PCB welding design for PCBA manufacturing prospective research

With the rapid development of computer simulation, PCBA is also developing towards high density and high reliability. Although PCB and PCBA manufacturing technology has been greatly improved, conventional PCB welding process will not have a fatal impact on product manufacability. However, for devices with very small pin spacing, due to the unreasonable design of PCB welding through-hole and PCB anti-welding through-hole, SMT welding process will be more difficult and PCBA surface mount processing quality risk will be increased.

In view of the potential problems of manufacturability and reliability caused by the unreasonable design of PCB welding aid and anti-welding through-hole, combining with the actual technology level of PCB and PCBA, the device packaging optimization design can be adopted to avoid the manufacturability problems. Optimization design mainly starts from two aspects. First, PCB LAYOUT optimization design; Second, PCB engineering optimization design.

PCB LAYOUT design

Package design is based on IPC 7351 standard package library and the through hole size recommended in the device specification. In order to design quickly, the Layout engineer gives priority to enlarge and modify the design according to the recommended through-hole size. The length and width of the through-hole design for PCB welding assistance are increased by 0. 1mm. The length and width of the anti-welding through-hole are also increased by 0 on the basis of the through-hole. 1 mm.

PCB engineering design

Conventional PCB anti-welding process requires the cover of the hole edge 0. 05mm, the middle resistance welding bridge of two through hole anti-welding is greater than 0. 1mm, as shown in figure 2 (2). In the stage of PCB engineering design, when the size of anti-welding through-hole cannot be optimized and the middle resistance bridge of two through-hole is less than 0. 1mm, PCB engineering adopts group through-hole window design.

PCB LAYOUT design requirements

When the distance between the edges of the two through holes is greater than 0. The through hole above 2mm should be designed according to the conventional through hole. When the distance between the edges of the two through holes is less than 0. At 2mm, DFM optimization design is needed. DFM optimization design method can help optimize the size of welding and anti-welding through hole. Ensure that the solder flux in the welding process forms a minimum solder bridge isolation throughhole when PCB is manufactured.

PCB engineering design requirements

When the distance between the edge of the through hole is greater than 0. 2mm above the through hole, according to the conventional requirements of engineering design; When the distance between the edge of two through holes is less than 0. 2mm, DFM design is needed, engineering design DFM method includes welding resistance layer design optimization and welding aid layer cutting copper treatment; The size of copper cutting must refer to the device specification. The through hole of the welding aid layer after copper cutting should be within the size range of the recommended through hole design, and the PCB anti-welding design should be a single-through hole window design, that is, cover the welding bridge between the through holes. Ensure that in the PCBA manufacturing process, there is a welding resistance bridge between the two through holes for isolation, to avoid welding appearance quality problems and electrical performance reliability problems.

Resistance welding film can effectively prevent short connection of solder bridge in the process of welding and assembly. For PCB with high-density and fine spacing pins, PCBA processing plant cannot guarantee local welding quality of the product if there is isolation between the pins on the barrier welding bridge. As for the PCB with high-density and fine-spacing pins that are shielded by barrier welding, the PCBA manufacturing factory now determines that the PCB has poor incoming materials and will not be put into production. If the customer insists on going online, the PCBA manufacturing factory will not guarantee the welding quality of products in order to avoid quality risks. It is predicted that the welding quality problems in the manufacturing process of PCBA factory will be solved through negotiation.

PCB engineering design requirements

According to the conventional welding protection engineering design, the size of the through hole of single border welding is larger than 0. 05mm, otherwise there will be a risk of blocking flux covering the welding aid layer. As shown in figure 5, the width of single border welding is 0. 05mm, meet the welding production and processing requirements. But the gap between the two stop-holes is only 0. 05mm, not meet the minimum welding resistance bridge process requirements. The whole array of pins on the chip is directly designed as group through-hole window in engineering design.

Actual welding effect

According to the engineering design requirements after the plate, and complete SMT patch. Through the functional test, the chip welding defect rate is more than 50%; After the temperature cycle experiment again, more than 5% defective rate can be selected. The first choice is to conduct appearance analysis of the device (20x magnifier), and find that there are tin slag and welding residue between adjacent pins of the chip; Secondly, the failure of the product analysis, found that the failure chip pin short circuit burned out.

Optimization scheme

PCB LAYOUT design optimization

According to IPC 7351 standard package library, the pass hole design is 1. 2 mm * 0. 3mm, welded through hole design 1. 3 * 0. 4mm, adjacent through hole center spacing 0. 65mm remains the same. Through the above design, single border welding 0. The size of 05mm meets the requirements of PCB processing technology, and the distance between adjacent welding edge is 0. The size of 25mm meets the requirements of stop-welding bridge technology. Increasing the redundancy design of stop-welding bridge can greatly reduce the risk of welding quality and thus improve the reliability of the product.

PCB engineering design optimization

The width of the through-hole is cupped and the size of the through-hole is adjusted. Ensure that the edge of the device's through hole is greater than 0. 2mm, more than 0 between the edge of the through hole. 1mm, the length of the through hole for welding aid and anti-welding remains unchanged. Meet the manufacturability requirements of PCB solderproof single through-hole window design.

Design verification

For the through-hole mentioned above, the through-hole and anti-welding design were optimized through the above scheme, and the space between the edges of the adjacent through-hole was greater than 0. 2mm, anti-welding through hole edge spacing is greater than 0. 1mm, the size can meet the needs of welding resistance bridge manufacturing process