PCB Technical - Surface treatment of PCB pads for surface mount MIC pcb substrate

Surface mount microphones,also known as MEMS (Microelectro Mechanical System) microphones, are microphones manufactured based on MEMS technology, mainly composed of sound pressure sensor chips,ASIC chips,sound cavity and RF suppression circuits. Surface mount MICs have been widely used in mid-to-high-end mobile phones of various brands in recent years because they can be assembled using surface mount technology and have strong stability.The solderable end of the surface mount MIC is the pad located on the back of the PCB substrate.

The surface mount MIC is usually located at the edge of the PCB and cannot be underfilled like the BGA devices on the mobile phone.Therefore,to ensure that the surface mount microphone does not fall off is mainly to ensure the strength of the solder joints. Summarizing the smt processing cases of these surface mount microphones,it is found that all MIC falling off is caused by cracking of the solder joint interface,and the cause of the cracking is greatly related to the solderable end of the MIC and the surface treatment of the PCB.

At present,the solderable ends of the surface mount MIC on the market are all gold-plated, including common nickel-gold (ENIG) treatment,nickel-palladium-gold (ENEPIG), nickel-gold electroplating and other surface treatment processes.However,PCB substrate mostly adopt ENIG,OSP,or selective ENIG processing methods.Readers familiar with ENIG surface treatment will immediately think of the problem of nickel corrosion. Yes, in ENIG solder joint cracking,nickel corrosion is a common factor leading to solder joint cracking,and it is no exception in surface mount MIC solder joints. But today, we will not talk about the problem of nickel corrosion,and talk about another problem that is easy to be overlooked-the area accumulation of AuSn alloy.

The case of surface mount MIC device falling off

A mobile phone product found the MIC to fall off after a roller test. The solderable end of the MIC device of this product adopts electro-nickel gold processing, while the PCB adopts ENIG processing (OSP+ENIG). Solder joint cracking occurs between the solder and the PCB pad. A large amount of AuSn alloy scattered in the solder can be observed on the separation surface of the solder joint. It can be seen from the cross-section of the solder joint that there are more AuSn alloys in the IMC formed at the upper and lower sides of the interface. The difference is that the AuSn alloy on the device side is closer to the solder at the IMC position, while the AuSn alloy on the PCB side is closer to the nickel layer. The PCB pad is OSP-treated solder joints, AuSn alloy is almost invisible in the solder, the IMC on the PCB side is a uniform and continuous Cu6Sn5 alloy, and the device side is still SnNi alloy, but at this time, no obvious AuSn is seen. The alloy exists.

This shows that when the solderable end of the MIC and the PCB pad are both gold-plated, the diffusion of Au in the solder is suppressed, resulting in the accumulation of AuSn alloy in the boundary area. However, the AuSn alloy with the boundary area will increase the brittleness of the solder joint and reduce the interface strength of the solder joint. The mobile phone manufacturer later changed the PCB manufacturer and reduced the gold thickness of the PCB's ENIG pad by 20nm. The results of the roller test showed that the failure rate of MIC drop was reduced from 20% to 3%, and the solder joint strength was significantly improved, but the failure rate was still unacceptable for mobile phone products.

Concluding remarks

At present, the solderable ends of surface mount MIC devices on the market are all gold-plated. Therefore, if the PCB is treated with ENIG or ENEPIG, it will encounter the problem of the accumulation of AuSn alloy boundaries. Although reducing the Au thickness of the PCB can significantly improve the strength of the solder joints, too much Au thickness reduction will bring about the problem of easy oxidation of the nickel layer and poor soldering. Therefore, reducing the Au thickness cannot fundamentally solve the problem.