Lead Free Process Development with Thick Multilayer PCBA Density in Server Applications

Although the EU RoHS legislation restricts the use of lead in electronics equipment, many high-end multi layer server printed circuit board assemblies (PCBAs) continue to be built with lead under the server equipment exemption. As the industry prepares to comply with the RoHS directive without the use of exemptions, several studies and research efforts continue to focus on expanding the lead free assembly process capabilities for these types of high density, thick PCBAs. In this space, a different approach is required to mitigate the often encountered technical challenges of a lead free process such as solder hole fill on PTH barrels, copper dissolution effects and reflow thermal profiling. The additional thermal mass on thick heavy assemblies’ further narrows the process windows to achieve the temperature profiles required. On these assemblies the printed circuit board thickness can often extend to over 0.130 inches with layer counts in excess of often 18 or more, comprising of 1 ounce and 2 ounce copper planes. These circuit board stack ups introduce an increased level of PTH solder hole fill difficulty which cannot be addressed by normal process optimization techniques. Furthermore during SMT reflow, the additional thermal mass from the PCB and number of large BGA devices generally increase the overall heat required in producing an optimized reflow profile condition to meet the solder joint attributes, while at the same time be constrained by the thermally sensitive components. These challenges requires new approaches to achieve optimization which will need to be considered at the conceptual stage of board lay out and component selection. This paper examines the effects of varying surface finishes, temperature sensitive component limitations, process parameters and the resulting interactions that affect the solder attach attributes. The study includes characterization of solder joint attributes from a time zero perspective and extends to accelerated temperature cycling with post stress characterization. Additionally, the intent of this work is to document the need to identify design and process options for applications where density and PCBA functions extends beyond the commercially developed lead free solutions. Introduction Industry & EU regulators to date have not established a firm implementation date for elimination of lead (Pb) in solders of server products. Many of these products which contain complex PCBAs that are dense, thick and with number of heavy copper planes, are amongst the most challenging products to convert to a Pb-free assembly process. In this work, a current OEM product in production and a new product in development are utilized as test vehicles to evaluate the assembly characteristics from different PCB surface finishes and identify the process parameters which optimize the yields for the Pb-free assembly process. The study follows the defined experiment through the primary soldering processes of Surface Mount Technology (SMT) and wave soldering. Solder joint attributes on selected samples are further characterized at time zero and post thermal stress conditioning. Case Study 1: Pb-free process feasibility on a current OEM product An OEM product currently in mass production with eutectic tin-lead (SnPb) soldering process was chosen for the initial Pbfree process feasibility study. The main objective of the study was to identify any critical gaps in the current assembly process and product design, as well as to identify areas which need additional investigation through follow-up studies. Test Vehicle Description The test vehicle used in this study was a double sided SMT and single side wave assembly which is shown in Figure 1. The assembly process and the raw materials which were based on SnPb metallurgy was converted to Pb-free for the experiment. The printed circuit board was 405 mm x 405 mm x 3 mm, consisting of 8 power and ground plane layers and 10 signal layers. The plane layers included 2 layers with 2 oz. copper while the remaining 6 layers were 1 oz. copper. Table 1 lists some of the Ball Grid Array (BGA) and Plated through Hole (PTH) component technology that were included on this test vehicle. As originally published in the IPC APEX EXPO Proceedings.