A publication of the National Electronics Manufacturing Center of Excellence August 2004

EMPF Director

Michael D. Frederickson
mfrederickson@aciusa.org


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PWB Qualification: Peel Testing
T
here are many ways to qualify a PWB manufacturer. Tests such as plated through-hole inspection, solderability, and cleanliness testing are common methods for assessing the general quality of a bare circuit board. There are however, additional tests that are not as well known that help in the understanding of a PWB manufacturers capabilities. These tests often yield quantifiable data that can be used to compare the performance of boards manufactured from competing board houses (Figure 3-1). Pull testing is one such test, and is used to assess the bond strength of copper traces (peel test) and the plated through holes (land bond strength integrity test). The method is used to determine the strength of a material in a uni-axial direction. Both the peel test and land bond integrity strength test determine circuit board manufacturing quality. Other uni-axial mechanical stress tests include shear and compression stress tests.

The IPC has recommended test methods for mechanical testing, to include peel testing, in section 2.4 of their TM-650 documents. The test performed at the EMPF is a modified version of these tests. To perform a trace peel test, a trace has to be raised and clamped by the digital force gauge. The circuit board is held fast to the testing surface, and the trace is then "peeled" perpendicularly from the surface of the circuit board. Specifications set by the IPC require a circuit board trace peel test to have a peel rate of two inches per minute.

The base-line of the test at the EMPF is a modification of several tests, which include peel strength of metallic clad laminates at ambient and elevated temperatures, and peel strength of flexible printed wiring materials.

The peel strength of metallic clad laminates at ambient temperature (IPC TM-650 2.4.8) was designed to determine the peel strength of metallic cladding. There are a few different ways this test may be performed depending upon a manufacturer’s need. Testing a sample in its received condition requires no additional preparation before testing. However, a thermal or chemical preparation may be applied to determine their effects on the peel strength of the metallic clad laminates. The samples would be stressed as required before the actual peel test, which is identical for all three types of metallic clad laminates at ambient temperature. IPC TM-650 2.4.8.1 (peel strength, metal foil, key-hole method for thin laminates) describes the procedure for the peel test. Regardless of preparation, each test requires a minimum of two strips measuring 0.125 inches wide (etched) per specimen. The tab to be clamped onto the load applicator is to be no longer then 0.5 inches long. The specimen is fixed to maintain a peel at an angle of 90°. A force is applied in the vertical direction at 2 inches per minute, and the specimen must be peeled for a minimum of 1 inch. The actual width of the test strip is then measured and recorded along with minimum load. Finally, the peel strength is calculated using an IPC supplied formula.

The test called peel strength of metallic clad laminates at elevated temperature (IPC TM-650 2.4.8.2a) is performed while submerged in a hot liquid. Two test strips are etched- once lengthwise for each clad side, and once cross-wise for each clad side. The dimensions of the test strip are the same as the ambient peel test. Before the test, the specimen must be pre-conditioned by baking for four hours at 125°C. The actual test is performed when the specimen is immersed 1 inch below the surface of the hot liquid. After the specimen is peeled, the sample is observed for laminate degradation.

The IPC TM-650 2.4.3 peel strength test applies heat to the sample via hot air. The method requires that the sample be placed into a hot air chamber after pre-conditioning for a minimum of 60 minutes. The specimen is then clamped and the test is performed in accordance with IPC TM-650 2.4.8.1. Upon completion of the peel test, the sample is observed for laminate degradation as it was with the hot liquid method.

For flexible printed wiring materials, IPC TM-650 2.4.9 defines the procedure for determining the bond strength of copper foil clad. Depending upon the test method, the sample is to be etched to 9 inches by 0.125 inches or cut to 9 inches by 0.5 inches. In all cases the peel must be mounted onto a sliding plate or free wheeling rotary drum test fixture to ensure that the peel is at an angle of 90° for the length of the peel. A minimum of 4 specimens, two from the machined direction and two from the transverse direction, must be prepared. Method A describes the procedure for received etched specimens. Method B is virtually the same with the exception of the sample having to be cut to the required size using a Thwing Albert sample cutter. Once cut, the specimens may follow Method A procedures. In methods C and D, specimens are thermally stressed by performing a solder float prior to peel testing. The specimens must first be conditioned by being dryed in a circulating oven maintained at 135°C for an hour. The samples are then floated (conductor side down) just beneath the surface of the molten solder at 288°C for at least 5 seconds. During immersion, the sample is agitated from side-to-side. The sample is then peeled as described in method A. Method D is executed in the same manner with the exception that the sample must be cut to the specified size. When utilizing methods E and F, samples are exposed to five temperature cycles prior to testing. As with methods B and D, F samples must be cut to a specified size.

All methods are evaluated by averaging the chart recordings for both specimens over the entire peel length. The width of the conductor is measured and recorded to calculate the peel strength in pounds per inch of width.

Peel and land bond strength testing can be performed with standard shear/tensile testing equipment. However, the test equipment must have the capability to maintain displacement at a constant rate. Some common electronics pull testing equipment maintains the force applied to the sample as a constant and neglects the rate at which the sample is displaced. This does not conform to the intentions of the peel strength test method. A low-cost alternative to expensive automated pull/shear testing machines are manual units affixed with sensitive force gauges. The operation of these types of tools is simple. The test specimen is clamped or fixed to the sample stage. The opposite end of the test specimen is then clamped fast to the digital force gauge. The digital force gauge is then raised or lowered manually (the EMPF’s unit utilized a hand crank wheel) to create a tensile or compressive force in the test specimen. However, these machines, like the automated constant force testers, do not displace the sample at a constant rate.

The EMPF utilizes a cost-effective modification to the manual equipment. An electric motor is applied to the hand crank using a timing belt/pulley assembly. The selected motor has enough torque to ensure the distance rate remains constant. The motor is affixed to a stand, ensuring that the motor will not move during testing. Power is supplied to the motor using a DC power supply with varying output voltage. The motor speed is adjusted by changing the voltage output on the DC power supply. Data is obtained by linking the digital force gauge with a spreadsheet program running on a near-by laptop computer. The data is graphed, and several basic calculations such as mean, median, and max, are performed. The following is a list of the components required to make a low-cost peel test:

  • DC power supply
  • DC high torque motor
  • Timing pulleys and belt
  • Electronic connection parts (quick disconnect wires/plug, three way switch)
  • Digital force gauge with laptop patch cable and spread sheet capability

Whether used as a means of comparison or quality control, peel testing is an effective method for determining the bond strength between the trace and the circuit board. The quantitative data that is obtained is useful when qualifying new or existing board vendors. One of the benefits of this test is that it is relativly inexpensive to perform in-house or at external laboratories. This modified pull-test makes the EMPF more capable of determining the quality of a PWB manufacturer by supplementing existing electronics manufacturing qualifying tests.


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