| Basic Knowledge |
| Why do we need strain test? |
Take the PCB board as the example, in the assembly and test processes like in-circuit test (ICT), cutting board, or final verification test (FVT) involve holding the board in some form of a fixture and then performing a task, it will sustain each kinds of stress from inside or outside.
Stress-induced solder joint failure is one of the most common modes of printed circuit board assembly (PCBA) failures. Today, along with widespread use of lead-free solder and compact ball grid array (BGA) components make the PCB board including components more fragile. Various assembly and test processes can cause solder joint failures on PCBAs by over-flexing them even before they are shipped out.
It has to design certain test to examine these strengths which apply higher than safely allowed stresses on the PCBs or not. However the strain test can represent the PCB board stress situation. It allows objective analysis of the strain and strain rate levels that a Surface Mount Technology (SMT) package is subjected to during PCB assembly, test and operation.
Lead-free solder: The Restriction of Hazardous Substances (RoHS) directive took effect in the European Union in July 2006. This directive restricts the use of six hazardous materials, including lead in the manufacture of various types of electronic and electrical equipment. The downside of this is that most of the lead-free solder alternatives have higher brittleness, and when it comes to assembly and test processes during manufacturing, the lead-free solder is more prone to cracks than the traditional tin-lead solder when subjected to stress.
BGA: Ball grid array (BGA) components offer many advantages over surface-mount packages including higher density of pins, better heat conduction and prevention of overheating because of its lower thermal resistance, and superior electrical performance because of the short distance – and thus lower impedance – between the package and the PCB. On the flip side, BGA components present some disadvantages including higher flexing and vibration compared to surface-mount components with longer leads, not very efficient at evenly distributing stresses, and are more prone to solder ball joint cracking, specifically on the inner rows. |
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