Comprehensive Guide to Lead-Free PCB Assembly: Step-by-Step Process

Jul 07,2020

Lead-free PCB assembly means PCB assembly that uses no lead during any phase of manufacturing. Lead is traditionally used in the PCB soldering process. However, lead is toxic and is therefore hazardous to humankind. Considering its consequences, Restriction of Hazardous Substances (RoHS) directive by the European Union prohibits the use of lead in PCB assembly processes. The replacement of lead with less toxic substances causes little differences in the PCB assembly process. This post gives step-by-step details about the lead-free PCB assembly process.

Guidelines for Lead-Free PCB Assembly

The lead-free PCB assembly process is divided into two fundamental parts namely, the pre-assembly process and the active assembly process. Steps involved in the lead-free PCB assembly process are detailed as follows.

Pre-assembly Steps

There are three fundamental pre-assembly steps involved in lead-free PCB manufacturing. These steps set the foundation for an error-free and precise PCB assembly. The pre-assembly steps of lead-free PCB assembly are as follows.

• Profiling:

  Profiling is a prototype resembling process. The manufacturers take a finished lead-free PCB as a prototype. This can be a functioning actual PCB or a non-functioning PCB or dummy parts. The stencils for assembly are traced by profiling. The lead-free assembly designs are compared to prototypes in order to ensure its compatibility for the assembly.

• Solder Paste-Inspection:

  Since the lead-free solder joint gives a metallic appearance, which is very different than a lead-based solder, it is important to inspect meticulously. The PCB profile and solder paste are inspected under IPC-610D standards to ensure the lead-free solder joints will be firm and solid. The moisture level is also tested in this step because, in lead-free soldering, the board is exposed to a high level of moisture as compared to traditional soldering.

• Bill of Material (BOM) and Component Analysis:

  In this process, the customer has to verify the Bill of Material (BOM) in order to make sure the components are made of lead-free material. Lead-free components are prone to moisture therefore should be baked in the oven by the manufacturer. Once the prerequisite steps are performed, the actual lead-free assembly begins.

Active Assembly Steps

In the active assembly process, the PCB assembly actually takes place. The steps involved in the active lead-free assembly are detailed as follows.

• Stencil Placement and Solder Paste Application:

  In this step, the lead-free stencil from the profiling stage is placed on the board. Then the lead-free solder paste is applied. Generally lead-free solder paste material is SAC305.

• Component Mounting:

  Once the solder paste is applied, the components are mounted on the board. The component placement can be done manually or by using automated machinery. This is a pick and place operation but the components used are required to be confirmed and labeled in the BOM verification stage. The machine or operator picks the labeled component and places it to the designated location.

• Soldering:

  Lead-free through-hole or manual soldering is performed in this stage. Regardless of the type of process, THT or SMT, the soldering must be lead-free.

• Board Placement inside Reflow Oven:

  The RoHS compatible PCBs are required to be heated at high temperature to evenly melt the solder paste. Therefore, the PCBs are placed inside a reflow oven, where the solder paste melts. Further, the boards are cooled at room temperature to solidify the melted solder paste. This helps fix the components at their position.

• Testing and Packaging:

  The PCBs are tested under IPC-600D standards. The solder joints are tested in this step. Visual inspection is followed by AOI and X-ray inspection. Physical and functional testing is performed prior to packaging.

For the packaging of lead-free PCBs, it is very important to use anti-electrostatic discharge bags. This is important to ensure that the final product will not suffer from static charges during transportation.

However, despite the thorough understanding of lead-free PCB assembly, it should be performed by experts for perfection. Creative Hi-Tech is a lead-free PCB manufacturing company that uses RoHS compliant FR4 materials for PCB manufacturing. They follow IPC-A-600D and ANSI/J-STD-001 standards for lead-free PCB manufacturing.     

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