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issue: February 2004 APPLIANCE Magazine

APPLIANCE Engineer®
Equipment Impacts of Lead-Free Wave Soldering


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by Jim Morris, Advanced Development Manager, Speedline Technologies, and
Matthew J. O'Keefe, Ph.D., associate professor of Metallurgical Engineering, University of Missouri-Rolla

The increasing popularity and use of lead-free solders has uncovered weakness areas in the wave solder equipment in operation today. The following evaluates the materials used to construct the solder unit of this type of equipment.

The desire toward making devices and appliances lead free will require appliance makers to change their electronics designs to utilize lead-free materials. This will in turn drive companies to review their current manufacturing equipment and electronic design. Furthermore, the treatment addressed here may prove useful to appliance engineers who are having trouble with stainless steel corrosion problems.

The increasing popularity and use of lead-free solders has uncovered weakness areas in the wave solder equipment in operation today, with the primary weakness being visible corrosion and short life of the materials used in the solder module. Various equipment manufacturers have proposed a variety of solutions to combat this problem, which range from doing nothing to entire pot and nozzle replacement using expensive alloys.


Figure 1. Failed Stainless Steel Solder Flow Duct
With safety foremost in mind, failure of the solder pot or vat is a more serious concern than failure of the internal components used to pump and form the waves. A failure in the pot material can create a severe safety hazard and cause injury. Failure of the other components may cause downtime or lost production, but will generally not create a safety hazard.

While it may be acceptable for some users to regularly replace the internal components of the soldering unit because of corrosion, replacement of the solder pot on a regular basis may be an entirely different story.

Equipment manufactured prior to the popularity of lead-free solders generally utilized welded 300 series stainless steel as the base pot material. Some manufacturers treated the stainless steel with a salt-bath nitriding process, and some manufacturers used cast iron for the pot components.

The majority of internal nozzle and pump components are manufactured from 300 series stainless steel, either treated with a nitriding process or otherwise left untreated. All of the materials have shown excellent life when used with tin-lead solders.

The installed base of wave soldering machines is large, and it is important to understand the impact to the equipment before switching to lead-free solder. Some machines may require very little change regarding materials, while other machines may require replacement of the entire soldering unit.

There are many varied solutions to prevent the degradation and corrosion. Some of the popular methods for the internal nozzle and ducting components are the following: all-titanium construction, nitrided stainless steel, nitriding QPQ coating (a designation that indicates a part has gone through a quenching process, then a polishing operation, and then quenched again), and ceramic-coated stainless steel. For the solder pot, the known alternatives being offered are cast gray iron, ceramic-coated stainless steel, and nitrided stainless steel.

In collaboration with the University of Missouri-Rolla Metallurgical Engineering Department, a variety of the common solder pot materials were tested and analyzed to determine the corrosion effects of high tin (scientifically known as Sn) exposure. Treated and untreated stainless steels were tested along with cast iron, titanium, and nitride-coated plain carbon steel.

Utilizing the data presented, the person responsible for wave soldering operations should be able to make good decisions as to what will need to be replaced in his or her existing wave solder machine. The data may also be used as a guide to assist in the selection of the proper materials used in a new machine.

Field Observations

Older equipment that has been drained and filled with lead-free solder has brought to light the severe corrosion that occurs to the internal stainless steel components. Experience has shown that corrosion occurs after as little as 6 months of operation in high uptime environments.

Figure 1 shows a typical failure of an unprotected stainless steel solder flow duct used in a lead-free application. Severe erosion and pitting of the metal has occurred, and some areas have lost all structural integrity. The most vulnerable stainless steel components are those in contact with flowing solder (i.e. pump impeller, ducts, and nozzles).

The effects on stainless also must be addressed. Stainless steel is resistant to corrosion due to a thin chromium oxide compound layer that forms on the surface. This layer is impervious to most materials, including common electronics solders using lead; however, solders with high concentrations of molten Sn will attack and dissolve the natural protective coating on stainless steels. When this layer is gone, wetting occurs. Once wetted, it is only a matter of time before the underlying material dissolves into the molten solder bath.[1]

 

Figure 2. SEM Image of Corrosion Pit

Figure 2 shows a scanning electron microscope (SEM) image of a sectioned sample of a corrosion crater from a stainless steel component operated with 97-percent Sn solder for approximately 1 year. Energy dispersive spectroscopy (EDS) compositional analysis was then performed in the SEM on the crater in 20-m increments, as shown in Table 1. Both indicate that a gradient of elements exist between the underlying stainless steel substrate and the external bulk Sn-Ag solder. This confirms that wetting to the stainless steel has occurred and that the underlying material is dissolving into the molten Sn bath of solder. [1]

 

Table 1. EDS Analysis of Corrosion Pit

Noting this, cast-iron effects must also be addressed. Solder pots made of cast iron that are currently being utilized in lead-free applications show what appears to be a slight wetting. Severe pitting and corrosion has not been observed in field use as it is seen on the components made from stainless steel.

