1. Introduction
Surface mount microphones, often referred to as MEMS (Microelectro Mechanical System) microphones, are devices produced using MEMS technology. They primarily consist of sound pressure sensor chips, ASIC chips, sound cavities, and RF suppression circuits. In recent years, surface mount microphones have gained widespread adoption in mid-to-high-end mobile phones across various brands due to their compatibility with surface mount technology and their impressive stability. The solderable end of these microphones is the pad located on the backside of the PCB substrate.
Typically, the surface mount microphone is positioned at the edge of the PCB and cannot be underfilled like BGA devices in mobile phones. Thus, ensuring that the surface mount microphone remains secure largely depends on the strength of the solder joints. A review of SMT processing cases for these microphones reveals that all instances of microphone detachment stem from cracks at the solder joint interface, with the cracking significantly influenced by the solderable end of the microphone and the PCB’s surface treatment.
1. Currently, the solderable ends of surface mount MICs available on the market are all gold-plated, utilizing common surface treatments such as nickel-gold (ENIG), nickel-palladium-gold (ENEPIG), and nickel-gold electroplating. However, most PCBs predominantly use ENIG, OSP, or selective ENIG processes. Those familiar with ENIG surface treatment will quickly recognize the issue of nickel corrosion. Indeed, nickel corrosion is a prevalent factor contributing to solder joint cracking, and this holds true for surface mount MIC solder joints. Today, however, we will delve into another often-overlooked issue: the area accumulation of AuSn alloy.
2. Case of Surface Mount MIC Device Detachment
A mobile phone product experienced MIC detachment following a roller test. The solderable end of the MIC in this case employed electro-nickel gold processing, while the PCB utilized ENIG processing (OSP+ENIG). Solder joint cracking was observed between the solder and the PCB pad. A significant amount of AuSn alloy was noted on the separation surface of the solder joint. Cross-sectional analysis revealed a higher concentration of AuSn alloy in the intermetallic compound (IMC) formed at both the upper and lower sides of the interface. Notably, the AuSn alloy on the device side was found closer to the solder at the IMC location, whereas on the PCB side, it was nearer to the nickel layer. The PCB pad, treated with OSP, showed minimal AuSn alloy presence in the solder, and the IMC on the PCB side was a uniform Cu6Sn5 alloy. In contrast, the device side still exhibited a SnNi alloy without any visible AuSn.
3. This indicates that when both the solderable end of the MIC and the PCB pad are gold-plated, the diffusion of Au in the solder is hindered, leading to AuSn alloy accumulation in the boundary area. However, this accumulation can increase the brittleness of the solder joint and diminish its interface strength. Subsequently, the mobile phone manufacturer switched PCB suppliers and reduced the gold thickness of the PCB’s ENIG pad by 20 nm. Following this adjustment, roller test results revealed a decrease in the MIC drop failure rate from 20% to 3%, significantly enhancing solder joint strength, although the failure rate remained unacceptable for mobile phone applications.
4. Concluding Remarks
Presently, the solderable ends of surface mount MIC devices on the market are all gold-plated. Thus, if a PCB is treated with ENIG or ENEPIG, it will inevitably face the issue of AuSn alloy boundary accumulation. While reducing the gold thickness on the PCB can markedly enhance solder joint strength, excessive reduction may lead to nickel layer oxidation and poor solderability. Therefore, merely reducing gold thickness cannot fundamentally resolve the underlying issue.
Surface mount microphones, often referred to as MEMS (Microelectro Mechanical System) microphones, are devices produced using MEMS technology. They primarily consist of sound pressure sensor chips, ASIC chips, sound cavities, and RF suppression circuits. In recent years, surface mount microphones have gained widespread adoption in mid-to-high-end mobile phones across various brands due to their compatibility with surface mount technology and their impressive stability. The solderable end of these microphones is the pad located on the backside of the PCB substrate.
Typically, the surface mount microphone is positioned at the edge of the PCB and cannot be underfilled like BGA devices in mobile phones. Thus, ensuring that the surface mount microphone remains secure largely depends on the strength of the solder joints. A review of SMT processing cases for these microphones reveals that all instances of microphone detachment stem from cracks at the solder joint interface, with the cracking significantly influenced by the solderable end of the microphone and the PCB’s surface treatment.
1. Currently, the solderable ends of surface mount MICs available on the market are all gold-plated, utilizing common surface treatments such as nickel-gold (ENIG), nickel-palladium-gold (ENEPIG), and nickel-gold electroplating. However, most PCBs predominantly use ENIG, OSP, or selective ENIG processes. Those familiar with ENIG surface treatment will quickly recognize the issue of nickel corrosion. Indeed, nickel corrosion is a prevalent factor contributing to solder joint cracking, and this holds true for surface mount MIC solder joints. Today, however, we will delve into another often-overlooked issue: the area accumulation of AuSn alloy.
2. Case of Surface Mount MIC Device Detachment
A mobile phone product experienced MIC detachment following a roller test. The solderable end of the MIC in this case employed electro-nickel gold processing, while the PCB utilized ENIG processing (OSP+ENIG). Solder joint cracking was observed between the solder and the PCB pad. A significant amount of AuSn alloy was noted on the separation surface of the solder joint. Cross-sectional analysis revealed a higher concentration of AuSn alloy in the intermetallic compound (IMC) formed at both the upper and lower sides of the interface. Notably, the AuSn alloy on the device side was found closer to the solder at the IMC location, whereas on the PCB side, it was nearer to the nickel layer. The PCB pad, treated with OSP, showed minimal AuSn alloy presence in the solder, and the IMC on the PCB side was a uniform Cu6Sn5 alloy. In contrast, the device side still exhibited a SnNi alloy without any visible AuSn.
3. This indicates that when both the solderable end of the MIC and the PCB pad are gold-plated, the diffusion of Au in the solder is hindered, leading to AuSn alloy accumulation in the boundary area. However, this accumulation can increase the brittleness of the solder joint and diminish its interface strength. Subsequently, the mobile phone manufacturer switched PCB suppliers and reduced the gold thickness of the PCB’s ENIG pad by 20 nm. Following this adjustment, roller test results revealed a decrease in the MIC drop failure rate from 20% to 3%, significantly enhancing solder joint strength, although the failure rate remained unacceptable for mobile phone applications.
4. Concluding Remarks
Presently, the solderable ends of surface mount MIC devices on the market are all gold-plated. Thus, if a PCB is treated with ENIG or ENEPIG, it will inevitably face the issue of AuSn alloy boundary accumulation. While reducing the gold thickness on the PCB can markedly enhance solder joint strength, excessive reduction may lead to nickel layer oxidation and poor solderability. Therefore, merely reducing gold thickness cannot fundamentally resolve the underlying issue.
