The shape of an assembly line
Once chips and other components exist, they have to be attached to printed circuit boards. A printed circuit board, or PCB, is a flat panel of insulating material with copper traces that connect components. The most common modern method is surface mount technology, where components sit on pads on the board’s surface instead of having leads pushed through holes.
A typical surface mount line is a series of machines joined by short conveyors, so boards flow from one station to the next. Understanding the order makes the whole thing easier to follow: print, place, solder, inspect, test.
Loading and printing
Bare boards are loaded from a magazine or a stack into the line. The first process machine is the stencil printer. A thin metal stencil with openings that match the board’s pads is laid over the board, and a squeegee pushes solder paste through the openings. Solder paste is a mixture of tiny metal particles and flux, a chemical that helps solder wet the metal.
Printing is a small step with large consequences. Too much or too little paste at a pad is a common source of soldering defects, so many lines add an inspection machine right after the printer to check paste deposits before components go on. Paste is also sensitive to storage and handling, and manufacturers’ instructions for temperature and working time are usually followed carefully.
Placement machines
Next come the pick-and-place machines, the machines most people picture when they think about electronics assembly. Components arrive on tapes wound on reels, or in trays, and a head with small vacuum nozzles picks each part, checks its position with a camera and sets it down on the paste.
Lines often use more than one kind of placement machine. High-speed machines handle small, simple parts in large numbers. More flexible machines handle large, odd-shaped or fine-pitch parts, where very precise positioning is needed. Many of these machines are, at heart, fast motion control systems, with accurate positioning axes and vision. The nozzles, feeders and cameras need regular cleaning and adjustment to keep placement reliable.
Parts too awkward for a machine, or low-volume items, may be placed by hand or by simple assisted stations.
Soldering
Placed components are only held by sticky paste at this stage. The board travels through a reflow oven, a long oven with several heating zones. The temperature rises along a planned profile, melting the paste so the solder flows and then solidifies as the board cools. Getting the profile right matters because boards and components can be damaged by too much heat, while too little heat gives poor joints. Many lines measure the profile with a test board that travels through the oven carrying temperature sensors. Oven behavior depends on good temperature control systems, and the oven’s heat patterns and airflow are a practical example of the same principles used in curing ovens elsewhere in industry.
Through-hole components, such as large connectors, may be soldered separately. Wave soldering passes the underside of a board over a flowing wave of molten solder. Selective soldering machines treat only chosen spots, which is gentler on nearby parts. Some work is still done by hand with a soldering iron, especially for repairs and small runs.
Cleaning and coating
Some products are cleaned after soldering to remove flux residue. Others use a flux that is left on the board by design. Boards used in harsh conditions may receive a protective conformal coating, applied by spray or dipping and then cured. These steps depend on the end use of the product.
Inspection
Defects on an assembly line are cheaper to find early than late, so inspection appears in several places.
- Automated optical inspection. Cameras compare each board to a reference, looking for missing, shifted or wrong components and visible solder problems.
- X-ray inspection. Some joints, such as those under a component, cannot be seen from above. X-ray equipment can reveal hidden voids, bridges or poor connections.
- Manual review. People still check flagged boards and handle judgment calls.
These tools are the electronics version of inspection sensors and feed the broader practice of manufacturing quality control. Inspection data is often traced back to specific machines so the cause of a defect can be found and fixed.
Test equipment
Inspection checks appearance. Test checks behavior. In-circuit testers use a bed of probes to contact test points and measure parts on the board. Flying probe testers use a few moving probes, which suits small batches because no custom fixture is needed. Functional testers power the finished board and check that it does what it is meant to do. Some products also go through burn-in or environmental testing, where they run in warm conditions to expose early failures.
Static electricity and the factory floor
Electronic parts can be damaged by static discharge, sometimes without any visible sign. Assembly areas typically use floor and bench materials that drain charge, wrist straps, and careful grounding systems. Humidity is often controlled partly for this reason. Components may also be sensitive to moisture and are sometimes stored in sealed bags or dry cabinets before use.
Where robots and software come in
Lines are linked by software that tracks each board by a barcode or code, records which machines touched it and stores test results. Robots are used for loading, unloading and moving carts, and for tasks like screw driving or final assembly, a field closely related to robotic assembly. Products with strict records requirements, such as those covered in medical device production, make this traceability especially valuable.
This is a general description. Real lines differ widely by product and volume, and operating or servicing any machine should follow the maker’s manual and the plant’s safety rules.