Mechanical design of the HP 48SX memory card and memory card connector

Hewlett-Packard Journal, June, 1991 by M. Jack Muranami

The mechanical implementation of the HP 48SX memory card and memory card connector addresses the goal of design and manufacturing leverage. For the HP 48SX, R&D was faced with the challenge of incorporating plug-in modules into a calculator for the first time since the HP 71 B (introduced in February 1984). The product required either development of a custom module such as those for the HP 41C, HP 75C, and HP 71B, or the purchase of an OEM module. The decision was made to source and qualify a module produced by an outside company to minimize HP design effort. As the search progressed, it became apparent that credit-card-size digital memory modules known as memory cards were becoming a loosely defined standard among electronic equipment manufacturers. By choosing this memory card form factor, we were able to use the expertise and resources of numerous manufacturers that were already in high-volume production. Other objectives, such as second-sourcing options and possible adherence to a future industry standard, could also be addressed.

Selection

The selection of one memory card manufacturer from a list of a dozen was assisted by other HP divisions. During initial supplier research, some of the manufacturers commented that the Rohnert Park, Spokane, Vancouver, Boise, San Diego, and McMinnville sites were all investigating memory cards. Information about card contact plating and connector soldering was obtained from Rohnert Park, while Spokane provided comparative data regarding reliability and price. After reviewing design specifications, reliability data, and quotations, R&D managers visited four manufacturers, One was selected to produce memory cards for the HP 48SX. The chosen design offers electrostatic discharge protection, mechanical strength, and long insertion life.

All of the suppliers offered memory card connectors designed for wave soldering and for strain relief using screws. To meet the goal of manufacturing leverage, the connector for the HP 48SX had to be surface mounted, be suitable for infrared reflow soldering, and be strain relieved using heatstakes. Taking into consideration the added design requirements of low height and accommodation of two memory cards, the need to tool a custom connector became apparent. The selected card manufacturer gave permission to start engineering discussion directly with the connector company that produced the standard connector designed for use with the card. By developing the new connector with the existing supplier, we gained access to data regarding design criteria, materials, and reliability testing without extensive investigation.

Card Design

Basically, the memory card is an expansion board packaged to provide protection from environmental factors. it consists of a printed circuit board, a frame, and steel panels. The card design chosen uses an edge-type contact in which 40 gold-plated fingers on a 0.01 7-inch-thick FR-4 printed circuit board are located on a 0.050-inch pitch. To maintain thinness, surface mount discrete devices and chip-on-board integrated circuits are mounted on the printed circuit board. For rigidity, the card employs a sandwich construction. The printed circuit board is laminated to an injection-molded PBT frame on the noncontact side. Two 0.010-inch full hard stainless steel panels are laminated to the frame on one side and to the printed circuit board on the other. A 0.058-inch-diameter compression coil spring mounted through the printed circuit board electrically ties the two panels together to provide ESD protection. The contact-side panel traps a 0.005inch-thick stainless steel shutter and two 0.039-inch-diameter tension coil springs. The springs keep the shutter closed over the contact fingers when the card is unplugged, providing protection from ESD and contamination. When the card is inserted into a connector, two formed tabs that are part of the shutter are pushed back to expose the contacts.

While the ROM card and SRAM card are similar in construction, the SRAM card incorporates several additional parts. Two 0.004inch-thick phosphor bronze flat contacts for a CR2016 lithium coin battery are heatstaked to the frame and make pressure contact to the printed circuit board. An injection-molded battery tray slides between the two panels and allows installation and replacement of the coin cell. A write protect switch consisting of a molded button and a flat spring is also trapped between the panels. The position of the tray and switch on the top edge of the card allows access while the SRAM card is plugged into the HP 48SX. This is necessary because the SRAM card must be powered by the calculator during battery replacement for memory retention.

The dimensions of the card are identical to those of a credit card at 3.370 inches by 2.126 inches. Overall thickness is 0.094 inch, which allows the HP 48SX to accommodate two stacked cards in the same thickness as one HP 41 C module. The SRAM card weighs less than nine tenths of an ounce including the battery.