Maximizing Lead-free Wetting

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By Richard Lathrop

As lead-free assembly ramps up, wetting of lead-free solder pastes is becoming the major performance tradeoff. Global efforts to increase lead-free wetting chemically have proven unproductive. The prospect of a “drop in” lead-free paste with respect to wetting looks improbable. This article reports the findings of numerous studies using quantitative wetting gauges to measure solder-paste wetting to the PCB surfaces. Although there are many lead-free alloys on the market, this article concentrates on the SAC (Sn/Ag/Cu) alloy, specifically 95.5/4/0.5 in a no-clean paste. In addition to wetting, solder defects and voiding also are included to determine the best overall lead-free reflow process.

The initial driver for the move to lead-free materials in electronics assembly rose from the concern of lead contamination in groundwater due to the leaching of lead from discarded electronic assemblies. Despite this concern, a 2003 European solder survey shows that current drivers among companies vary. In fact, only 14% of those surveyed were moving toward lead-free assemblies for the environmental reason. Instead, reaction to legislation was listed as the prime cause for this migration1.

The industry-wide move toward lead-free assembly is occurring in the absence of a “drop-in” replacement for Sn/Pb solders. Lead-free alloys require higher reflow temperatures due to the higher alloy liquidus temperatures (217°- 219°C for SAC alloys) than that of eutectic Sn/Pb (183°C). Numerous studies have reported lower wetting performance, higher solder-void content and a duller fillet finish. Wetting performance of lead-free solder pastes is a direct result of the wetting angle of a bulk alloy. The wetting angle of a SAC alloy is steeper over copper than Sn/Pb. Wetting angle differences over gold are considerably less. To counteract this alloy’s physical properties, chemists have been increasing the activator level in the flux with minimal impact. From a process standpoint, there is room for improvement, although experts tend to agree that matching Sn/Pb wetting may not be obtainable.

Redefining the Process

Finding the best process and material set is relatively simple when working with Sn/Pb. There are many options for solder pastes in both no-clean and water-clean chemistries. Board finishes that yield excellent results are plentiful and paste sensitivity to reflow profiles are rare. With lead-free, we need to re-evaluate all main elements of the process and carefully select the solder-paste formulation. To accomplish this, sensitive and quantitative tools are needed. Failure to reach this can result in excess rework due to wetting-related defects, such as misaligned components, tombstones, solder beads and balls.

Phase 1: Finding the Best Profile

To begin the process of specifying the optimal profile, material suppliers must carefully study the cause-and-effect relationship of the reflow profile and key performance attributes with the lead-free formulations they are offering. This process begins with quantitative tools to test key reflow attributes such as wetting, solder voiding, solder defect generation and even solder spitting or splattering that can contaminate connector fingers and wire-bonding pads within close proximity to solder joints. Finding a profile that yields the best performance in all of the categories of attributes may not be possible, but selecting the profile that yields the best overall performance is mandatory.

Wetting Tests

Quantitative solder-paste performance tests have been developed and used for paste benchmarking, process improvements and material selection using the Benchmarker II test board.2 There are four wetting areas in the design (Figure 1). In Area A, the gap between each adjacent pad is increased by 1 mil, rendering the difficulty of creating a bridge between printed pads of solder paste more difficult as the gap increases. This creates a horizontal wetting thermometer. Gaps range from 10 to 85 mils. Areas B, C and D gauge pad wettability by printing varying coverages of solder paste onto round BGAs, wide rectangular SOICs and narrow rectangular fine-pitch pads, respectively. These areas are inspected to find out which pads the reflowed solder was able to extend to the pad ends. The smaller the coverage, the more distance the solder must wet to reach the pad ends - ­increasing the difficulty of complete pad wetting. These tests provide three different pad geometry-wetting thermometers. Results are reported in wetting points based on the paste coverage. Profile 2 yielded the strongest wetting. The profile screening tests were done in an air reflow atmosphere over an ENIG (Electroless Nickel Immersion Gold) board finish.

Figure 1. Wetting Gauges

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Passive Component Defect Tests

There are four passive chip “stress test” areas designed on the test board. Figure 2 shows a section of the test board for 1206 chips with 0805, 0603 and 0402 test areas using similar design logic. The first column is a series of chip pads varying the gap between the pads to the full range recommended by the IPC.3 The second column shows various stencil aperture designs that can be found on customer designs. The third column has intentional aperture misalignment. These different pad and aperture design elements are intended to push formulations to generate solder beads, balls and tombstones, as well as provide a test media to evaluate stencil and pad design solutions to correct these problems. A solder bead is a large solder ball that forms on the side of chip components. The origin of these beads can vary somewhat, but mainly they are due to having excessive solder under the center of the chip prior to reflow. Profile, inerted reflow, pad and aperture design can all contribute to solder beading. The number of chips with as little as one bead are recorded. Inspection also is done for solder balls. Because the test board has no ground plane or internal traces, it is possible to backlight the board and easily inspect for solder balls. The solder balls are usually found in the “gutter” of exposed laminate between the solder mask and solder pad. Occasionally a solder ball is found some distance from the solder joint, possibly from solder spitting during reflow due to paste or chip outgassing. The number of chips with as few as one solder ball is recorded.

