The Museum of HP Calculators
by Thomas B. Lindberg
THE PACKAGING OF THE HP-71B COMPUTER is heavily based on the proven designs of the HP Series 10 Calculators and the earlier HP-75 Computer.1 The styling follows a similar horizontal format, offering a block QWERTY keyboard with a numeric pad on the right side. Fig. 1 shows an exploded view of the HP-71B and identifies many of the components discussed below.
The HP-71B mainframe is formed by two subassemblies: the top case assembly and the bottom case assembly. The two subassemblies are electrically connected by a flexible circuit, which is reflow soldered in place using focused infrared light. As with the HP-75 Computer and the Series 10 Calculators, aluminum overlays in the case halves are used to provide parallel ground planes for effective shielding of the CMOS circuitry against electromagnetic interference and electrostatic discharge.2
The top case assembly consists of the keyboard and the liquid-crystal display. The CPU, display drivers, and clock circuitry are mounted in this subassembly. The bottom case assembly contains circuitry for handling both ac and battery power, as well as all ROM and RAM. Besides the battery compartment and piezoelectric beeper, the bottom case assembly provides four ROM/RAM module ports, a card reader port, and an HP-IL (Hewlett-Packard Interface Loop) interface module port. When installed, these optional modules are fully contained within the HP-71B's mainframe to maintain its compact silhouette.
The injection-molded keycaps are separated from the underlying key contacts by an elastomer membrane to enhance tactile feedback and to protect the underlying circuitry from dust. The key contacts for the keyboard are based on snap discs connected in a daisy-chain fashion. The display is mounted to the printed circuit board using elastomeric connectors and a rigid stainless-steel clip for mechanical shock protection, a method similar to that used on Series 10 Calculators.
The custom 4-bit chip set plays a significant role in the mechanical packaging. A typical 8-bit computer system would require at least 30 signal lines to connect the standard ICs required. With HP's custom chip set, eleven signals provide the same interconnection function, allowing trace routing and connections to be made with fewer lines. The high-pin-count CPU and display drivers use the same flatpack surface-mounted-device packages developed for the Series 10 Calculators. The low-pin-count RAM, ROM, card recorder, and HP-IL ICs use the hybrid packaging method developed for the HP-41C Handheld Computer.3
In the top case assembly, the display and key contacts cover a major portion of the printed circuit board, restricting the use of discrete components and leading to the application of surface-mounted devices. A total of eleven surface-mounted resistors, capacitors, and inductors plus four ICs packaged in 72-lead flatpacks are placed on this board. Placement is done by a robot using optical registration to improve location tolerances and reduce the need for manual touch-up. Silk-screened polymeric resistors are used for electrostatic discharge protection of the key contacts, an application where these components' loose resistance tolerances do not affect performance.
Twenty IC chips are mounted in the bottom case assembly to provide the HP-71B's built-in 16K RAM and 64K ROM. The memory density requirement in the available 6.1-cm-by-2.6-cm area exceeds the capabilities of dual inline and/or flatpack packages, and hence requires the use of hybrid packaging. Four ICs each are chip-on-board mounted on the same polyimide hybrid boards designed for use in the plug-in memory modules. These hybrid subassemblies are stacked on the motherboard, with their phosphor-bronze pins inserted like single in-line packages. Other lead components are also loaded before the entire motherboard is drag soldered.
The optional plug-in memory modules presented the challenge of how to stuff a maximum number of memory chips into a rugged package with a total volume less than 6 cubic centimeters. The same hybrid subassemblies used for the mainframe memory are employed with beryllium-copper spring contacts for the electrical connection to the mainframe. The hybrid subassembly (Fig. 2) with one to four memory chips--up to 4K bytes of RAM or 64K bytes of ROM per module--is encased in plastic by ultrasonic welding.
The optional 82400A Card Reader Module (Fig. 3) provides an inexpensive and highly portable mass memory for the HP-71B mainframe. Handpulled magnetic strips (cards) are the storage medium. A one-piece housing that also serves as the card guide allows alignment of the magnetic head without adjustment. Although the basic card reader design was originally developed for the HP-71B, the first version appeared in the HP-75 Computer.4 Both designs use the same head and magnetic cards with prerecorded timing tracks. Battery life is conserved since these tracks eliminate the need for the constant read/write speed previously provided by a motor. In addition, the card bit density of 315 flux reversals/cm is roughly double that of HP's earlier motor-driven readers. The packaging makes extensive use of surface-mounted components, which are placed by a robot and vapor-phase reflow soldered. The entire mass storage device with 20 discrete components, two hybrid-packaged ICs, magnetic head, and connectors occupies a volume less than 35 cubic centimeters.
The 82401A HP-IL Interface Module (Fig. 4) provides the interface between the HP-71B's 4-bit mainframe bus and HP's serial loop for interfacing portable devices. Hybrid packaging is necessary to achieve the high functional density. Two ICs are mounted chip-on-board along with eleven lead and surface-mounted components on a 3-cm by-3.5-cm printed circuit board. The circuit board assembly is enclosed by ultrasonically bonded plastic and a stainless-steel cover. The total module volume is 20 cubic centimeters.
With the exception of the silk-screened polymeric resistors, none of the design techniques or manufacturing processes are actually new to HP's Portable Computer Division. Instead, the packaging of the HP-71B represents a refinement of the existing technologies. For example, while vapor-phase soldering, snap-disc keyboards, and crimped liquid-crystal display assemblies have all been previously used at this HP Division, they have not all been used on the same assembly. It is the combination of these technologies that makes the HP-71B Computer the most powerful computing device for its size.
There are countless people who made the idea of the HP-71B become a reality. Special appreciation is directed to the following for their major contributions. The industrial design of the product was done by Ed Liljenwall. Russ Paglia, assisted by Marc Baldwin, did the initial mainframe design. The design was further developed by Allen Wright and Jerry Steiger. The memory module design was done by Allen Wright and developed by Jerry Steiger and Dave Smith. The HP-IL module design by Glenn Goldberg was refined by Larry King. The card reader was designed by Tom Hender. Thanks is extended to John Mitchell and his tooling group for their major support during the project.
1. L.S. Mason and G.G. Lutnesky, "Packaging a Portable Computer," Hewlett-Packard Journal, Vol. 34, no. 6, June 1983, p. 12.
2. G.J. May, "Electrostatic Discharge Protection for the HP-75," ibid, p. 14.
3. J.H. Fleming and R.N. Low, "High Density and Low Cost with Printed Circuit Hybrid Technology," Hewlett-Packard Journal, Vol. 31, no. 3, March 1980.
4. K.R. Hoecker, et al, "Handpulled Magnetic Card, Mass Storage System for a Portable Computer," Hewlett-Packard Journal, Vol. 34, no. 6, June 1983.