Microstrain: It is a dimensionless physical quantity. When a PCBA is subjected to external force, a deformation will occur in the PCBA. The tensile strain becomes positive, and the compression shortens strain is negative. The general limit in the industry is 500μe. Principal strain: The maximum and minimum orthogonal strains in a plane, and the shear strain in the direction perpendicular to each other is "0".
Strain rate: is used to describe the degree of how fast the strain changes. The amount of change in strain divided by the time interval over which this change was measured. The strain rate is also used to measure the risk of component rupture, and is mostly used to measure the rupture risk of BGA tin points. For MLCC, strain is mainly used to measure the rupture risk of components. If the strain rate is generally not required by customers, the limit value is generally 100000μe/s.
By quantifying the strain at the position of the MLCC in the process through the strain test, it is very convenient and intuitive to know that the MLCC has a relatively large strain in those processes, and the position of the MLCC in the same process has a relatively large strain.
Knife-pass splitting causes MLCC to crack. A manufacturer of coffee machines, in the process of shipping a coffee machine in a batch, received dozens of machines with the same defect. After circuit analysis, it was found that the defect was caused by MLCC C134. Through slice analysis, it is found that the crack on C134 is a typical mechanical stress crack, which is caused by PCB deformation.
The strain test results (P&D Strain) are as follows: the maximum principal strain value is 2269.3μe, far exceeding the current industry standard for MLCC strain ±500ue, the MLCC has a high risk of rupture and has become a potential killer of PCB failure
When it was found through the strain test that C134 failed due to the strain in the cutting process and the cutting process, the company found the supplier of the cutting machine and adjusted the equipment. The results of the strain test are as follows:
It is concluded that the strain test is performed on the PCBA generation process, and then the generation equipment or jig is adjusted according to the analysis of the test results, so as to reduce the impact of external mechanical force on the PCBA, effectively control the risk, and improve the failure rate.