First Step of Assembling a Printed Circuit Board-Incoming Material Inspection

How do you assemble a printed circuit board? What are the steps of PCB assembly? Today, we will introduce the first step of assembling a printed circuit board in our factory: Incoming Material Inspection.

  • Incoming Material Inspection for SMT

Incoming material inspection is crucial to guarantee the quality of PCB assembly. If there are any issues with the incoming materials like PCBs, components, and soldering paste, there is no way to resolve them during the assembly process. Therefore, incoming material inspection involves inspecting PCBs, components, solder pastes, and other materials used to facilitate the assembly.

  • SMD Components Inspection

The main test items for SMD components include solderability, soldering resistance, lead coplanarity, and usability. There are two testing methods for solderability. One is the wetting test, and the other is the dipping test, which is commonly used in assembly. The dipping test involves using a stainless clip to grasp the components and immersing them into a soldering pot at a constant temperature of 235+/-5 degrees Celsius for 2+/-0.2 seconds or at 230+/-5 degrees Celsius for 3+/-0.5 seconds (lead-free soldering requires 250-255 degrees Celsius for 2.5+/-0.5 seconds). The soldering condition is then checked under a microscope magnified 20-40 times. If 90% or more of the component leads are soldered, the soldering is accepted.

The basic steps of the soldering resistance test are similar to those of the solderability test but with different conditions. Reflow soldering requires 235+/-5 degrees Celsius for 10-15 seconds (265-270 degrees Celsius for 10-15 seconds for lead-free soldering), and wave soldering requires 260+/-5 degrees Celsius for 5+/-0.5 seconds (270-272 degrees Celsius for 10+/-0.5 seconds for lead-free soldering). Afterward, the components’ surfaces, packages, or soldering joints are checked for any cracks, deformities, color changes, or embrittlement under a microscope magnified 40 times. The test result is considered positive if the electrical test conforms to the parameters in the datasheet.

Physical checks are also performed on the components, including:

Checking for oxidation or pollutants on the tip or surface of the lead with the naked eye or microscope.

Verifying that the values, specifications, part numbers, tolerances, and outlines match the production request.

Ensuring that the leads of SOT, SOIC, and QFP are not deformed and that the leads’ coplanarity is less than 0.10mm for components with multiple fine-pitch leads.

Confirm that the marking will not come off for any components that require cleaning and that their functions and reliability will not be affected.


  • Printed Circuit Board Inspection

The testing for PCBs mainly covers manufacturability, PCB quality, and pad solderability, including:

Verifying that the PCB soldering pads and their sizes, solder mask, silkscreen, and the position of vias meet the design requirements of SMT.

Ensuring that the PCB outline sizes are uniform. The tooling holes and fiducial marks should meet the equipment requirements in the assembly lines.

Checking that the Bow and Twist of PCB are less than 1%.

Ensuring that the PCB is not wetted or polluted. Any damp or contaminated PCBs should be baked dry or cleaned.


  • Soldering Auxiliary Materials Inspection

The essential auxiliary materials include soldering pastes, surface mount adhesives, rod solder, flux, and cleaning agents. They are crucial to ensuring assembly quality. Ideally, they should be tested before assembly using related equipment and instruments. However, many assembly factories in China do not perform such inspections. Fortunately, YaHeng Circuit Co., Ltd. is proud to be one of the few that do. Generally, the quality of auxiliary materials can be verified by purchasing them from reliable sales channels from the manufacturer. Secondly, their package, part number, production date, or product lifecycle should be checked to ensure they match the needs. Thirdly, their appearance, color, and odor should be normal. Additionally, their performance during production should be monitored, and any issues should be promptly addressed with the supplier.


At YaHeng Circuit Co., Ltd., such inspections are conducted in two aspects. Firstly, solder testing and evaluation are based on material characteristics and process performance. Material characteristics include viscosity, alloy particle size and shape, flux content, halogen content, and insulation resistance. Process performance refers to characteristics in production like soldering balls, wettability, and solderability. Secondly, flux inspection covers various items such as appearance, physical stability, density, non-volatile matter content, pH value, halogenide, weldability, aridity, corrosion, surface insulation resistance, copper erosion, ionic contamination, and mold growth.

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