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Selecting housing materials for DDR4 memory module

Posted: 01 Apr 2014 ?? ?Print Version ?Bookmark and Share

Keywords:DDR4? synchronous dynamic random-access memory? SDRAM? PCB? Surface Mount Technology?

These days, the electronics industry is paying significant attention to green design. Besides looking for more efficient energy consumption, OEMs are increasingly restricting the use of halogens in flame retardants in plastics used for such items as connector housings.

The memory that supports next-generation Green Design must therefore meet the diverse demands of higher performance, increased power density, improved reliability, low power consumption and avoidance of substances of hazardous concern.

DDR4 is the latest type of synchronous dynamic random-access memory (SDRAM) now coming onto the market. It has higher clock frequencies and data transfer rates than DDR3, and is not backwards-compatible, due to, among other things, differences in signalling voltages and physical interface. The technology also puts higher demands on materials used to house and insulate the memory modules and sockets.

Various high performance thermoplastics may be used to injection mould socket housings for the new DDR4 generation of memory modules. Specifiers need to pay close attention to important differences in the performance of these materials, since they have critical effects on key parameters in the finished part, such as warpage, connector reliability, pin retention forces, and compatibility with the printed circuit board (PCB).

The common termination methods used for DDR4 are: Surface Mount Technology, SMT, which will be the main termination method in the future; Pin Through Hole, PTH (currently a mainstream technique, mostly applied in desktops); Pin-in Paste (used mainly in All-In-One PCs); and Press Fit (used in Telecoms). Depending on the OEM and specifics of the board design such as the number of PCB layers, any of the above DDR4 connector types can be applied.

Differences in the details of connector designs can directly trigger the choice of the housing material. For Pin-In-Paste and SMT designs, for example, very high temperature resistant plastics are a must because they have to withstand the reflow soldering step during assembly. Here, the connector is exposed to lead-free assembly temperatures in the range of 260-280C, with some spots reaching even higher temperatures. Connector housing materials must be able to withstand the peak temperature for around 10s. Furthermore, materials must show a proper balance of low moisture absorption and high surface tension. This avoids the formation of blisters which can appear during high IR-reflow processing temperatures.

DDR4 connector requirements
In the figure 1, the left star diagram shows the performance of materials tested for SMT DDR4 connectors intended for mounting onto the PCB using SMT. Designs require housing plastics of the highest temperature resistance and mechanical performance. Zero blistering during reflow soldering and excellent co-planarity are the two key qualifiers (Q). Insufficient performance in either of these two criteria excludes a material from use in DDR4. The diagram on the right shows data for PTH press fit designs. Somewhat less critical design parameters are the so called differentiators (D).

The design and type of connector determines the set of materials that can be considered for the housing. For the SMT/ULP (Ultra-Low Profile) connectors, these materials are: Liquid 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 body and the PCB, and the flow properties of the housing material, which are coupled to the stress built into the housing during injection moulding.

To achieve good co-planarity of the connector on the FR4 or latest halogen-free 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.

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