Study on Thermal Compensation Treatment Method of Strain Measured by Resistance Strain Gauge Method in Reflow Soldering Process

Accurate measurement of strain in PCB assembly process is the basis for establishing process baseline and ensuring the manufacturing reliability of electronic products. The strain in PCB assembly process mainly includes mechanical strain and thermal/mechanical strain. Reflow soldering process is one of the most common thermal/mechanical strain operation processes in PCB assembly. In this paper, the strain of reflow soldering process is measured by resistance strain gauge method, and two technical methods are used for thermal compensation treatment. Based on the comparative analysis of multiple groups of strain data and the strain measured by strain gauge method and TMA method, the applicable thermal compensation method of the current test system is optimized, which provides a technical reference for the thermal strain compensation treatment of reflow soldering.

I. Introduction

Due to its strong adaptability, easy to master and the ability to measure stress/strain under complex process operating conditions, the resistance strain test method has been widely used in the assessment of stress-strain damage risk in the assembly process of electronic products, and has formed relevant industry standards, such as IPC-9704 Printed Circuit Assembly Strain Gage Test Guideline 2012. However, ensuring the accuracy of strain test data and controlling test errors are still one of the prerequisites that must be solved in the application of this method, and they are also the aspects that often make mistakes in practical engineering applications. As for the control of strain measurement error, Li Yan et.al analyzes the strain measurement system error of resistance strain gauge from the error caused by the change of sensitivity coefficient K of resistance strain gauge, the error caused by mechanical hysteresis, the nonlinearity of electric bridge and the measurement error caused by long conductor[1]. However, the above theoretical error analysis work is not enough to directly guide the control of stressstrain test error in electronic product assembly process. Because the electronic assembly process involves multiple processes, including mechanical stress processes such as flying probe test, depanelling, screen fastening and so on, and thermal stress processes such as wave soldering and reflow soldering. As we all know, the output of the strain gauge is not only related to the strain of the specimen, but also related to the temperature change of the specimen. The additional error caused by the change of the ambient temperature at the measurement site is called the temperature error of the strain gauge. The basic reason mainly comes from the influence of the temperature coefficient of the sensitive grid resistance of the strain gauge and the influence of the linear expansion coefficient of the specimen material and the resistance wire material. Although there have been a lot of studies on thermal compensation for strain measurement by resistance method, there are still some differences and application misunderstandings[2]-[6]. Liang Likai. et.al compares and analyzes the advantages and disadvantages of the compensation method for temperature error in resistance strain gauge measurement--bridge temperature compensation method and strain gauge self-compensation method (selective self-compensation strain gauge and combined self-compensation strain gauge) and their application fields[2], while Lan min analyzes the misunderstanding of bridge temperature compensation method and considers that for statically determinate structures under non-uniform temperature field, the temperature strain compensation of the working strain gauge obtained from the compensation strain gauge is somewhat invalid due to many influencing factors in the strain test, such as the material change of the tested structure, the temperature gradient of each measuring point, the structural constraints, etc.,. which indicates the compensation method needs to be designed and analyzed according to the actual situation.