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July 23, 2012

SMT Component Glue Strength

Is there a recommended product or procedure to test surface mount component glue strength?


Experts Comments

There are two basic alternatives:
  1. Use a shear tester and shear the component off using a flat-faced rod to push the component parallel to the board surface. The shear tester should report at least the maximum force value at fracture. A force curve can be useful but is not a requirement.
  2. Use a torque tester that records the maximum value at failure.
The first method can be difficult to implement where testing is to be performed on actual assemblies due to access issues. It also requires more expensive equipment, but can yield more data, such as the "brittleness" of the adhesive. It allows calculation of the shear strength of the adhesive bond if the bond area is known.

The second method can and does yield very good data, but we can't easily calculate the shear strength of the bond or material from the data, only the relative strengths of the bonds. Taking the torque data also requires a tool that can grip the component without damaging the component body instead of failing the adhesive. For chip components, this is normally a rod with a machined slot wide enough to accommodate the part. When testing adhesive strength, it is also useful to record the failure mode. Three failure modes are possible:
  1. Adhesive failure between component and adhesive.
  2. Cohesive failure within the adhesive.
  3. Adhesive failure between the adhesive and PWB.
A fourth failure mode, cohesive failure of the PWB (usually the solder mask) is possible when bond strengths are very high, but in practice is almost never seen. Finally, it may be useful to record the amount of voiding in the adhesive joints. One major cause of low strength is excessive outgassing of the adhesive, causing a sponge-like appearance of the sheared joint.
Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
There IS a test procedure within the IPC-TM-650 standard, which is an invaluable document but yours absolutely FREE at http://ipc.org/4.0_Knowledge/4.1_Standards/test/2.4.42.pdf; available right now just because the IPC likes people like you!

IPC-TM-650 is a compilation of many different test methods. The one I believe you are looking for, in case the link does not work, is IPC-TM-650-2.4.42 "Torsional Strength of Chip Adhesives". You can download this test method (as well as many, many others) at this location. If by any chance you cannot access it, let me know directly and I will help you (Richard.stadem@gd-ais.com). This method uses a 1206 size SMT capacitor and a device called a Waters Manufacturing Torque Watch, and its use is also described within TM 2.4.42. You will need to make some type of holding fixture as well. Please note that you may be able to lease the Torque Watch. Contact Waters for information.

To further answer your question, on all chipbonder Technical Data Sheets (and most other epoxies and adhesives) the lap sheer strength is listed. You can compare different material properties simply by reviewing their TDSs. However, please understand that just because a given chipbonder has a higher sheer strength than another does not necessarily mean it is better than others; with every property gain, you lose something elsewhere. Those with higher shear strength may also be more brittle, and have a lower stress cracking tolerance. Some may transmit (or insulate) flexure stresses from the PWB to the chip component better than others. You have to understand what all of the material properties mean in order to select the one best for your particular product's field application.

Note that there is no acceptance criteria; IPC only provides a standard means of testing, they cannot always tell you what is "good" or "bad". Further qualification evaluation and testing may be called for in addition to this test method.
Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
There are two adhesion strengths that are typically measured for SMT adhesives. There is the so called green strength that measures the ability of the uncured adhesive to holed a part in place. The Siemens test this involves assembling a board placing it on a table which is on an inclined plane the table with the board is allowed to accelerate down the plane and hit the stop. The movement of the components if any is the measured.

For cured adhesive the most common way is to use a torque gauge with a suitable fitting to go over the component. the gauge is then rotated and the torque required to remove the component recorded.
Neil Poole
Senior Applications Chemist
Henkel Electronics
Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.
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