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Crystal Plastic (LCP); polyphthalamide (PPA) PA10T, polyamide PA4T and LCP/polyphenylensulfide (PPS) blends; for the PTH/ Press fit design, these are the polyamides PA4T, PA46 and PA66 as well as PPAs. Effects of CLTE, flow and HDT on coplanarity Warpage in a connector happens when the connector loses its co-planarity before or while being soldered onto a PCB. Such warpage is a complex phenomenon, driven by various parameters such as the heat distortion temperature (HDT) of the material used for the connector housings, a difference in the coefficient of linear thermal expansion (CLTE) between the plastic width and height, and the higher pin count of DDR4 connectors. After molding, there is little difference in warpage between connectors made in LCP and connectors made in PA4T and PA46. After assembly to the PCBs, however, LCP housings show a significant warpage, often with a change (or ‘flip’) in the direction of the warpage, which makes any prediction and warpage correction in the design virtually impossible. Even blends of LCPs with PPS, in which the higher stiffness of the PPS leads to some improvement in warpage, and which have been used in DDR3, do not meet the required co-planarity levels in DDR4. When the connectors are made in PA4T or PA46, however, the warpage after assembly is significantly lower and Temperature Variable Attenuators TVAs from the recognized leader in high reliability resistive components offer: • Case size 0.150” x 0.125” x 0.018” • Choice of three temperature coefficient of attenuation (TCA) values: -0.003, -0.007, -0.009 • Attenuation values from 1-10 dB • Planar design with solderable or wire bondable terminations • Lower signal distortion, phase change and intermodulation compared with active circuit temperature compensation When the mission is critical, choose State of the Art. State of the Art, Inc. ResisTive PRoduCTs Made in the USA. body and the PCB, and the flow properties of the housing material, which are coupled to the stress built into the housing during injection molding. To achieve good co-planarity of the connector on the FR4 or latest halogenfree PCBs, there has to be a close match in CLTE between the board and the connector housing material. In addition, a combination of high stiffness and high heat distortion temperature (HDT) under load is required to ensure low warpage after reflow soldering. The CLTE mismatch between the FR4 board and PA4T is lower than between FR4 and LCP. The difference is even more noticeable when mismatches with the new generation of halogen-free (HF) PCBs are considered. The CLTE of an HF PCB is still closer to that of PA4T, and yet further away from LCP. In producing good quality DDR4 connectors while keeping costs to OEMs affordable, manufacturers look for housing materials that have the highest possible flow while meeting other key design requirements such as mechanical properties or colorability. Use of a high flow material enables the use of injection molds with a high number of cavities, increasing productivity. At the same time, use of a high flow material leads to less incorporated stress in the housing. If a housing has stress built into it, that stress will cause the housing to warp when it is exposed to high temperatures during connector assembly. As a result, it is possible that signal pins at both ends of the connector will lose their electrical contact to the PCB. Traditionally, when a molder or connector manufacturer is looking for high flowing materials, LCPs have often been the material of choice. From a simple flow perspective, LCPs show the best performance, followed by PA46 and PA4T. But while LCPs have done an acceptable job in connectors up until the DDR3 generation, from DDR4 onwards all LCPs are failing in warpage. The reason is the significantly higher design complexity, the thinner walls, lower www.electronics-eetimes.com Electronic Engineering Times Europe March 2014 41


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