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Lead-free Development in Server Applications
Materials Tech |
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Authored By:L. G. Pymento, W.T. Davis, IBM Ben Kim, Surangkana Umpo, Celestica TranscriptAlthough RoHS legislation restricts the use of lead in electronics equipment, many high-end multi layer server printed circuit board assemblies 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. This paper examines the effects of varying surface finishes, temperature sensitive component limitations, process parameters and the resulting interactions that affect 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. SummaryAlthough 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. ConclusionsFrom the above case studies, we note that for thick PCBAs used in server applications, the approach for ensuring product reliability requires a combination of design elements and process development. Each card assembly will have different attributes and technology elements, so it will require customization of process parameters and chemistries to achieve the optimum solution. Temperature sensitive parts at SMT require active monitoring. Through profile iterations and assembly techniques, a profile which satisfies both the requirements of the component supplier and Pb-free process temperatures may be developed for many temperature sensitive components. However, when all attempts to meet these requirements have been exhausted, alternate components or assembly processes which do not violate the temperature profile requirements must be identified. Achieving the PTH barrel fill requirements on thick PCBAs is one of the technical hurdles for a full Pb-free product transition and this should be a key focal point during early product design. It was demonstrated that design factors can create conditions on a large thick PCBA which results in low peak temperature in the barrel, resulting in poor barrel fill. Contact time at wave soldering has been identified as an important variable and those working on Pb-free assemblies are encouraged to perform process trials that include variation on wave direction to evaluate temperature conditions and PTH hole fill. Additionally, there exist in the market today newer technology and equipment, as well as alternative lead free alloys, which are worth exploring, as supplements to the ultimate, holistic wave soldering techniques for the often complex design of server PCBAs. The authors and companies involved in this study continue to investigate these through additional collaborative efforts. Through early involvement with designers, selection of ideal components and optimized design will allow for processes to be developed that optimizes the temperature limits and solder attach properties. It is also suggested that if test vehicles are chosen for experimentation, they should closely represent the actual product so valid inference can be gained from the study. Initially Published in the IPC Proceedings |
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