A Look at the RoHS PCB Assembly Process

Since its introduction, the Restriction of Hazardous Substances Directive (RoHS) has brought in immense changes to the PCB manufacturing process. The directive conveys that a variety of hazardous substances should not be used in the PCB manufacturing process. One of the important substances mentioned is lead, which is known for creating brain disorders, and affecting the nervous system. In light of the directive, PCB manufacturers changed to a lead-free production process. In this post, we provide the details of this lead-free RoHS PCB assembly process.

Pre-Assembly Points

Before any manufacturer begins the assembly process, there are a few things to be taken care of.

  • The assembly process is performed at high temperatures. Hence, it is important that the circuit board substrate, and the PCB components have the capabilities to withstand high temperatures.
  • The level of moisture that the boards are exposed is higher than lead PCBs. Hence, it should be noted that the shelf life of the materials will be shorter, as compared to leaded PCBs.
  • The customer should verify that the Bill of Materials (BOM) has the correct list of lead-free parts. If components are exposed to moisture due to the poor quality packaging, then the manufacturer will have to bake the components before assembly.

RoHS PCB Assembly Process Steps

The following steps will help you understand how the lead-free assembly process is performed. In this post, we shall be focusing on PCB assembly.

  • Install the Board Stencil and Apply Solder Paste: Take a lead-free circuit board and apply the design stencil to it. Then apply lead-free solder paste to the board. Generally the solder paste is an alloy known as SAC305. The board and the stencil are soldered together. A visual inspection of the board is performed to ensure that the solder coverage is optimal, and that the board can be used in the manufacturing process.
  • Perform Component Placement: Component placement is performed using a pick and place machine. The parts that are used are those specified in the BOM. Every component has a unique identification number. The pick and place machine is programmed to identify the component through its number. The machine picks up the correct part, and places onto the board.
  • Create a Board Profile for the Reflow Process: When the component placement phase has been completed, the board needs to be placed in the reflow oven. However, sometimes, certain components like BGAs, silicon circuits, or heat slug parts may not be able to withstand the reflow oven temperatures. In such cases, it is a good idea to create a dummy lead-free PCB with mechanical samples. This will allow you to test the reflow heating process, and find the optimum temperature profile to perform the reflow process.
  • Place the Board in the Reflow Oven: Once you have completed the profiling step, you can place the original PCB into the oven and perform the reflow process. The heat from the oven will melt the solder paste, which will form a metallic board between the circuit traces and the components.
  • PCB Inspection: The reflow process is that last phase in the assembly process. Post this, you need to get the board inspected. First, perform a visual inspection of the board to ensure that the solder joints are solid. You can also perform X-ray or AOI inspection to ensure that all the components are working, and that the PCB will perform as expected.
  • Package the PCB for Delivery: Once the inspection phase has been completed, the PCB should be packaged properly before delivery. Utilize anti-electrostatic discharge bags to pack the PCB. This will ensure that final product will not suffer from static during transport.

 By understanding all the above points, customers as well as novice manufacturers will have a clear idea of how the RoHS PCB assembly process is performed. This will help both groups to communicate better to improve the PCB manufacturing and assembly processes.