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RELIABILITY UPDATE FOR SN100C

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RELIABILITY UPDATE FOR SN100C

This report is the first of a five part series highlighting the results of recent reliability testing comparing the Nihon Superior SN100C nickel stabilized tin/copper alloy to tin/lead and SAC alloys. The full report is available by contacting Jim Wertin at jwertin@fctassembly.com.

One of the primary concerns in the industry today is whether converting to a lead free process from tin/lead will have an impact on the reliability of the assembly. Much testing has been performed in the last 10 years on lead free joints in anticipation of the potential switch by the industry but there are still concerns about the applicability of the results. Most of the testing has been to procedures developed over time for tin/lead alloys and the issue is whether there will be the same correlation between test results and actual product life cycle of lead free products in the field. As reliability is a complex issue, and not just thermal cycling, a number of tests were performed to attempt to simulate the environment in use.

The testing included vibration, thermal shock, HALT testing, salt fog, temperature/humidity, thermal cycling at multiple ranges, hand solder repair with mixed alloys and thermal cycling. Vibration and thermal shock results will be discussed in this report. There were two vibration studies performed with SN100C, one as part of the Joint Group for Pollution Prevention (JG-PP) lead free project on SN100C wave solder through-hole parts and one conducted by the American Competitive Institute (ACI) on surface mount components soldered with SN100C reflowed solder paste.

The Joint Group for Pollution Prevention (JG-PP) is a government supported consortia that has developed a project to build a class 3 electronic assembly with various lead free alloys to compare their performance in high reliability testing to the 63/37 tin/lead currently used for military and aerospace applications. The alloys chosen are listed in Table 1 below and were based on the lead free alloys either being used in mass production or being considered by industry experts.

Table 1

In addition to manufacturing the boards with lead free alloys, the project also tested parts that originally were soldered with tin/lead and then removed and replaced with a lead free alloy. This simulates an assembly returned from the field that would be repaired with the lead free alloy.

The components utilized in the projected included chip components, CLCC's, PLCC's, CABGA's, PDIP's, TQFP's, TSOP's, BGA's, and Hybrids. The wave solder testing was performed on PDIP's as an example of typical through-hole component technology used in the industry as highlighted in the table below.

The vibration testing was performed by Boeing, Phantom Works and the results indicate that the SN100C not only outperformed the SAC alloy but also Sn63Pb37. The table below details the rating for the vibration results. Vibration testing simulates the stresses a joint encounters during assembly and use, such as board bending during depaneling and attachment.

The theory for the improved performance for SN100C is the greater elongation percentage during tensile testing. Although SN100C has lower gross tensile strength than both SAC and 63/37 alloys, the elongation percentage is better as detailed in Table 2. This allows the joint to stretch without breaking for joints requiring the solder alloy to take the stress, such as through-hole joints, surface mounted chip resisters and capacitors, BGA’s, and leadless components.

Table 2

Insert table 11- both

With the positive results in vibration testing of SN100C through-hole joints, a test was performed on reflow joints manufactured with SN100C solder paste. The testing by the American Competitiveness Institute (ACI) compared SN100C boards with Sn63Pb37 control boards. The board is shown below.

Test Conditions

Results:

• All vibration samples passed continuity testing. None of the vibration samples exhibited an increase in resistance greater than 500 mOhms.
• No visible cracks or separations were detected on the solder joint exteriors (Figure 3) after testing.

Table 2. Maximum resistance change during vibration testing (mΩ).

Figure 3. Board 1070-003 (SN100C) after vibration testing.

Thermal Shock Testing

Thermal Shock testing was performed by Boeing Phantom Works for the JG-PP project. The exposure profile for the joints were 1000 cycles of -55° to 125°C with dwells of 15 minutes (see below).

Results showed only 3 failures with the PDIP’s, two SnPb and one SnAgCu. No manufactured SN100C PDIP’s failed at 1000 cycles.

SN100C has been patented by Nihon Superior and is offered worldwide through licensed manufacturers including FCT Solder for North America. We offer SN100C for wave solder and lead tinning, reflow solder paste applications, and SN100CL for lead free hot air solder leveling (HASL) of P.C. Boards.

We will be including in future newsletters, many of the advantages our current customers have discovered in switching to SN100C as their lead free alloy of choice and we welcome your comments and questions. We look forward to working with you to make your transition to lead free as successful as possible. 

For more information about our products, please visit our web sites at www.finelinestencil.com for stencil products and www.fctassembly.com for solder products.

All files for stencils, templates and PCB carriers should be sent to sales@finelinestencil.com to assure immediate attention.

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