We outlined what can be checked in an EMS factory before, but we didn’t go into the quality control points of what is really special about them — the SMT (Surface Mount Technology) machines.
In China, I have visited all types of PCB assembly factories — from the worst to the best in class. Many do not apply the right QC points in an adequate manner and do not maintain the equipment preventively — they end up with high defect rates and unhappy customers.
All SMT flowlines should have a number of different test stations either built into the line itself or as peripheral to the line where samples can be taken from the line for testing.
The common test stations that are built into a typical SMT flowline would be:
- Bare board testing – this can be achieved with an Image Subtraction Method.
- Solder Paste inspection – options include high speed camera through 3D scanning.
- Pick and Place – Visual System and scan to ensure nozzle has picked the correct component before placing it.
- Automated Optical Inspection (AOI) – a camera scans the PCBs for variety of surface feature defects such as scratches and stains, open circuits, short circuits, thinning of the solder as well as missing components, incorrect components, and incorrectly placed components.
- X-Ray Inspection – used to inspect features, which are typically hidden from view.
- Flying Probe – This has a simple fixture to hold the board and contact is made via a few probes that can move around the board and make contact as required. These are moved under software control so any board updates can be accommodated with changes to the software programme.
All the inspection processes listed above are designed to identify faults during production and if the operational process is set up correctly, identification of a fault would trigger a line stop that would prevent faulty products from further process steps in the flow line.
The fact that all the in-line inspection equipment helps prevent faults from an assembly point of view does not take away the fact that a functional test is still required in order to establish if the PCB actually functions and conforms to specification.
The functional tester should be designed in conjunction with the customer’s engineering department to ensure all test features and system integrations are included within both the physical design and the software driving the tester.
From the system integration aspect, elements such as user interface, security access, input methods, output processes, switch timing, operator safety and other customer specifics need to be included in the functional test rig.
Key Features of the Functional Test
- Identify errors in all functional aspects of a PCB
- Identify failure thresholds in a PCB when both upper and lower voltages are applied
- Simulates the operating environment for the PCB
- Verify speed, frequency, oscillations, and other on-board timing elements
- Verify all input and output features conform to specification
- Establish all communication features are operational
By the time the PCBs reaches the functional test stage, everything should have been inspected and checked from a physical point of view with all the in-line test equipment we first mentioned. This is where the PCB is checked to ensure all the best practices and optimization of processes that have been implemented throughout the production process has paid off and everything works together as functioning PCB.
The functional test is the final step in the manufacturing process where the finished PCBs are tested and the process acts as a Go or No-Go decision point on the production batch.
Obviously, a factory that catches errors early (through a sensible application of the quality gates we mentioned) will save on repair and on components. It is good for quality and goof for costs!