PCB Tech - Research on PCB Solder Mask Design and PCBA Manufacturability

With the rapid development of modern electronic technology,PCBA is also developing towards high density and high reliability. Although the level of PCB and PCBA manufacturing processes has been greatly improved at this stage, conventional PCB solder mask processes will not have a fatal impact on product manufacturability. However, for devices with very small device pin spacing, due to the unreasonable PCB solder pad design and PCB solder mask design, it will increase the difficulty of the SMT soldering process and increase the risk of PCBA surface mount processing quality. In view of the manufacturability and reliability hidden problems caused by the unreasonable PCB soldering and solder mask design, combined with the actual process level of PCB and PCBA, the manufacturability problem can be avoided through the optimized design of device packaging. The optimization design mainly starts from two aspects. One is the optimization design of PCB LAYOUT; the second is the optimization design of PCB engineering.

PCB LAYOUT design

According to the IPC 7351 standard package library and refer to the recommended pad size of the device specification for package design. In order to design quickly, Layout engineers give priority to increase and modify the design according to the recommended pad size. The length and width of the PCB soldering pad design are increased by 0.1mm, and the solder mask pads are also different in length and width based on the soldering pad. Increase by 0.1mm.

PCB engineering design

The conventional PCB solder mask process requires to cover the edge of the solder pad by 0.05mm, and the middle solder mask between the two solder pads is larger than 0.1mm, as shown in Figure 2 (2). In the PCB engineering design stage, when the size of the solder resist pad cannot be optimized and the solder resist bridge between the two pads is less than 0.1mm, the PCB project adopts the group pad window design process.

PCB LAYOUT design requirements

When the edge distance between two soldering pads is greater than 0.2mm or more, the package is designed according to the conventional pad; when the edge distance between the two soldering pads is less than 0.2mm, DFM optimization design is required, DFM The optimization design method has the optimization of soldering flux and solder mask pad size. Ensure that the solder resist in the solder mask process can form the smallest solder mask bridge isolation pad during PCB manufacturing.

When the edge distance between the two soldering pads is greater than 0.2mm or more, engineering design shall be carried out according to the conventional requirements; when the edge distance between the two soldering pads is less than 0.2mm, DFM design is required. The engineering design DFM method has solder mask design optimization And the soldering layer copper removal treatment; the copper removal size must refer to the device specification, the soldering soldering layer pad after copper removal should be within the size range of the recommended pad design, and the PCB solder mask design should be a single-pad window design, That is, the solder mask bridge can be covered between the pads. Ensure that in the PCBA manufacturing process, there is a solder mask bridge between the two pads for isolation to avoid solder appearance quality problems and electrical performance reliability problems.

PCBA process capability requirements

The solder mask can effectively prevent the solder bridge from shorting during the soldering assembly process. For PCBs with high-density fine-pitch pins, if there is no solder mask bridge between the pins for isolation, the PCBA processing plant cannot guarantee the local soldering quality of the product. For PCBs with high-density and fine-pitch pins without solder mask isolation, the current PCBA manufacturing factory processing method is to determine that the PCB is poorly supplied and will not be on-line production. If the customer insists on going online, the PCBA manufacturing factory will not guarantee the welding quality of the product in order to avoid quality risks. It is foreseen that the welding quality problems that occur during the manufacturing process of the PCBA factory will be negotiated and dealt with.

PCB engineering design requirements

According to the conventional solder mask engineering design, the size of the single-sided solder mask is required to be greater than the size of the flux pad by 0.05mm, otherwise there will be the risk of the solder mask covering the flux layer. As shown in Figure 5 above, the width of one-sided solder mask is 0.05mm, which meets the requirements of solder mask production and processing. However, the edge distance between the two resist pads is only 0.05mm, which does not meet the minimum solder resist bridge process requirements. The engineering design directly designs the entire row of pins of the chip as a group-pad type window design.

Actual welding effect

After making the board according to the engineering design requirements, and complete the SMT patch. It is verified by functional tests that the chip has a defective soldering rate of more than 50%; after passing the temperature cycle experiment again, a defective rate of more than 5% can be screened out. Firstly, perform appearance analysis of the device (20 times magnifying glass), and find that there are tin slag and residues after soldering between the adjacent pins of the chip; secondly, analyze the failed products and find that the failed chip pins are short-circuited and burned.

PCB LAYOUT design optimization

Refer to the IPC 7351 standard package library, the solder pad design is 1.2mm*0.3mm, the solder resist pad design is 1.3*0.4mm, and the center distance between adjacent pads is 0.65mm unchanged. Through the above design, the single-sided solder mask size of 0.05mm meets the PCB processing process requirements, and the adjacent solder mask edge spacing of 0.25mm size meets the solder mask process. Increasing the redundancy design of the solder mask bridge can greatly reduce the welding quality risk., Thereby improving the reliability of the product.

The width of the soldering pad is cut copper, and the size of the solder mask width is adjusted. Ensure that the edge of the two soldering pads of the device is greater than 0.2mm, and the edge of the two soldering resist pads is greater than 0.1mm, and the length of the soldering soldering pad and the soldering resist pad remain unchanged. Meet the manufacturability requirements of PCB solder mask single-pad type window design.

Design verification

In view of the above-mentioned problem pads, the pads and solder mask design are optimized through the above solutions. The edge spacing of adjacent pads is greater than 0.2mm, and the edge spacing of solder mask pads is greater than 0.1mm. This size can meet the solder mask process. need.

Test yield comparison

After optimizing the solder mask design from the PCB LAYOUT design and PCB engineering design, the organization re-invested the same number of PCBs and completed the placement and production according to the same manufacturing process.

Through the above, the optimization scheme is verified to be effective and meets the product manufacturability design.

Optimized design summary

In summary, the chip with the device pin edge spacing less than 0.2mm cannot be designed according to conventional packaging. The width of the soldering pad in the PCB LAYOUT design is not compensated, and the length of the soldering pad is increased to avoid the reliability problem of the soldering contact area. If the soldering pad is too large and the distance between the two solder mask edges is too small, priority should be given to copper removal; for the solder mask that is too large, the solder mask design should be optimized to effectively increase the edge width of the two solder masks to ensure PCBA Welding quality assurance. It can be seen that the coordination between soldering flux and solder mask pad design plays a decisive role in improving PCBA manufacturability and soldering through rate.