Rework methods for stacked PCB SMT assemblies.

摘要

The basic trend of the electronics packaging industry is a movement to smaller more compact assemblies. Devices get smaller and package densities increase. However, there are limits as to how small and dense things can become before problems arise. Also, new ideas arise as to how to utilize the space on a PCB. Without any more room to add new components, or the technology to decrease the size, a common idea is to build upwards. Just as apartments can be stacked one on top of each other in New York City, components can be stacked on top of each other on a PCB. Though an advantage that a PCB has over a city is that, in certain situations, it is possible to stack PCBs themselves on top of each other, instead of trying to stack the components.;When working with stacked components or stacked PCBs, there are two general concerns during the production of the assembly; (1) added reflow cycles, and (2) added heat. In order to build a stacked component, at least one extra reflow cycle is required for the stacking. This same concept applies when stacking PCBs. It is generally accepted that adding extra thermal cycles can adversely affect the reliability of the product. Reflow temperatures must increase due to the additional density of doubling, tripling or quadrupling the component size, or adding extra PCB layers. With the recent change to lead-free solder pastes, these temperatures are already pushing the limit of heat tolerance for the components. Extra care must to be taken to remain within the acceptable windows for producing stacked assemblies. These windows close even tighter if a stacked assembly needs to be reworked.;With the growing cost of materials and the complexity of assemblies, especially with stacked components or PCBs, scrapping units is costly. Thus, a reliable and repeatable rework process can be very valuable. The challenge is to develop this process while avoiding the excess heat and multiple reflow issues. The goal of this thesis is to develop a tactical rework process for reworking a triple stacked PCB assembly.;There is already a rework method in place for this particular assembly, but it tends to use a "brute force" method of heating. Also, according to the operators, the success rate of this method is below fifty percent. The current method utilizes a rework station using infrared radiation. The profile appears to melt every joint in the entire assembly to perform the rework. Though the method does work, a more strategic approach may increase the rework success rate. At the very least, because the new process is using a different rework station, the rework capacity on this very important product will be increased by this new method.;A method using hot air convection was successfully implemented to rework the three-stack assembly. Because of the high copper density in the PCB layers it was not possible, with the available equipment, to keep the joints on the top PCB layer from reflowing, but this was still better than reflowing the entire assembly. The first 4 assemblies reworked had some issues, but after the final adjustment of the rework process 3 out of 3 assemblies passed system test after rework. This process also helped find issues with the data retention test part of the functional test procedure.