Figure 2. Chip Defect “Stress Test”

Tombstones also are occasionally generated. Profile testing results indicated fairly similar defect levels when plotted against a scale of all potential defects.

X-ray Voiding

Based on the assumption that the ideal quantitative void measurement method will use BGA analysis software and a symmetrical Z-axis reflow structure, the “sandwich” concept was developed. This approach simulates the worst conditions of a solder joint for voiding, under the component where flux evacuation is the most difficult. This idea was developed out of a quest for a quantitative method of determining the voiding percentage on a Ceramic Column Grid Array (CCGA).4 For solder-paste void benchmarking, a dedicated pad test area was included in the Benchmarker II test board. This allows the testing of solder pastes on standard PCB surfaces such as Entek OSP (Organic Solder Protectant) and ENIG. This makes a copper sandwich, resulting in cylindrical structures that permit highly quantitative void analysis with standard BGA analysis software. For each paste, profile and board finish, 108 preforms are placed over solder paste, reflowed and then X-rayed. The X-ray data are compressed into a single “point scale” to facilitate comparisons. These points are calculated as:

Points = (≤4% - ≤6%)

For Test Phase 2, 1-mm pitch BGAs were included using a similar point assessment. Profile 2 yielded the most points (lowest voiding) of all four profiles tested (Figure 3).

Figure 3. Voiding Results

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Phase 2: Board Finish and Reflow Atmosphere

This phase explores the effects of board finish and reflow atmosphere. The wetting tests, solder defect tests and solder voiding tests were repeated with the addition of gauging BGA voiding on ENIG (gold), Entek (copper), immersion tin and immersion silver finishes. Reflow atmosphere also varied. A seven-zone oven was purchased with an O2 doping feature. This bleeds into the central portion of the oven metered amounts of air during nitrogen reflow. Oxygen content was measured in Zone 7 (reflow) and adjusted from the pure nitrogen O2 level of 25 ppm to 50, 100, 200, 400 and 800 ppm. In total, Phase 2 of testing had four board finishes in seven reflow atmospheres.

Wetting

When reviewing the wetting results, the ENIG finish was expected to have the strongest wetting. The low wetting response of the immersion silver, as well as the relative lack of contrast in the O2 doping experiments, however, was not anticipated. ENIG wetting was nearly 100 times that of immersion silver. The tin finish came in third and the OSP (Entek) was last. On the silver immersion boards, dewetting was observed.

Solder Defects

The two precious metal-based board finishes yielded the least overall defect levels. Again, the levels of O2 in the reflow section had little effect on defect levels, except when comparing very low O2 (25 ppm) to pure air. The largest contrast was noticed when comparing the solder ball results of tin - the poorest-performing finish, to Entek (copper) - the best performer for solder balling.

Solder Voiding

As with the wetting results, solder void results were as expected. It was predicted that the ENIG finish would have the lowest voiding from numerous internal studies. The high voiding of the immersion silver, however, was not predicted. When the data were compiled, plotted and first examined, it was theorized that the copper preforms and the silver pad finish combined with the acids in the flux system to form a type of battery during the preheat, resulting in a by-product that expanded into visible voids during reflow. Visual examination of the X-rays, however, revealed that the solder pads without copper preforms also had significant voiding. The presence of the preforms had amplified the voiding. The BGAs used for this study were Sn/Pb.

Conclusions

The best overall profile for the formulation studied is a ramp-to-spike style profile with a slightly extended liquidus time. Profiles with extended liquidus times have proven valuable for soldering to tin-plated (lead-free) components in the field, even with Sn/Pb solder pastes. Board surface finish had a greater effect on overall lead-free performance than the reflow atmosphere O2 content. Results also showed that ENIG was the most suitable board finish testing in regard to wetting and lowest solder voiding. Immersion tin, with only slightly better wetting than OSP coated copper (Entek), had high solder balling and BGA voiding. Immersion silver did not demonstrate the strong wetting as ENIG. It also produced the highest levels of solder voiding of all of the lead-free finishes tested.

* The author thanks Steve Fritzinger and Mitsuru Kondo for their support.

References

1. Prismark 2003 (Soldertec of Tin Technology Ltd. European solder survey), 12/2003.
2. Richard R. Lathrop, Defining Solder Paste Performance via Novel Quantitative Methods, Apex 2003 Proceedings, Anaheim CA, 3/2003.
3. IPC-SM-782, Surface Mount Design and Land Pattern Standard, IPC, 4/1999.
4. Richard R. Lathrop, Avoiding The Solder Void, Apex 2003 Proceedings, Anaheim CA, 3/2003.

Richard Lathrop, technical service manager, SMT materials, Heraeus Circuit Materials Division, may be contacted at (610) 825-6050, ext. 265; e-mail: rlathrop@4cmd.com.