Materials Testing

Laboratory testing was conducted on representative samples exposed to 97-percent Sn solder at different temperatures and various exposure times. Material samples were fabricated into strips approximately 1-in wide by 3-in long. These strips were then immersed into a static bath of 97 percent Sn solder and examined at 2-, 4-, and 8-week intervals for signs of wetting. Baseline pictures were taken prior to running tests.[2]. Some materials rely upon a coating to provide increased corrosion resistance. Since it is likely that components in the solder unit may be scratched during routing maintenance, a scribe mark was added to the samples to determine the effects in field use. The materials tested and test criteria are noted in Table 2.

 

Table 2. Matrix for Testing

Melonite® coating is one form of treatment that is used on stainless steel and other materials to improve properties.[3] It is a salt-bath nitriding process used to improve the surface properties of ferrous metal parts. This particular nitriding process has an extra quenching step. This last quenching operation enhances the corrosion protection properties and sets this method apart from other common nitriding processes.[4]

The nitriding coating consists of two layers, a compound layer and a diffusion layer. The compound layer consists of e-iron nitride (Fe3N) that is a hard, chemically stable material and is primarily responsible for the improved corrosion resistance. The diffusion layer is g-iron nitride (Fe4N) and is responsible for improved material fatigue strength.

 
Figure 3. SEM Image of Nitride-Coated Substrate

Test Data and Discussion

Macroscopic images are provided for some of the samples exposed to the Sn-Ag solder for the 2-, 4-, and 8-week exposure period at 250°C.

Figures 4, 5, 6, and 7 show the ability of the QPQ Nitriding process to protect the underlying substrate of the stainless steel and carbon steel materials. Macroscopically, all coated samples showed very little wetting, except in the area that was scratched. The unprotected 304 and 316 stainless steels (SST) had significant wetting on all samples. Plain carbon steel samples were tested as a possible "lower cost" material for solder unit internal components. Plain carbon steel exhibited good resistance to corrosion when protected by the nitriding coating. Results indicate that coated plain carbon steel will give equivalent corrosion protection to the stainless steel materials.

Click here to view Figures 4, 5, 6, and 7.

 


Figure 8. 100x Light Microscope Image of Coated 304 SST
Samples were examined microscopically to determine if any corrosion effects could be found. Under magnification, the coated 304 SST sample shows that the nitride layer remained intact and continues to protect the underlying material (see Figure 8). Some pitting was observed on the surface of the nitride layer.

Figure 9 shows a close-up view of the 316 SST sample after exposure to Sn-Ag solder. Visually, the samples did not show any signs of wetting or corrosion. Under magnification there appear to be "fingers" of material protruding into the nitride coating from the substrate side. These fingers do not appear on the baseline samples. It is suggested that the austenitic SST is diffusing nitrogen from the Nitride compound layer. Once extended to the surface, the Sn will attack and undercut the protective coating and eventually cause failure.[2]


Figure 9. 50x Light Microscope Image of Coated 316 SST
It is interesting that the 304 SST samples did not show the presence of these fingers that cause degradation of the protective layer. Further investigation needs to be done to determine if this phenomenon is only applicable on the 316 alloy. Given the theory proposed, similar diffusion in the 304 alloy samples would be expected.

Higher temperature testing was conducted on the 316 stainless steel coated samples using temperatures of 250°C, 350°C, and 450°C. At the elevated temperatures, none of the samples showed significant wetting.[2] Under microscopic examination, the austenite diffusion phenomena was much more pronounced than the testing done at the lower temperature levels. Undercutting of the Nitride coating was observed for the 316-coated sample at 350°C (see Figure 10).

 


Figure 10. Image Showing Undercutting at 350°C Exposure
This testing indicates that the service life of coated SST components is finite. The coating significantly improves the corrosion protection compared to non-coated stainless steel, but will eventually degrade at some point in time.

The gray cast-iron samples were visually wet after 2 weeks of exposure. Light microscope images were taken of the 8-week exposed sample to determine the extent of the corrosion.

Microscopic images of the gray cast iron sample show a "damming" action created by the graphite flakes in the cast-iron material (see Figure 11). Since Sn will not dissolve the graphite flakes, less iron surface area is available for attack by the Sn-Ag solder, thus, slowing the rate of corrosion to a very low level.

