With the rapid development of modern electronic technology, PCBA is also developing towards high density and high reliability. Although the current PCB and PCBA manufacturing technology level has been greatly improved, the conventional PCB welding process will not be fatal to the product manufacturability. However, for devices with very small pin spacing, the unreasonable design of PCB welding pad and PCB blocking pad will increase the difficulty of SMT welding process and increase the quality risk of PCBA surface mount processing.

In view of the potential manufacturability and reliability problems caused by the unreasonable design of PCB welding pad and blocking pad, the manufacturability problems can be avoided by optimizing the device packaging design based on the actual process level of PCB and PCBA. Optimization design mainly from two aspects, first, PCB LAYOUT optimization design; Second, PCB engineering optimization design. Package design according to IPC 7351 standard package library and refer to the pad size recommended in the device specification. For rapid design, Layout engineers should increase the size of the pad according to the recommended size to modify the design. The length and width of PCB welding pad should be increased by 0.1mm, and the length and width of block welding pad should be increased by 0.1mm on the basis of the welding pad. Conventional PCB resistance welding process requires that the edge of the pad should be covered by 0.05mm, and the middle bridge of the two pads should be larger than 0.1mm. In the design stage of PCB engineering, when the size of solder pads cannot be optimized and the middle solder bridge between the two pads is less than 0.1mm, PCB engineering adopts group solder plate window design treatment. When the two pad edge spacing greater than 0.2mm pad, according to the conventional pad packaging design; When the distance between the edges of the two pads is less than 0.2mm, the DFM optimization design is needed. The DFM optimization design method is helpful for the optimization of the size of the pads. Ensure that soldering flux in soldering process can form a minimum barrier pad when PCB is manufactured. When the edge distance between the two pads is greater than 0.2mm, the engineering design shall be carried out according to the conventional requirements; When the distance between the edges of two pads is less than 0.2mm, DFM design is needed. The DFM method of engineering design includes design optimization of welding resistance layer and copper cutting of welding aid layer. The size of copper-cutting must refer to the device specification. The copper-cutting pad should be within the size range of the recommended pad design, and the PCB blocking welding design should be single-pad window design, that is, the blocking bridge can be covered between the pads. Ensure that in PCBA manufacturing process, there is a blocking welding bridge between the two pads for isolation, to avoid welding appearance quality problems and electrical performance reliability problems. Welding resistance film in the process of welding assembly can effectively prevent welding bridge short connection, for high-density PCB with fine spacing pins, if the open welding bridge between the pins is isolated, PCBA processing plant can not guarantee the local welding quality of the product. For PCB isolated by open welding of high-density and fine spacing pins, the current PCBA manufacturing factory determines that the incoming material of PCB is defective and does not allow online production. In order to avoid quality risks, PCBA manufacturing factory will not guarantee the welding quality of products if the customer insists on putting the products online. It is predicted that welding quality problems in the manufacturing process of PCBA factory will be dealt with through negotiation.

Case Study:

Device specification book size, device pin center spacing: 0.65mm, pin width: 0.2 ~ 0.4mm, pin length: 0.3 ~ 0.5mm. The size of solder pad is 0.8 * 0.5mm, the size of solder pad is 0.9 * 0.6mm, the center spacing of the device pad is 0.65mm, the edge spacing of solder pad is 0.15mm, the edge spacing of solder pad is 0.05mm, and the width of unilateral solder pad is increased by 0.05mm. According to the conventional welding engineering design, the size of unilateral welding pad should be larger than the size of welding pad 0.05mm, otherwise there will be the risk of welding flux covering the welding pad. As shown in Figure 5, the width of unilateral welding is 0.05mm, which meets the requirements of welding production and processing. However, the distance between the edges of the two pads is only 0.05mm, which does not meet the technological requirements of the minimum resistance welding bridge. Engineering design directly design the whole row of chip pin design for group welding plate window design. Make board and finish SMT patch according to engineering design requirement. Through the function test, the welding failure rate of the chip is more than 50%. Again through the temperature cycle experiment, can also screen more than 5% of the defective rate. The first choice is to analyze the appearance of the device (20 times magnifying glass), and it is found that there are tin slag and welding residues between the adjacent pins of the chip. Secondly, the failure of the product analysis, found that the failure of the chip pin short circuit burned. Refer to IPC 7351 standard package library, the design of the help pad is 1.2mm * 0.3mm, the design of the block pad is 1.3 * 0.4mm, and the center distance between adjacent pads is 0.65mm. Through the above design, the size of unilateral welding 0.05mm meets the requirements of PCB processing technology, and the size of adjacent welding edge spacing 0.25mm meets the requirements of welding bridge technology. Increasing the redundancy design of welding bridge can greatly reduce the welding quality risk, so as to improve the reliability of products. The width of the auxiliary welding pad is copper-cut, and the size of the resistance welding pad is adjusted. Ensure that the edge between the two pads of the device is greater than 0.2mm and the edge between the two pads of the device is greater than 0.1mm. The length of the pads of the two pads remains unchanged. It can meet the manufacturability requirement of PCB resistance welding single plate window design. In view of the above mentioned pads, pad and resistance welding design are optimized by the above scheme. The edge spacing of adjacent pads is greater than 0.2mm, and the edge spacing of resistance welding pads is greater than 0.1mm, which can meet the requirements of resistance welding bridge manufacturing process. After optimizing welding resistance design from PCB LAYOUT design and PCB engineering design, organize to resupply the same number of PCB, and complete mounting production according to the same process.