A phone assembly line is often described as one line, but it is really four lines joined together. SMT runs at a different pace, sub-assembly runs at a different cleanliness level, final assembly is manual and test is a bottleneck. Treating them as one conveyor creates buffers in the wrong places.
Key takeaways
- Separate SMT, sub-assembly, final assembly and test with buffers sized to their own variation.
- ESD control is continuous, but the transport and the fixtures change between zones.
- Test is usually the bottleneck; plan its capacity first.
- Packing should be close to final test to avoid a second handling step.
Zone 1: SMT and board-level test
SMT lines run at their own speed and are usually linked by magazine loaders and unloaders, not by a single conveyor. The output of the SMT line should feed a buffer before board-level test. The buffer decouples the SMT line from the test line and allows the SMT line to run through a test stoppage.
The transport between SMT and test is usually a magazine conveyor or an AGV. Keep the boards in magazines to protect the components and to avoid handling damage.
Zone 2: sub-assembly
Sub-assembly includes display, camera, speaker and battery modules. These are small, sensitive and often have their own fixtures. A speed chain line with small carriers works well, but the carriers must be ESD-safe and the stations must have wrist strap points.
Sub-assembly is often manual and has more variation than SMT. The buffer between sub-assembly and final assembly should be sized to the variation, typically 15–30 minutes of output.
Zone 3: final assembly
Final assembly is the longest manual section. It is usually a speed chain line with fixture plates that hold the phone body. The stations are organised by takt, and the line is balanced to the slowest station.
The fixtures should locate the phone by the body, not by the screen, to avoid pressure on the display. The carrier should have a cover or a soft liner to protect the screen during transport.
| Zone | Transport | Typical buffer | ESD level |
|---|---|---|---|
| SMT | Magazine / conveyor | 30–60 min | EPA |
| Board test | Magazine / tray | 15–30 min | EPA |
| Sub-assembly | Speed chain, small carriers | 15–30 min | EPA |
| Final assembly | Speed chain, fixture plates | 10–20 min | EPA |
| Test | Conveyor or tray | 20–40 min | EPA |
| Packing | Belt or roller | 10–15 min | Clean area |
Zone 4: test and packing
Final test is usually the bottleneck because the test time is fixed by the product, not by the operator. If the test takes 60 seconds and the line takt is 12 seconds, you need five test stations in parallel, or a test cell with five fixtures. Plan the test capacity before the line layout.
Packing should be adjacent to test so that a passed unit goes directly into the box. A second transport step after test adds handling and the risk of mixing passed and failed units.
ESD continuity
ESD control must be continuous across the zones. The floor, the carriers, the workbenches and the wrist straps should all connect to the same earth reference. Measure resistance at the boundaries, where a carrier moves from one zone to another, because that is where the ground path is most likely to break.
Ionisers may be needed in areas where insulators cannot be eliminated, such as display films. Place them so that the airflow reaches the product, and maintain them on a schedule.
Frequently asked questions
Can SMT and final assembly share one conveyor?
Usually not. The transport, the carrier and the environment are different. A buffer and a transfer between the two zones is more practical.
How much buffer is needed before test?
Enough to cover the longest test stoppage, typically 20–40 minutes of line output. More buffer adds work-in-progress without improving flow.
What is the biggest risk in a phone line?
Handling damage to the display and ESD damage to the board. Both are reduced by fixtures and grounding, not by inspection.
Send your process flow and output target to our engineers through the contact page; we will help size the zones and the buffers. See the consumer electronics section for line examples.
