CRACK ELIMINATION STUDY
August 2019 - September 2019
Problem Statement
To determine a root cause and solution to cracking that was occurring along the screw bosses of PCB jamnut receptacles during installation.
The righthand image displays high resolution images of cracked screw bosses.
Constraints
I sought to determine a solution that would not require any design changes to the connector itself.
My solution needed to work on both versions (sizes) of the connector over a wide range of PCB thicknesses.
The two connector sizes had a screw boss size difference of approximately 10%. ​​
Testing & Analysis
My initial work involved developing a torque test that mimicked the normal and extreme use of this part to better understand what conditions caused cracking.
The image to the left displays the torque test rig I used. ​​This rig consisted of a "mock PCB" test board that was clamped to a rigid surface. A torque driver was used to manually install the connectors at a set torque value.
Alongside testing, I researched the material properties of the connector and the characteristics of thread cutting screws. I also used tolerance stack ups to analyze screw engagement.
Based on my findings, I developed a customized thread cutting screw that would reduce stress during torqueing. I validated this design by performing a series of torque testing, strip (failure mode) testing, pull-out force testing, and thermal cycling (testing outsourced).
Results & Conclusions
By modifying the screw design, I observed a massive reduction in cracking, but detected stripping in the screw bosses
This was easily prevented by also lowering the installation torque ​range to prevent over torqueing
These modifications were an effective solution, however, proper consideration of material characteristics during the design stage could have prevented the issue from arising. ​