DFM Rules for Smartphones: An Analysis of Yield on Extremely Dense Assemblies

Handheld portable products such as smartphones are trending toward smaller form factors while simultaneously increasing in functionality to keep up with consumer demands. This is achieved in part by decreasing the size of components and increasing the density of the circuitry. These unique product needs drive different Design for Manufacturing (DFM) recommendations than those that are in use for larger products – while for larger products, reworkability is paramount, for handheld portable products, high first pass yields and fitting the required functionality into an appropriate form factor are of greater concern in many cases. This paper summarizes a new test vehicle designed to emulate a next-generation smartphone product. One of the goals of this project was to study the effect of pad design and component spacing on assembly yield. The test vehicle includes a representative range of component types including 01005 and 0201 discretes, 0.3mm pitch CSPs, Package-On-Package, QFNs, and RF shields. For selected components, different pad designs were included on the board, allowing a direct comparison of the various options and recommendations for the optimal pad designs. In addition, a range of component to component spacings were used on the board, ranging from spacings in common use in today‟s products to extremely aggressive spacings that push the limits of the PCB manufacturers. The test vehicles were inspected after assembly, and yields were determined for the various component to component spacings studied to determine what the limitations are and to update DFM rules specific to the needs of extremely dense handheld portable products. The results of the yield study will be presented along with the analysis of the implications for the DFM rules. Introduction It has long been considered good practice in the electronics industry to conduct a “Design for Manufacturability” (DFM) analysis of a new printed circuit assembly prior to releasing it to manufacturing. Ideally, DFM guidelines are used in the design of a product to ensure that few if any changes are required as a result of the review process. Companies base their DFM guidelines on years of experience in manufacturing various products, and these guidelines are critical in manufacturing high yielding products. DFM guidelines tend to be widely applicable to many types of product, but with the emergence of extremely dense handheld products such as smartphones, some limitations have become apparent in traditional DFM guidelines. In order to pack a maximum amount of functionality into an extremely small form factor, smartphone designers routinely have to break the types of spacing guidelines typical of most other electronic products. While there is a sound basis for the guidelines that have been established historically, they simply do not accommodate the needs of very dense assemblies. Many of the existing DFM guidelines are established with a view to enabling a cost effective, high yielding rework process for each component on the board, should it be necessary. The needs of hot air rework equipment are typically the limiting factor when defining DFM guidelines primarily due to features such as component to component spacings. This paper describes the results from a project designed in part to re-visit design for manufacturability guidelines with the needs of handheld portable products, particularly smartphones, in mind. This project focused on determining which pad designs gave the highest possible first pass assembly yield for the type of extremely miniaturized components that are in use or will soon be in use on smartphone type products, such as 01005 discretes and 0.3mm pitch CSPs. The minimum spacing between different component types was also studied with a view to extending existing DFM guidelines down to absolute minimum levels. Primary attach assembly yield was the main criteria used to determine acceptable limits – most handheld portable products are manufactured in very high volumes, so first pass yield is critical. For some products, rework may be carried out if the value of the assembly makes it worthwhile, while for other products, rework is not economically feasible, and defective assemblies are discarded. While the focus of the study is primarily on first pass assembly yield, the intent is to study rework as well to determine whether further limitations on factors such as component spacing are required if a viable rework process is necessary. Prior studies have been conducted in some of the areas of focus for this project. Considerable work has been done to determine the ideal pad dimensions for 01005 discretes and in developing a viable screening process for these extremely small components. Much of this development work to date has been performed on test vehicles that have only 01005 discrete components on them or where analysis has been focused purely on 01005 components on a test vehicle that is not specifically tailored to the needs of handheld portable products. 1,2 As originally published in the IPC APEX EXPO Conference Proceedings.