Issue link: https://iconnect007.uberflip.com/i/1479191
SEPTEMBER 2022 I PCB007 MAGAZINE 63 are fine. However, microvias typically fail during reflow and the resistance change can only be detected during the peak reflow temperature cycle. A microvia failure at peak temperature will cause an open, and plastic deformation in the microvia will create permanent damage at the target land microvia interface. What makes this type of failure so frustrating is that the open does not exist at ambient room tempera- ture so chasing down the type and cause of the failure is a lengthy process that can impact both engineering cycle time and overall product development costs. Actions taken to reduce the microvia fail- ures have included increasing the microvia diameter and reducing the aspect ratio. These actions and the new rules resulting from them haveing reduced fail- ures. The benefits of these rules are described below. • Increasing the microvia diameter increased the interface area. › A small increase of a microvia from 0.1 mm to 0.127 mm increases the surface area by 56%. › A wider surface area increased the survivability of a microvia if the same material expansion was exerted on the larger target pad interface. › Reducing the aspect ratio typically reduced the overall dielectric thickness. – This thickness reduction decreased the material expansion potential which improved the microvia survivability. › Stacked microvias required a lower aspect ratio than a single microvia for consistent 6X reflow passes. While these new rules have worked for most builds, failures have still occurred. Most of the fail- ures occurred with dielectrics that exceeded 0.15 mm/.006" or if high resin content prepreg was used. When failures did occur, the typical solution was to try and increase the microvia diameter, reduce the dielectric and rebuild the parts with the hope that the rebuild would pass. The Traditional Stackup and Material Selection Processes and How They Affect Microvia Reliability During the traditional stackup and material selec- tion processes, the importance of microvia reliabil- ity is often not considered. This is due, in part, to not clearly understanding the stackup and material selection process in general and, specifically, how microvia reliability factors into these processes and why it needs to be addressed so early on. Material selection for a stackup may be specifi- cally defined by the PCB designer. In this case, each ply of prepreg and each core construction is defined and a stackup is provided to a PCB fabricator. When a stackup is not provided by the PCB designer, the fabricator is allowed to make the material selection. In many cases, the fabrication drawing can simply state that lead-free FR-4 material is to be used by specifying IPC-4101/126. The simplest instructions are that there are no dielectric callouts on the fabri- cation print and a PCB fabricator can select any core thickness or prepreg glass style that will accommo- date the required overall thickness and, if required, impedance requirements. These basic generic instructions open the door to infinite stackup vari- ations. A PCB designer or the electrical engineer could make all the stackup decisions by specify- ing the resin system brand; the core and prepreg openings, and the core constructions and prepreg types. But these practices are standardized within the industry and are dependent upon the compa- ny's design practices as well as the experience level of the designer or engineer. PCB planners will select dielectric woven glass styles that are compatible with laser drilling. Another consideration for planners is to select resin rich pre- preg to fill internal plated layers that are found in HDI designs. Electrical engineers will select low loss and spread glass weave to improve the electrical performance of high-speed designs. Stackups will favor a high resin content and spread glass com- bination to meet fabrication and electrical perfor- mance needs. PCB007 To read the rest of this white paper, click here. Figure 1: This depicts the image of a resin-rich spread glass material. In this image, the white portion is resin and the woven glass is gray.