One purpose of pull testing is to confirm the strength of materials on an electronic printed wiring board (PWB).  An example would be the annular rings and traces attached to the surface of the PWB.  The results provide information about the reliability of the lands (annular rings), traces, and solder joints of the assembly.  For instance, if the solder cracks during testing, perhaps poor wetting was an issue.  Setting up this test is crucial for obtaining good reliable results.  The process is delineated in the five steps below: 

1)  Cut the trace near the land so it is not attached to the remainder of trace leading to it. 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 2.4.21 suggests cutting the trace at least 6mm from the land to prevent disturbing the bond of the land. 

2)  Use the proper amount of heat while soldering.  Excess heat during the soldering process has the potential to weaken the traces and pad around the test area, therefore not giving reliable results.  The heat of the tip should be chosen based on the type of solder selected for testing.  For a 63/37 SnPb solder, 260°C is sufficient.

3)  Perform the pull test with solid wire.  Solid wire is stronger than leads of a component.  Weak wire might snap before reaching your standards threshold of force.  By using a solid wire which is as close at possible to the diameter of the through hole, the likelihood of the wire breaking is significantly reduced.  If the wire breaks, this is not considered a failure.  Re-solder the wire and test again.   

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4)  Do not clinch the wire.  Clinching the wire will add strength to the joint making it more difficult for the land to pull away from the board if it has weak land bond strength. 

5)  Apply the load perpendicular to the land.   Pulling the load perpendicular to the land ensures that the force is being pulled in the proper direction results in a true land bond strength measurement.  

Pull testing is commonly used for board qualifications.  The failure of the pull test should be specified in the procurement document.  If it is not, use the applicable standard.  IPC TM-650 2.4.21 states that if the land around the unsupported area is loosened, it fails.

The EMPF also has the capabilities to perform several other board qualifying tests.  Tests may include cross-sections for board registration and plating thickness, solderability testing such as wetting balance and re-work, and cleanliness tests such as the ionograph and ion chromatography.  More information about these services can be found on the EMPF website, http://www.empf.org/. or by calling the EMPF technical staff at (610) 362-1320.

References:  IPC TM-650 2.4.21