The Museum of HP Calculators
HP began designing a new CPU called Saturn and series of calculators to follow the HP-41C but these designs became quite complex so they designed another series of midrange calculators based on improved HP-41C technology as a short-term measure. The CPU for these new machines included 61Kbits of ROM, 2.2Kbits of RAM, an LCD driver and a low-battery detector on-chip. To improve on previous designs, HP developed a new CMOSC process with design goals including:
Previous HPs had a flexible sheet under the keyboard to protect the electronics from minor spills and dirt. The 10C series took this further by having a single large sheet of film under the keyboard that wrapped around the electronics and overlapped at the back. This sheet consisted of three bonded layers: the outer layers were nonconductive, but the center layer was conductive. There was also an inner conductive film surrounding the main circuit board. These films gave the CMOSC chips a high degree of electrostatic protection.
Picture of an opened HP-15C (~16K)
Picture of an opened HP-15C with outer and inner films opened (~74K)
The majority of the calculator body housed only the keyboard. All of the electronics where built onto a small flexible circuit film that was bonded to the back of the LCD. A plastic fastener held the end of the circuit film to the keyboard. The entire LCD/chip module sat on foam rubber pads (small white squares seen in the picture above) to protect it when the calculator was dropped.
LCD/Flexible circuit module chip-side (~37K)
LCD/Flexible circuit module LCD-side (~11K)
A rubber sheet with a stainless steel snap disk layer attached to the lower side sat between the protective film and the rigid keyboard circuit. The disks connected the outer circle of the circuit board to the center point when the keys were pressed. The top of the calculator was heatstaked to the keyboard circuit board in 47 places to produce a rigid and consistent keyboard feel.
Picture of snap disks on rubber sheet (~19K)
Top side of keypad circuit board (~25K)
The bottom half was pressed into the top half along slanted surfaces to make a tight fit and attached with four screws which were then covered with the rubber feet to produce a finished appearance. The result was a very small, solid and durable calculator.