| A Show of Strength |
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| Written by American Competitiveness Institute | |||
| Monday, 31 March 2008 19:00 | |||
For pull testing, setup is crucial to get the right results.
Pull testing is common for board qualifications. One purpose of pull testing is to confirm the strength of materials on a PWB; for example, annular rings and traces attached to the PWB surface. The results provide information about the reliability of the lands (annular rings), traces and solder joints. For instance, if the solder cracks during testing, perhaps wetting was at issue. Test setup is crucial for obtaining reliable results. The process is delineated in these five steps:1. Cut the trace near the land so it no longer attaches to the remainder of trace that leads to it (Figure 1). If the trace is not cut, the land under observation will not be represented properly. The pull test would involve the entire trace, not the specific area of interest. IPC-TM-650, method 2.4.21 suggests cutting the trace at least 6 mm from the land to prevent disturbing the bond of the land.1  2. Use the proper amount of heat while soldering. Excess heat during soldering has the potential to weaken the traces and pad around the test area, and can affect the reliability of the results. Select the tip heat based on the solder type selected for testing. For an SnPb37 solder, 260°C is sufficient. 3. Perform the pull test with solid wire (Figure 2). Solid wire is stronger than component leads. Weak wire might snap before reaching the standard’s threshold of force. Use of a solid wire as close as possible to the through-hole diameter significantly reduces the likelihood of wire breakage. If the wire breaks, this is not considered a failure. Re-solder the wire and test again.  4. Do not clinch the wire (Figure 3). Clinching the wire will add strength to the joint, making it more difficult for the land to pull from the board if it has weak land-bond strength.  5. Apply the load perpendicular to the land (Figure 4). Pulling the load perpendicular to the land ensures the force being pulled in the proper direction results in a true land bond strength measurement.  A pull test (and what constitutes failure) should be specified in the procurement document. If not, use the applicable standard, such as IPC-TM-650, method 2.4.21. Reference
The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.
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