Available corrosion data shows that cast iron will corrode at a rate of 0.25 mm/year when exposed to molten Sn at 300°C.[6] At this rate, it is expected that there will be many years of acceptable service life given the 10- to 12-mm thickness of these components.

Titanium samples exhibited no effects of corrosion or wetting after exposure. This is expected, as titanium forms a passive oxide on its surface, and the titanium-Sn phase diagram indicates that titanium has no solubility in Sn at the temperatures common to soldering.[7] Of all samples tested, titanium had the most resistance to corrosion attack.  

Figure 11. Gray Cast Iron Showing Graphite Flakes and "Damming" Action

Summary of Test Results

Based upon testing, field data, and experience, better decisions can be made concerning the use of lead-free solder in wave solder equipment. Up-front costs, maintenance costs, safety, and reliability must be considered when determining the best selection.

Based upon corrosion resistance alone, titanium is by far the best material. The likelihood of being able to afford an entire wave solder unit manufactured entirely out of titanium is very low. It is estimated that the purchase price of a new wave solder machine with an all titanium solder unit would be double the cost of a regular unit.

Unprotected stainless steel, either 304 or 316, is not suitable for long-term use in tin-rich, lead-free solder. Samples of these materials are readily wet after a relatively short-term exposure to solder. On the other hand, when protected by the Melonite coating process, the testing indicates that an extended life will result over the non-protected stainless steel. It is important to note that the nitriding coatings do not protect the stainless steel indefinitely - they only delay the eventual corrosion. It is difficult to make predictions on the actual life, but from field experience, 3 to 5 years is not unreasonable when protected with a nitride type coating.

For comparison, unprotected stainless steel has degraded to the point of failure after as little as 6 months of use. The key to a long life is to avoid scratching or damaging the nitriding protective coating. Once the coating is damaged, corrosion of the substrate will be accelerated. Though not evaluated in this examination, we would expect similar results when ceramic-coated stainless steel is used. Gray cast iron quickly wets with lead-free solder but exhibits a damming action that slows the corrosion to a very low level.

Conclusion


Table 3. Summary of Recommended Uses for the Variety of Materials Under Evaluation.
CLICK for full-size table

Tin-rich, lead-free solder can be used in pre-existing and new wave solder machines if appropriate materials are used in the construction of the soldering unit. Older machines utilizing unprotected 304 or 316 stainless steel solder pots should not be used with lead-free solder. Nitride-coated stainless steel is a cost-effective material to use for the internal solder module components, but care must be taken to not damage the surface coatings during maintenance. Nitriting- or other coated-stainless steel is only recommended in safety critical areas when frequent inspections are made to identify degradation of the material. Due to the difficulty of such inspections, safety critical components such as the solder pot should be produced from titanium or gray cast iron, but not stainless steel.

 
This is an edited version of a paper presented at IPC's (the Association Connecting Electronics Industries) Surface Mount Equipment Manufacturers Association Council's APEX 2003, held March 29-April 2 in Anaheim, CA, U.S. It was awarded the "Best U.S. Paper Award."
 
About The Authors
Jim Morris is the advanced development manager for Speedline Technologies in Camdenton, MO, U.S. He has a BSME, an MBA, and holds several patents in the field of electronics assembly and manufacturing.
Dr. Matt O'Keefe has B.S. and Ph.D. degrees in Metallurgical Engineering from the University of Missouri-Rolla (UMR) and the University of Illinois, respectively. He worked for AT&T Microelectronics, AT&T Bell Laboratories, and the Air Force Research Laboratory before his current appointment as an Associate Professor of Metallurgical Engineering and Research Investigator in the Graduate Center for Materials Research at UMR.
 
 

References

1. Blair, Cook, and Hartman. "Interaction of 304L Stainless Steel with Lead-Tin and Lead Free Solder." Met316 Report, May 11, 2001.

2. Carter, Johannes, and Yenicek. "Tin Solder Corrosion of Wave Soldering Components." Met316 Report, December 12, 2001.

3. Melonite‚ The Answer for Wear, Corrosion and Fatigue Problems; Degussa Corp.

4. Kolene Corp. web site (www.finishing.com/kolene/qpq); last accessed December 1, 2002.

5. Birschbach, Davenport, and Cavins. "Melonite QPQ and its Application to Wave Soldering Equipment." Met316 Report, May 11, 2001.

6. ASM Handbook of Corrosion Data, Second Edition, 1995, pgs. 499-504.

7. Massalski, T. B., Editor-in-Chief. "Binary Alloy Phase Diagrams: 2nd Ed.," ASM Intl., 1990, Vol. 2: pgs. 160, 370.

 

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