What is Halogen-Free Solder Paste? Print E-mail
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Written by Karl Seelig and Michael Burgess   
Monday, 04 May 2009 13:32

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And do independent lab results support its use?

Increased interest in halogen-free assemblies is a result of Non-Government Organizations (NGOs) exerting pressure on electronics manufacturers to eliminate halogens. The NGOs’ primary focus is on resolving global environmental issues and concerns. As a result of an increase in the enormous e-waste dump sites around the world, NGOs are pushing consumer electronics manufacturers to ban halogen-containing material in order to produce so-called green products. Not only are these sites enormous, but the recycling methods are archaic and sometimes even illegal. This stockpiling and dumping has created growing political and environmental issues. To deal with this issue, the question of why halogens are a focal point must be addressed.

As a safety measure, halogens are added to organic materials as a fire retardant. The common halogens used only emit bromides under elevated temperatures, decomposing and releasing bromine to extinguish fires. These are toxic as well as corrosive when decomposing. However, they are benign at ambient temperatures. The jury is out on the replacement products for these brominated organics, with uncertainties about long-term health exposure and environmental impact.

Rather than replace halogens with potentially equally harmful substitutes, it would be more logical and effective to address this issue at the e-waste sites. If modern recycling processes were applied to these sites, this environmental problem could be minimized. Methods such as shredding followed by fluid bed separators could yield higher value returns for the recyclers and mitigate the impact of the halogens.

Nevertheless, electronics OEMs are feeling pressure from NGOs and are moving toward halogen-free electronics assemblies. That, in turn, trickles down to the suppliers of electronics assembly materials, board materials and components. The general trend is that every component subassembly on the board must be halogen-free. Other committees refer to these component subassemblies as substances, articles and/or preparations. Solder paste and flux products are included in these restrictions. Some electronics OEMs require solder pastes to be tested; some want the flux tested; yet others want combinations tested.

Solder paste that creates the electrical and mechanical connection on a circuit board now constitutes the article. The rules of sample preparation regarding dilution should apply, as they are consumed in one unit. So what is halogen-free? Many committees, consortia and organizations are working on this issue. Some organizations have published maximum limits as the determining factor. These can be 900 ppm of either bromine or chlorine and a combined total of 1500 ppm. Others have set 1000 ppm of either bromine or chlorine. There is an attempt to have a defined limit on halogen content (Table 1).

Table 1

How does one determine if a solder paste is halogen-free? The most popular test method is known as “oxygen bomb,” which is a combustion test immediately followed by ion chromatography. This is an environmental test procedure and may be performed in a variety of methods. The most popular seems to be BS EN 14582:2007; although other test methods are available, including EPA SW-846 5050/9056 or JPCA ES-01-2003.

Table 2

We conducted round robin testing to determine repeatability of halogen-free testing results to EN14582:2007. We manufactured a batch of solder paste medium (“NC-A”) intentionally doped with 13,000 ppm bromine, as well as a completely halogen-free version (“NC-B”). All samples were chlorine- and fluorine-free. Solder paste was manufactured with these two mediums; the medium itself for NC-A was also sent for testing. The solder paste was composed of 89% metal, with the balance being the medium. We prepared these three sample types and sent them to six independent testing laboratories around the world. Results are shown in Table 2 and Figure 1.
Based on testing performed by six different laboratories, all of which used the same method, it is readily apparent that the precision of this testing method is poor.

Fig. 1

An interaction between the metal alloy and flux medium in a solder paste was also observed. Figure 2 shows a notable difference between the NC-A flux medium made intentionally with 13,000 ppm bromide added and the NC-A SAC 305 solder paste of the same medium. Because solder paste is 11% by weight medium and 89% by volume metal (in this case), one might expect a 11% drop in reported bromide or 1430 ppm, which is not the case. Additionally, there is no consistency in the net difference reported between laboratories, further suggesting lack of repeatability in the test method. This also points out that testing of the medium alone for compliance is not recommended because the bromide levels are reduced or consumed when mixed with the alloy in the solder paste.
When these mediums are tested to J-STD 004, no halides are found. However, the current IPC test does not detect halogens. To fail the IPC test, the halogen has to be ionic and soluble in water/alcohol. Based on the test results following this IPC standard, these pastes and mediums are halogen-free.

Fig. 2

Further testing would need to be completed to determine if a reflowed residue contains halogens, as all of the above were run on unreflowed paste. This further complicates the current test methods because of the extra step to extract the reflowed residue and the potential of compromising sample integrating. If further reduction or consumption of the halogen content takes place, then soldering fluxes and paste would have virtually no impact on a board, component or subassembly relating to halogen content of the final product.

Table 3

SIR results for the halogen-free versus the halogen-containing products in this study indicate the halogen-containing product has slightly better SIR values than the halogen-free equivalent. This is due to the relatively low amount of organic activators required to interact with the halogen to get acceptable soldering results, whereas the halogen-free products require a much higher concentration of organic activators that have a slightly negative effect on SIR values. Table 3 and Figure 3 compare the SIR values of the two pastes used in this study.

Fig. 3

The electronics assembly industry is being pushed in the direction of halogen-free materials. Already, many manufacturers around the world are pushing suppliers to provide materials to meet the criteria of this latest industry buzzword. However, many questions concerning this issue remain: If we do want halogen-free, what are the gauge R&Rs of the test procedures? How critical is 1500 ppm versus 900 ppm? Is this a test that represents the real impact of the halogenated fire retardants? Should the halogen content only be tested on the final assembly? Brominated fire-retardants were introduced to eliminate the more toxic antimony oxide previously used. What are the impact and the danger of the replacements? How pertinent is this to electronics assembly when the real issue is illegal dumping of e-waste? The results of the testing regarding this topic create more questions than answers. Until these questions are answered, the attempt of OEMs and their suppliers to meet the requirements of NGOs to provide halogen-free products is confusing, unduly expensive, disadvantageous, and potentially more harmful than supplying halogen-containing materials.

Karl Seelig is vice president of technology and Michael Burgess is strategic account manager at AIM (aimsolder.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Tuesday, 05 May 2009 10:06
 

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