In our quest to create the best fertility charting system in the world, last week Kati and I visited a thermometer factory in Shenzhen, China to see how digital thermometers are made. We were pleased to see a clean and relaxed working environment inside this factory, and the workers seemed in good spirits. The only complaint I have is that high-pressure air was being used without adequate ear protection, so if we did business with this factory, we would bring that up!
This factory makes over 1 million thermometers per year and we learned tons! So here it is ladies and gentlemen – how an electronic thermometer is made from soup to nuts!
First, the factory buys the following components from other vendors:
- Empty stainless steel cans for the thermometer tips
- Special conductive medical grade epoxy
- ABS (Thermoplastic) and Thermoplastic Rubber (TPR) beads for injection molding the thermometer body
- Un-populated Printed Circuit Boards
- The thermometer Integrated Circuit/Micro-Controller Unit
- The screen and backlight
- The battery
- The little spring that holds the battery in place
- Thermistors (temperature sensitive resistors) from Taiwan
- Sheets of foam that are stamp-cut into little circles and squares (read on to see what these are for)
- Buzzers (to make the thermometer beep!)
Now that all the needed components are at the factory, here are the next set of processes that need to happen before final assembly:
- The thermometer bodies are injection molded on site out of ABS. They are then over-molded in a separate injection molding machine with TPR (a flexible polymer) to make the flexible tip. Next they are screen printed with the logo (if any) and taken to the assembly line.
- The thermometer tips (stainless steel cans) arrive empty, and a worker squirts a tiny bit of conductive epoxy into each one. The epoxy starts to harden as soon as it it mixed and becomes very hard within 6 hours. So I imagine they pre-fill a batch of tips twice per day. The pre-filled tips are then taken to the production line.
- The screen covers (lens) are molded on-site and then have a screen printed frame applied to them and are taken to the assembly line.
- The PCB’s (Printed Circuit Boards) arrive from the PCB factory and are populated with a pick-and-place machine, and soldered using an oven. Next the IC/MCU (Chip containing the thermometer program) is placed and wired using another machine. The placement and wiring of the MCU is checked, and the completed circuit boards are function-tested on a testing rig before the ICU is covered with black epoxy to protect it
These completed assemblies are taken to the assembly line for final assembly.
Ok now all the components are ready for assembly on the assembly line. Here are the processes we witnessed on the assembly line:
1. The thermistors arrive bare with just the leads on them. A worker picks up each one with a pair of tweezers and twists the leads coming out of the thermistor into a little coil. The next worker picks up a tiny piece of foam with tweezers and inserts it into the coil formed by the thermistor leads. This is so the next worker can stuff the thermistor-and-foam ball into the pre-filled thermometer tips and ensure the thermistor will end up at the tip of the can and embedded in the conductive epoxy.
2. The next worker feeds the thermistor leads through the end of the injection-molded thermometer bodies. And the next worker squirts medical grade epoxy into the thermometer tips to seal in the thermistor-and-foam-ball, and then inserts the tip onto the end of the body to complete the probe assembly. The next worker cleans of any excess epoxy.
3. Next they insert the buzzer with a little bit of glue, and fit a tiny injection-molded flexible piece onto the top button to complete the button assembly before gluing in the screen cover, and inserting the screen and backlight into the body.
4. Next they insert the completed PCB on-top of the screen assembly and solder the buzzer and thermistor leads onto the board. Contacts for the screen and button are closed automatically when the PCB is inserted into the body.
5. At the next stage they solder the little spring onto the board to press against the battery, and glue the back part of the thermometer body to the front part to seal in the screen cover, screen, backlight, and PCB.
6. Finally they insert the battery, and screw on the battery cover. The assembly is now done and a worker gives each thermometer a visual inspection and functional test before it goes to QA.
The first QA step is to place the thermometers into a water bath. To calibrate the water bath, they use a special reference thermometer that is calibrated to +/- 0.01C against a large mercury thermometer once per month (this thermometer is little and pink in the photo). Then a batch of completed thermometers are inserted into the water bath and a timer is started. Within 10 seconds they must all register the temperature of the water bath within +/-0.1F (in this case 98.7+/-0.1F) to pass the QA step. We asked them about BBT thermometers and they said they would have to run a QA process at +/- 0.05C which would require a more accurate water bath and different components within the thermometer to get to this level of accuracy.
The thermometers that pass are then given a wipe down, visual inspection, and sent to be packaged. At the packaging stage there is a QA step that pulls product from the line every hour and tests it. If problems are found they have to test all the product from the last hour to see what is going wrong. They told us this is required to keep their FDA certification.
They also told us that the FDA shows up around once every 4 years and inspects them to make sure all the required Quality Assurance processes are in place.
And that’s how a digital thermometer is made in China. We were amazed at the amount of manual labor that goes into the process.
So next time you’re at the store looking for a digital thermometer you’ll know how they are made. Please post any questions in the comments and I’ll answer them! – Will
P.S. Looking for a basal thermometer? Wink, our bluetooth basal thermometer, is fast, highly accurate, fits comfortably in your mouth, and automatically syncs your temperatures with the Kindara app.