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Watch Crystal Soldering |
| There are two distinct standards of construction for parts that are cased in a metal cylinder. Which one is used will determine what the soldering condition should be for the part. The base is generally glass and this is a press fit into the can. Terminal leads are of Kovar with solder plating. |
| Parts built for operation over commercial temperature range, usually -10°C to +60°C, are expected to be hand soldered or wave soldered within a relatively short time. Typically, the solder temperature should be less than 230°C, the contact time should be less than 5 seconds and solder should not reach closer than 2 millimetres from the can. Storage temperature for these parts is -30°C to +70°C. If the can is to be secured to the printed circuit board, this should be done with a thermal conducting adhesive. |
| Parts built for industrial temperature or -40°C to +85°C will survive reflow or vapour phase soldering. With these methods of soldering, the peak temperature should not exceed 230°C and for no longer than 10 seconds maximum. The temperature of the vibrating element inside the can should not exceed 150°C. Storage temperature is –55°C to +125°C. The can of these parts may be soldered to the printed circuit board, subject to the foregoing conditions. |
| Plastic encapsulated watch crystals for surface mount contain cylinder parts in 1x5mm or 2x6mm cans. These can be either temperature grade. The legs exiting from the can are welded to an alloy lead frame and the complete assembly is injection moulded with a thermal insulating plastic, epoxy or silicon filler. |
| The high ESR of a 32.768KHz watch crystal when compared to higher frequency devices means that there can be low impedance paths due to flux or other surface contaminants which must be removed to ensure satisfactory operation of the circuit in which it is used. Washing of the printed circuit board may introduce low impedance paths either on the board, or within the device encapsulation itself, until both are thoroughly dried. Any conformal coating that is applied to the board after cleaning and drying should be high impedance and non-reactive. |
| The seal between glass plug and cylinder is currently made of lead (Pb). This seal will be superseded by a tin-copper alloy alternative when production processes are revised. The legs will also be tin-plated as an alternative to solder with a lead content. |
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