceedings, MCMS.

ROSS B. BERNTSON, DAVID W. SBIROLI and JEFFERY J. ANWEILER may be contacted at Indium Corp. of America, 1676 Lincoln Ave., Utica, NY 13503; (315) 853-4900; Fax: (315) 853-1000; E-mail: RBB@indium.com, Dsbiroli@indium.com and Janweiler@ indium.com; Web site: www.indium.com.

Figure 1. Wetting speeds of a solder formulation tested at different temperatures. Impacting factors include alloy type, temperature, flux vehicle and reflow atmosphere.

Figure 2. A linear temperature ramp profile. With no plateau soak zone, some spattering resulted for all solder/flux materials.

Figure 3. A high-temperature soak profile. The loss of solvent raises the viscosity of the remaining flux, thereby minimizing solder spatter.

Figure 4. Summary of spatter results for each material on a six-up array of memory modules. The level of spatter measured on bare boards is diminished significantly on populated boards.

Small Explosions

Solder spatter has a variety of causes and is not necessarily the result of hot or molten solder explosively outgassing during reflow. For example, by observing procedures to ensure optimum cleanliness during solder paste printing, a spattering problem can be reduced or eliminated.

Any means by which solder paste spheres can be deposited on gold fingers - and remain during the reflow process - can produce spatter. This includes:

1.Failure to wipe the underside of a stencil between prints (dirty stencil).

2.An improperly cleaned misprint.

3.Careless handling during the printing process.

4.Excessive moisture in the board material or board contamination.

5.A radical ramp rate (in excess of 4°C per second).

In the case of the latter causes, violent flux outgassing can cause small explosions in molten solder joints, prompting solder particles to become airborne in the reflow chamber and scatter about the PCB, contaminating a connector`s gold fingers. The same is true for entrapped moisture in the board material, which can have the same effect as outgassing flux. Similarly, foreign contaminants on the board surface can contribute to solder splatter.