Factory Assembly Flow


The image that comes to our mind when we hear factory is lines of workers sitting neatly along the assembly line piecing components together. However that is only a small part of the entire factory flow. The goal of manufacturing is high yield, hence lots of inspections and testings are placed along each stage to ensure quality. In this text, we will take a tour of a simplified factory flow for a embedded system electronics.


Before the factory starts cranking, there are few manual pretest that needs to be carried out ahead of time such as:

  • Components Orientation check
    • Commonly known as the pin 1 check
    • Pin1 is usually marked by a dot on the IC package

Stuffing options

    • Verify components from the released schematic are indeed populated on the board.

Factory Overview

Factory Test Flow

Block description

Material In house

    • refers to all the electrical and mechanical components that are needed for the assembly to start. Missing any of them prevents factory from starting.

Stencil Inspection

    • Checks elasticity of the stencil after long term usage. Replace the stencil if elasticity of the stainless stencil is reduced under certain limit.

Incoming Quality Check (IQC)

    • Mechanical parts dimension check (i.e tolerance)
    • Mechanical parts cosmetic check (i.e scratch)
    • Antenna specification check
    • Electrical module functional check

Solder Paste Dispenser

    • This machine applies solder paste onto the PCB board by using a squeegee that automatically spreads a glob of solder paste material over a stencil with PCB underneath.

Solder Paste Inspection Machine

    • This machine checks the solder paste fill ratio over the component pads on the PCB. If solder paste volume on pad is under certain recommended ratio, the stencil or solder paste material needs to be adjusted.
      • Note: Under fill solder paste over a component pad reduces the quality of solder joint compromising reliability.

SMT Pick and Place Machine

    • This is where a electronics components gets placed on the the PCB boards. A small suction tip picks up tiny electronic components from a pre-loaded tray (often comes in the form of reel packaging), and place the component into designated area on the PCB board.

Reflow Oven

    • A PCB with mounted components sitting on top of solder paste go through this temperature controlled oven that melts the solder paste based on a predefined temperature heating curve
      • Note: This temperature profile is specifically designed to quality of solder bonding.

Automatic Optical Inspection

    • At the output of reflow oven, the assembled PCB (PCBA) is inspected for any moved or/ and missing components that were introduced during the reflow process.
    • This machine needs to be programmed with a golden reference assembled board to define pass and fail limit.

Test Software Image download station

    • For more complex embedded system products, a special light weight test software with special test commands are required to work with factory test equipment for start test sequence and data collection.

In Circuit Test (ICT)

    • In the context of factory floor test flow, this tests encompassing verifying device boot, power up voltages, inter circuit communication, subsystem basic functionality, etc.
      • Please see fundamental->manufacturing->quality assurance for more information on ICT


    • This is where the operator assembles all pieces of product together. Generally, the assembly and functional tests are done together to verify functionality the partially assembled product piece by piece.

Functional Circuit Test (FCT)

    • This test verify the functionalities of the product during assembly process. It ensures that no parts of circuit is damaged during assembling.
      • Please see fundamental->manufacturing->quality assurance for more information on FCT.

Final Software Image download station

    • For more complex embedded system such as Phone, TV, etc. A final customer facing software image needs to be downloaded onto the product.

Outgoing Quality Check (OQC)

    • This tests the final cosmetic and usability of the device before they are packaged and sent for shipment. For large volumes, sampling from a large batch (i.e out of 1000) is typically used save time and cost.

Pack out

    • This is where the assembled product are put into it's package and send out for shipment.

Summary and Conclusion

  • Pretests such as component orientation and stuffing options are required before factory start
  • Quality checks on incoming material as well as outgoing product are required for manufacturer to achieve high yield.
  • Solder paste and automatic optical inspection are essential to build a reliable and functioning mother board.

Pretests and repeated inspections and testing are required factory to deliver a high yield product. In the highly competitive hardware market, high yield process reduce costs, improves quality, and shorten the time to market.