Issue link: https://iconnect007.uberflip.com/i/1499197
78 PCB007 MAGAZINE I MAY 2023 tape. e process takes time and is prone to human error, which creates further potential for pan- els to get damaged. The Leaderless Approach If leaders are not desired, the next solution must come from the conveyor design. Frequently, the solution is to provide greater con- veyor density—meaning fewer gaps in the conveyor and less like- lihood for an error to occur. e main down- side is that the etch rate on the bottom side can be inhibited by the conveyor since it is harder for spray to reach the panel. Surface tension is another issue that comes with leaderless transport of thin materials. Oen in wet processing equipment, rollers ensure the chemicals in each wet processing section are contained within their modules and don't leak into other modules. Sometimes with thin material, the liquid on the panel cre- ates enough surface tension to cause the panel to stick to the rollers and wrap around it. To avoid this, use either lightweight rollers or roll- ers with reduced surface area. Conclusion If you wish to move away from using the leader method, know that wet processing of thin materials and substrates is a complex pro- cess that requires specialized equipment. While specialized equipment can aid in the transpor- tation of these materials, it is not enough to guarantee error-free wet processing. Although there are some restrictions, I foresee leaderless thin material transport soon becoming an issue of the past. PCB007 Christopher Bonsell is a chemical process engineer at Chemcut. To read past columns or contact Bonsell, click here. the board. at is why wet-processing con- veyors are designed this way today (Figure 1). Gaps are required to conduct normal wet processing, which creates some difficulties. Any gap in the conveyor provides a chance for a corner to dip downward. e thinner the material, the more likely this will happen, especially when a liquid is sprayed onto the top of it. When this happens, it can cause that edge or corner to fold or "crinkle." is hap- pens because of the uneven movement among all parts of the panel. Once a corner drops down into a gap, that part of the board loses velocity in the forward direction, while the rest of the board keeps a consistent forward velocity as the conveyor moves along. The Leader Method Although these complications can be a sig- nificant obstacle, they do not stop flex PCB manufacturers from using wet processes. e leader method has been commonly used for many years. With it, a rigid board is taped (using chemically resistant tape) to the lead- ing edge of the thin material that needs to undergo a wet process. is ensures that the leading edges of the flexible circuit do not drop into a gap and thus form an error. Since this drooping issue is the main problem of thin material transport, the method tends to provide consistent results. Despite this, most manufacturers of flexible circuits want to get away from using leaders because of the man- ual labor needed to apply and remove the Figure 1: Standard wet processing conveyor.