PCB007 Magazine

PCB007-July2019

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30 PCB007 MAGAZINE I JULY 2019 shipping conditions and user environments. Many electronic devices—such as network cards HDDs in the server system—tend to be- come loose, resulting in poor contact problems due to the severe shock from the transporta- tion and material handling processes. Differ- ent design variables—such as hard gold plat- ing thickness on the pin, motherboard locking mechanism, and damping plate—are experi- mented and reviewed in this article. A shock measuring device was used to monitor in real-time the acceleration, duration, and di- rection of shock in large stationary or moving systems in the transportation and transferring process. Poor contact issues happened on some mod- els of desktop, AIO, and server computer sys- tems. After removing the top cover of a comput- er system, some accessories—such as memory and NIC cards—were found to be partially dis- engaged from their normal interconnect posi- tions (Figure 1). An example of a contact inter- connect defect rate for a specific experimental test vehicle is shown in Table 1. In most of the cases, these contact problems may not be per- manent but can be quickly resolved by double insertion of the interconnect system. Although not the main topic of this study, another source of contact interconnect prob- lems [1] is coming from particles or fibers from raw material, manufacturing, or the user en- vironment can be observed from time to time in DIMM socket pins and circuit board contact pads. These foreign materials can create a bar- rier for proper contact between pad and socket (Figure 2). In one extreme case, soft white plas- tic particles were smeared on the contact sur- face, creating a risk of intermittent contact or open circuit (Figure 3). FTIR organic chemical analysis indicated that the fibers were rayon/ cellulose, which is a common material from various sources—such as cloth and gloves— which are difficult to clearly implicate in a fail- ure. However, the white particles are most like- ly polyethylene from plasticizer—a fatty acid that poses an interconnect concern (Table 2). To avoid the accumulation of fibers and par- ticles on contact pads, there are many changes required in environment control and manage- ment for sensitive interconnect devices, such Figure 1: Example of partial loose contact of a daughter card from a press-fit connector. Figure 2: Dust and fiber accumulated in DIMM slot (L) and particles found near DIMM contact pads (R). Table 1: Defect rate of a series of computer server systems.

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