Most people like surprise gifts, and I’m no different. The excitement of not knowing exactly what is coming can be exhilarating. For example, on your birthday or during the holidays, the anticipation of the gift can be almost unbearable. Regardless of what the gift turns out to be, you are usually pleasantly surprised. Surprises aren’t always pleasant, however, especially when it's not in the form of a gift. Discovering that the PCB you designed and have been anxiously waiting to receive from your manufacturer is unusable is an unpleasant surprise that no one wants to receive. In this post, I compare PCB quality control options so that you can ensure that your next board is built correctly.
To minimize or even eliminate the possibility of that unpleasant surprise, you should ensure that your manufacturer institutes good PCB quality control (QC) measures. In fact, asking your contract manufacturer (CM) about their commitment to QC and the QC techniques they use should be a part of your process while selecting a PCB fabrication and assembly services provider. To help you assess how much a CM prioritizes PCB quality control, let’s examine some techniques that are typically employed to ensure that your board is built correctly.
PCB Quality Control for Fabrication
The first stage of the PCB manufacturing process is the fabrication of the bare board (without components). The quality of materials used for PCB construction is an important concern here. However, the priority is ensuring that the layout remains true to the design while adhering to the tolerance restrictions for board edge clearance, drill holes and spacing between pads, traces and other elements. To ensure these QC issues are addressed, your CM may perform the following tests:
- Design for Manufacturing (DFM) - The design file is checked against the CM’s equipment constraints. Most violations need to be corrected before the board can be constructed.
- Electrical Testing
- Flying Probe E Test - This is a non-invasive test to check continuity and is typically used for prototype runs.
- Bed of Nails Test - This is a traditional in-circuit test (ICT) for production runs. It utilizes an electronic fixture with multiple pins to contact test points on the PCB.
- Manual Inspection - Engineers visually inspect the board for errors or excess debris that may be remaining on the surface.
- Automated Optical Inspection (AOI) - Although the level of equipment used varies from CM to CM, this testing is more reliable than manual visual inspection and is repeatable. AOI can identify the following problems.
- Line width violations
- Spacing violations
- Excess copper
- Missing pads or other element
- Shorts circuits
- Cut traces or pads
- Drill hole breakage (via outside of pad)
- Time Domain Reflectometry - This test is used to measure the characteristic impedance of the device under test (DUT). This is especially important for differential traces where impedance mismatches can cause signal distortion.
- Sawing - This destructive test, which involves cutting the board in half or sections, is performed to visually inspect of the layer stackup.
Besides ensuring that high quality materials are used for the boards themselves, you also need to determine which of the above QC measures meet your minimal standards for PCB fabrication QC. Note that many fabricators only perform DRC and manual inspection for prototyping.
PCB Quality Control for Component Procurement
The next step in the PCB manufacturing process is to attach components. Component procurement QC begins with your component selection. It may be tempting to let cost be the primary guide in making these selections; however, there are other considerations. Optimizing component procurement requires good QC because any delays in sourcing your components can stall the PCB manufacturing process or require future design changes. To minimize potential problems, the following QC steps can be taken:
- BOM Validation - These are checks to ensure the components in the BOM match the PCB design.
- Manufacturer Part Number (MPN) verification - MPNs may be incorrect and can result in the acquisition of the wrong component. A single incorrect digit may result in a wrong package type. This error results in loss of time and additional cost.
- Component Reference Designator verification - This is the unique identifier in the BOM. If this is incorrect, the number of components acquired may be erroneous.
- Component Manufacture Status checking - This ensures that the component will be available for future production.
- MPN - Footprint match - An error here may violate solder relief and component spacing requirements. If component and footprint are not matched will stall assembly.
- Kit accuracy - If kit components are inaccurate, extensive delay and extra costs will result.
- Use of reliable suppliers - This ensures that counterfeit components are not used.
- Documentation review - Review component shipping documentation for errors.
- Visual Inspection - This helps ensure that the component matches your order and documentation.
- Solderability Testing - This is an accelerated “aging” process applied to samples to ascertain solderability.
- Component Tolerance Check - This test ensures that the components actually meet the datasheet tolerance ranges.
You may include these considerations as part of the PCB QC for component procurement to ensure that the correct components are available for assembly. As for fabrication, you need to determine the minimum requirements that will satisfy your expected level of QC.
PCB Quality Control for Assembly
As the final stage of the PCB manufacturing process, assembly is the last point where any defects or errors can be discovered before the board is sent to you. Thus, part of PCB quality control for assembly involves performing a preliminary check of the previous two stages. Testing during this stage is typically more intensive than during fabrication. Some QC techniques that may be performed during PCB assembly include:
- Visual Inspections - These are performed throughout the assembly process to check for excess solder, unattached components and other issues that may require rework.
- Microscopes are often used to improve visual inspections.
- Cameras may be used to take photos for distributed viewing and additional analysis.
- X-ray Inspection - These provide non-invasive high quality imaging of the PCB’s internal quality.
- AOI - For assembly, higher quality imaging (as compared to fabrication) is used to check connections and solder quality.
- Sawing - For assembly this test is used to primarily inspect vias. Only recommended for large-scale production runs.
- Resistivity of Solvent Extract (ROSE) Testing - This a quick and reliable test for determining the “cleanliness” of a PCB. It provides a quantitative measure of the contamination level on the board.
- Electrical Testing - Although these tests are used for fabrication as well, they test for different criteria during assembly. Due to the lack of components during fabrication, you can only check conductivity. For assembly, however, you can examine parameter levels since components are present.
- Flying Probe E Test
- Bed of Nails Test
The PCB Design Development Checklist
When determining the techniques you want to include in your contract manufacturer’s QC for assembly, an important decision is whether to perform invasive or non-invasive testing. Invasive testing is usually a part of a rigorous destructive physical analysis (DPA) of the board that often includes non-invasive testing as well. If errors are discovered, board redesign might be required.
|Tempo‘s commitment to PCB QC
At each stage of the PCB manufacturing process, there are many options for QC. Because different CMs vary in terms of QC techniques, you must fully research your CM’s commitment to PCB QC. This will help ensure that your boards are built correctly with the level of quality that you expect.
At Tempo Automation, delivering high quality PCBs with speed, precision and transparency is our mission. We employ advanced QC for all stages of our turnkey PCB manufacturing process. This includes a patented software BOM verification process: using AOI and X-ray inspection to ensure that your board has the right components and is free from defects. We begin by partnering with you during design to ensure that your DFM is aligned with our manufacturing equipment and there are no unpleasant surprises when you get your boards back. To facilitate this, we enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on QC or what techniques and tests we incorporate into our manufacturing process to ensure the quality of your PCB, contact us.