PCB Assembly Testing Methods for SMT Solder Joint Quality

Back when I was in school, I spent a lot of time working on my car. I could not afford a new one, and I avoided putting it in the shop whenever I could. One of the repair jobs that I enjoyed most was working on the exhaust system, because it gave me an opportunity to do some welding. Although it was a hot job, I relished trying to make the smoothest joint that I could. A secure connection and a smooth joint usually translated into a good quality weld.

Welding is a lot like soldering components on a circuit board in that there are tell-tale signs that indicate a good solder joint. For example, a good solder joint will have solder applied to both elements being joined, and there will be no air pockets. The importance of having good solder joints to PCB quality control is well-known, as bad joints can lead to unreliable signal transfer or tombstoning and no signal flow at all. Let’s examine what is meant by good SMT solder joint quality and then take a look at some PCB assembly testing methods your contract manufacturer (CM) can employ to ensure your board exhibits it.

What Constitutes Good SMT Solder Joint Quality?

Example of good solder joint quality

Example of good solder joint quality

In the figure above, an example of a PCB with several types of components mounted is shown. In each case, a good solder joint has been established, which means that the component is securely attached to the board, and:

  • component pin and footprint pad are aligned;
  • pad area is completely covered with solder;
  • component pin area in contact with pad is completely covered with solder;
  • no excess solder covers component;
  • solder joint is smooth;
  • solder joint does not contain voids; and
  • solder does not cover solder mask.

The list above are indications that your design’s surface mount device (SMD) has been soldered well; however, it is not exhaustive, as traces that run within or through your board, better known as vias, cannot be completely examined from the board’s exterior. For this, you must rely on the quality of your CM’s printed circuit board assembly process.

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How to Ensure Solder Joint Quality?

It is true that the quality of your board’s solder joints rests with your CM. More specifically, it rests with your CM’s assembly process and commitment to quality. A good indication of this commitment is reflected in the number and types of tests that are employed during your board’s assembly. This may include the following:

PCB Assembly Testing Methods to Ensure Good Solder Joint Quality
PCB Assembly Testing Method Why it is Important
Manual Visual Inspection These should be done at virtually every stage of the assembly process and provide a quick assessment of the success of the previous step.
Automated Optical Inspection (AOI) This testing method is employed during fabrication and assembly and provides resolution greatly superior to visual inspection for checking solder joint volume.
X-ray Inspection This method allows for inspection of the board’s interior. As such, vias for BGAs and other packages where pads cannot be seen from the surface can be examined.

All of the testing methods above are not employed by every CM. However, the advantages of using AOI and  X-ray inspection for solder joint quality should probably be weighed when choosing your PCB assembly services. In addition, you should employ steps to help your CM optimize your board’s assembly.

Tempo‘s Custom PCB Manufacturing Service
  • ISO-9001, IPC-600 and IPC-610 commitment to quality certifications.
  • Accurate quote in less than 1 day.
  • Performs entire turnkey process in as fast as 3 days.
  • Emphasizes DFM to eliminate time-consuming back-and-forth design corrections.
  • Sources components from the most reputable suppliers in the industry to reduce procurement time.
  • Performs multiple automated inspections during assembly to ensure PCB quality for prototyping.
  • Provides support throughout the PCB manufacturing process, beginning with design.
  • Smooth transition from prototyping to production.

At Tempo Automation, the industry leader in fast, efficient PCB prototyping and low-volume production, we institute many automated and manual PCB assembly testing methods to ensure that all aspects of your board construction are of the highest quality, including solder joints. By partnering with you and understanding your design intent and objectives we can deliver not only the best results, but also the best manufacturing experience for your project.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

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 PCB assembly testing methods for solder joint quality, contact us.

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