Max, Min, Diagonal, Rate?

 

Strain testing for PCBAs is an essential tool to improve reliability of modern electronics. Minimising the strain during production processes and handling reduces the potential for latent failures in components and the PCB itself. Micro-fractures can pass production ICT (in-circuit test) but fail some weeks, months or years later in the hands of the customer. Warranty claims or a reputation for poor product performance and reliability can be very costly.

The details of how to run a test can be found elsewhere, within our own literature, videos and hands-on training. Our products, including our instrumentation and software, enable best-available support for this process. It is very important to understand what strains are required to be reported, and the potential dangers from not following the protocols.

Most commonly a three-element rosette is used for capturing the strains on the board. This results in three channels of measured strains. However, these quantities are coincidental to the actual strain on the PCBA. A previous article covers the potential large errors between the measured and calculated principal strains. For PCBA testing there are also two other metrics, diagonal strain and strain rate.

 

Diagonal strain

 

Defined as the maximum strain in the direction of grid 2 (the diagonal gauge at 45° to grid 1) or a strain at 90° (grid 1 + grid 3 – grid 2). This is described in some literature as the main cause of damage at the corner of a BGA. Unfortunately, this can be significantly lower than the maximum principal strain.

 

Strain Rate

 

In some cases damage can be caused by a rapid change of strain, rather than a fixed maximum. Most commonly calculated as a change of strain per unit of time per data point (point-by-point analysis).

 

Maximum and Minimum principal strain

 

A strain field always has a maximum and minimum principal strain. The angle (in relation to grid 1) can also be found, useful for failure diagnostics. Full details of the calculations can be found in our technical literature.

 

 

Absolute strain

 

It should be noted that what is really required is the maximum change of strain, and because the values can sometimes be negative the Maximum Principal Strain can be smaller (less negative) than the Minimum Principal Strain. For this reason, both Max and Min are normally reported, or look for absolute values (ABS function in spreadsheets).

 

Examples:

Calculated Strains

 

In the example below five plots are shown. Note that the Maximum Principal Strain (blue) is a smaller value than the Minimum Principal Strain (yellow) as they are both negative. The Absolute Principal Strain (green) in this example is the mirror of the yellow trace. Note that the Diagonal Strain is a significantly smaller value than the Absolute Principal Strain (500 vs 600 microstrain). The trace at the bottom shows Absolute Principal Strain Rate.

 

 

PCBA Reports

 

The same data above can be reported according to IPC/JEDEC 9704A.

 

Strain Rate Chart

 

The diagonal guidance line shown on the chart is from the IPC/JEDEC guidance for SnPb solder (Sqrt(2.35/(PCBAThickness)) * (1900-300*Log10(StrainRate))). The flat line is a 500 microstrain custom value. Strain guidance can come from the component manufacturer, customer, or from your own characterisation testing on your specific PCBA design.

 

 

Strain Report

 

This report shows pass/fail and the details of the peak value. Note that the Minimum Principal Strain shows FAIL based on a fixed 500µƐ Strain Guidance value whereas the Max Principal Strain and Diagonal Strain show a pass.

 

 

Conclusion

 

Damage to components on a PCBA can be a result of several factors in combination with the correct data interpretation, and it is up to the measurement engineer to ensure the relevant values are reported. Using a combination of max, min, diagonal and absolute strain values is essential. This will enable the engineer to make the right adjustments and decisions.

 

Summary

Micro-measurements provide a complete one-stop shop for PCBA measurements, including pre-wired strain gauges, installation tools and accessories, and our StrainSmart systems and software. The comprehensive and easy-to-use software has integrated PCBA strain reports, and can be fully customised to reflect the specific PCB thickness and desired strain metric as a fixed value or equation. The above charts, reports and data were acquired and reported using our PCBA measurements demonstration rig, System 7100 and StrainSmart 7100 software. No additional hardware or software was used.