Protecting Your PCB Design From Machine Assembly Defects

Created: February 9, 2018
Updated: September 25, 2020

PCB assembly arms pushing against each other on blue background.

As much as I enjoy designing electronic products for my clients, I immediately turn to repair centers when my own laptop breaks down, trusting the expert technicians who have dealt with a variety of issues. But when my laptop returns with a torn cover and a missing screw, it just proves that human error is inevitable. Unfortunately, machines aren’t infallible either.

When you choose to get your PCB assembled by machines instead of manual labor, you expect your assembled PCB to be free of defects. But in reality, perfection is virtually unachievable when it comes to PCB assembly. Even with the top equipment, a small percentage of your boards might occasionally face quality issues. However, being aware of issues can help you optimize your design to minimize or even prevent PCB assembly defects:

PCB Assembly Defects

There is plenty you can do to check a manufacturer before your PCB moves into assembly and production stages like scheduling an on-site review or an audit. Sometimes, though, no matter how thorough your inspection process is a machine is going to have some fault or encounter an unexpected error that may leave your design in jeopardy. Here are some of the problems you might run into:

1. Solder bridges

One of the most PCB assembly defects that could cause serious damage when powered up is solder bridges or shorts between the leads of fine pitch components. Shorts are usually tiny and easily escape visual inspection. Shorts during PCB assembly can be caused by various factors. For instance, component pads that are too wide with little gap between them can result in solder bridges. Shorts can also be caused by excessive amount of solder placed on the pads due to incorrect stencil specification.

Board with beads on the back arranged in a pattern
BGA Defects Is Impossible To Detect By Human Eye

2. Open Joints

Unless you’re using a magnifier, open joints on the PCB pad are almost impossible to detect. A visual check will indicate whether all the surface mounted components are properly soldered. However, even a tiny gap between the component lead and the solder pad is enough to prevent the electronics from functioning correctly. In addition, a stencil that is too thin can also cause lesser solder paste to be deposited and hence the open joints.

3. Component Shifts

While the possibility of component shifts in PCB assembly is low, they do happen and are usually obvious to the careful eye, except with packages like Ball Grid Array (BGA) where the leads are at the bottom of the component. A rework to realign the component requires desoldering the component and manually soldering it again. This may cause further quality issues as the success of these tasks depends on the skills of the technician. Component shifts are usually caused by physical factors such as bent leads or high vibrations in the conveyor.

How to Reduce or Even Prevent PCB Assembly Defects

On paper, your design looks great. You follow up with a working prototype that shows promise of a great product. Yet, if you did not take the right precautions to minimize PCB assembly defects, your efforts could be wasted. Here is what you can do to reduce or prevent PCB assembly defects:

1. Ask About Visual Inspection Capability

Do not assume that your PCB assembly supplier is equipped with visual inspection technologies or performs any quality checks at all. Learn about their quality check process and inquire whether their visual inspection system will be sufficient for your PCB. For instance, a PCB with BGA components will require an X-Ray scope to scan the balls of solder located beneath the component.

Magnifying glass icon for inspection
A good visual inspection system will identify post assembly defects.

2. Get the Right Stencil Fabricated

When you’re not manufacturing in high volume, the cost of PCB stenciling can substantially reduce your profit margin. It is tempting to fabricate a stencil that is smaller and thinner but this could backfire with open joints between your component lead and pads. Instead, make sure your PCB stencil is fabricated properly based on your pad size to ensure the right amount of solder paste is deposited.

3. Ensure Component Footprint Has the Right Size

Getting the right pad size for fine pitch SMD components can be tricky. Make the pad too thin and you won’t have enough surface area for the solder to hold. Increase the width too much and risk causing solder bridges. Most SMD components provide a suggested footprint within the datasheet. It is important to create the footprint accordingly.

Prepare for Potential PCB Assembly Defects

Throughout your PCB design process, keeping PCB assembly defects in mind can help prevent, or at least reduce potential issues. The right PCB design software can help you adhere to complex design rules while minimizing your overall turnaround time. With features like design history compatibility and customizable component creation, CircuitStudio® can help you develop an assembly-ready design.

Having quality issues with your assembled PCB? Get advice from an expert at Altium.

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