Improving Lead-free Joint Quality with Nitrogen
December 31, 1969 |Estimated reading time: 5 minutes
Nitrogen inerting can reduce defects in lead-free reflow soldering. However, it is claimed that many solder pastes either do not need nitrogen, or work equally well in air. This article compares joint quality when reflowed in nitrogen vs. air.
By Hiew Pang Ling and Paul Stratton
Nitrogen inerting has been reported to reduce defects in lead-free reflow soldering, particularly those that can be attributed to poor wetting.1 However, many solder paste manufacturers claim their alloys either do not need nitrogen during reflow, or work equally well in air.2,3 While some of these pastes can produce acceptable joint quality, they are susceptible to process window narrowing, and some cannot produce high-quality joints - even under the most advantageous conditions. Manufacturers must consider if a more active, higher-residue, more expensive fluxing system is appropriate for their product.4
One Asian-based consumer electronics manufacturer conducted trials to determine if a paste that performed poorly in air could be improved using nitrogen inerting. Four pastes from major producers were compared. Commercial boards were reflowed in air and nitrogen at two purity levels. The boards were then visually inspected for joint quality using the manufacturer’s standards. Previous experience with nitrogen suggested that low oxygen levels could give rise to defects associated with excessive wetting; therefore, the minimum oxygen level considered was 2,000 ppm.
Experiment
The experimental work was carried out on an active SMT line. Nitrogen was supplied in a portable cryogenic container and the reflow oven was tuned by its manufacturer. Reflow temperature settings (from the data collected using a temperature profiler, after the oven was heated) were:
Preheat 150º-190ºC for 80 sec.;
Reflow above 220ºC for 37-40 sec.;
Peak temperature of 235ºC
Oven zone settings were:
1st side: 160ºC/170ºC/180ºC/180ºC/185ºC/255ºC/233ºC; speed: 0.92 m/min.
2nd side: 160ºC/170ºC/180ºC/185ºC/185ºC/257ºC/233ºC; speed: 0.88 m/min.
The slight differences in settings between the first- and second-side reflow were meant to allow for the additional components present when the second side was reflowed. All pastes used were lead-free Sn96.5/Ag3.0/Cu0.5 (SAC 305), and were qualified for use. Pastes A-D had been used previously for different products manufactured at that site, and included:
The trial ran for four days, one for each type of paste. Unfortunately, several problems occurred on the first day that made the results from Paste A erratic. Because of this, results for Paste A were discarded and are not reported in this article.
Figure 1. Test board: first side, immediately before reflow.
The board tested (Figure 1) was the main board from a consumer electronics product. Components on this board included 1608/0603 resistors and capacitors, 0.5-mm QFP, 0.4-mm QFP, and 0.5-mm BGAs. Each frame consisted of two identical sections (boards) with different sides facing up and down. Thus, each board needed to be reflowed twice. Although this board was used as the test board, it had been running in mass production using Paste D, and occasionally Paste A.
Because the experiment involved four different pastes at three oxygen levels (210,000 ppm, 4,000 ppm, and 2,000 ppm), there were 12 conditions. For each condition, five frames (each consisting of two boards joined together) were reflowed on the first side, and then again on the second side. When the frames were broken apart, there were 10 boards, reflowed on both sides, for each condition.
Solder quality was assessed using visual inspection of selected points on the board (Table 1). Some examples of how the inspection criteria were applied are shown in Figures 2a/b and 3a/b.
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Figure 2a/b. Inspection point 1: reasonable cover of the leads (l); poor lead coverage (r).
In general, less oxidation was observed on the solder joints (shinier); less oxidation of the copper pads and less flux residue for all the boards running in nitrogen was observed. These joints look cleaner and had better solder spreading, cover, and fillet angle than those reflowed in air - indicating better wettability and solderability.
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Figure 3a/b. Inspection point 6: good solder fillet (l); less-than-ideal solder fillet (r).
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Inspection Point #1: Joint Quality
This component was known to be difficult to solder in air, even with the best-performing paste (Paste D). Figure 4 shows that inerting with nitrogen containing 2,000-ppm maximum oxygen improved the performance of the worst-performing paste in air (Paste C). However, it should be noted that the performance of the best paste was also improved by a similar amount.
Figure 4. Joint quality at inspection 1.
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Inspection Point #2: Solder Dot Quality
Both Pastes B and C showed significant improvements as the oxygen level decreased. On the other hand, printing problems occurred with Paste D because of paste-viscosity issues, and it was able the print the dot on only a few boards.
Inspection Point #3: Shorts
With the exception of Paste A reflowed in air, there were few shorts on the test pads. Paste A improved - demonstrating no shorts at 2,000 ppm.
Inspection Points #4, 5, & 6: Joint Quality
The trends at these inspection points were similar to those observed at inspection point 1 (Figures 5 and 6). Generally, Paste D performed better than the other pastes in air, though still very well - especially at inspection points 1 and 6. As the oxygen level decreased, the performance of all pastes improved (some more dramatically than others) to produce superior joints. In 2,000-ppm oxygen conditions, other pastes caught up to the performance of Paste D. Compared to Paste D in air, the other pastes surpassed its performance under nitrogen conditions. Both Pastes B and C, when run in nitrogen, performed better than Paste D in air. Paste B sometimes performed as well as Paste D in the same nitrogen atmosphere.
Figure 5. Joint quality at inspection 4.
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Figure 6. Joint quality at inspection 5.
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Conclusion
This study demonstrated that not only can nitrogen inerting bring the worst-performing paste up to the level of the best-performing paste in air, but it also can improve the performance of the best paste over a range of conditions.
REFERENCES
This article was originally published at IPCWorks, February 2006.
1Amey Teredesai, Srinivasa Aravamudhan, Joe Belmonte, and Richard Szymanowski, “Self-centering of Offset Chip Components in a Pb-free Assembly,” www.speedlinetech.com.
2http://www.kester.com/en-us/leadfree/lfsfaq_smt.aspx.
3 www.mbouk.co.uk/solderpaste.htm.
4 Peter Biocca, “Flux Chemistries and Thermal Profiling: Avoiding Soldering Defects in SMT Assembly.”www.loctite.com.
Hiew Pang Ling, senior process specialist, Malaysian Oxygen Berhad, may be contacted via e-mail: pang-ling.hiew@mox.boc.com. Paul Stratton, manager, Controlled Atmospheres Technologies, BOC, may be contacted via e-mail: paul.stratton@boc.com.