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Fig. 2: HDT-A (1.8MPa) of various polymers. The temperature range highlighted in yellow is required in order to enable low warpage and to avoid collapse of the side walls of the connector. well below the 0.1mm specification. Moreover, both polyamides do not show any flip in the direction of warpage. A key indicator of warpage is the Heat Distortion Temperature, or HDT, of the used housing material. HDT defines the temperature where the polymer becomes soft and starts to deform under a specific load. Figure 2 shows the HDT-A (1.8MPa load) of various materials which have been tested for DDR4 connectors. The yellow bar indicates the range of HDT values that is border line for good warpage performance during the assembly process of connector assembly. Materials with an HDT below the bar cannot be used for connectors attached to the PCB with reflow soldering. Materials with an HDT inside the yellow bar are likely to require significant design and manufacturing efforts (such as mechanical fixation by clamps during assembly) in order for them to be used. Materials with an HDT above the yellow bar show low warpage even without any fixation to the board. Only PA46 and PA4T show high enough HDTs to ensure high reliability during reflow soldering assembly. HDT also has a very important role on the reliability of a DDR4 connector. An HDT that is too low could lead to the side walls of the connector collapsing slightly during assembly on the PCB. This collapse will increase the required memory module insertion and removal forces, hence increasing the cycle time. During insertion or removal of the socket, in the worst CIRCUIT PROTECTION & ENCLOSURES case, cracks could occur at the thin parts of the connector. Pin retention force Pin retention force (or “contact retention to housing” force) holds a terminal in a housing cavity. This prevents terminal back-out, or the terminal coming loose. Typically, locking devices called tangs secure the terminal against the housing walls using spring-like pressure. The “contact retention to housing” specification describes the force required to remove a properly seated terminal. The reduction of pitch size from 1mm in DDR3 to 0.85mm in DDR4 increases the challenge for pin retention forces (DDR4 requires at least 0.3kgf/pin). Since memory connectors are specified for approximately 25 insertion/removal cycles, a high pin retention force is essential for the quality and reliability of the connector and entire board during its use in the field. Upon removal of the memory module, the connector housings must remain firmly attached to the pins. Figure 3 shows the outstanding performance of PA4T for ULP and SMT designs, providing the highest possible pin retention forces before and after soldering. For the pin retention forces of PTH connectors, PA46 has been found best in class, keeping the required 0.3kgf/pin also after soldering. Other materials such as PPAs (PA10T, PA6T/66) or PA66 may provide sufficient force during connector assembly, but show a strong decay below the specified 0.3kgf/pin after soldering. Conclusions The challenging design of DDR4 and the various changes from previous DDR3 technology have significantly increased the application requirement for mechanical strength, pin retention force and flow. Due to its outstanding combination of flow and mechanics, PA46 has been found to be the most suitable material for PTH and Press Fit designs. PA4T, with its approximately 25°C higher melting point combined with a higher surface tension and a lower moisture absorption, is considered the material of choice for SMT and ULP designs. Warpage in connectors is increasing in importance with the growing trend towards SMT, ULP and VLP designs, the growing number of DDR sockets per server board, and the risk that warpage in a connector could even bend an entire PCB. Both PA46 and PA4T show outstanding performance in co-planarity. Warpage combined with insufficient mechanical strength disqualify LCPs from the upcoming DDR4 technology. Legislation governing the use of hazardous materials, such as the European Union RoHS (Restriction of Hazardous Substances) Directive 2011/65/EU, is providing a further spur for the use of thermoplastics whose flame retardant properties do not depend on halogenated additives. PA46 and PA4T polyamides are available that contain neither halogens nor phosphorus-based flame retardants. They will ensure full future compatibility with customer requirements, avoiding the need for any further requalification at either the connector manufacturer or the OEM. Fig. 3: Pin retention forces of various connector housing materials before and after soldering exposure. Left: Materials for SMT and ULP design; Right: Materials for PTH and press fit design 42 Electronic Engineering Times Europe March 2014 www.electronics-eetimes.com


EETE MAR 2014
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