Five Types of Soldering Defects in PCB Assemblies and How to Avoid Them

The world we live in is not always perfect and it isn’t uncommon for defects to occur from time to time. The rubber used in a tire may have a defect leading to a blowout, or a water pipe could have a defect from manufacturing, resulting in a leak. Those are just a few of the defects that we’ve seen over the years and surely if we all got together we could add a whole lot more. The important thing to learn is how to find those defects, repair them, and then change design and manufacturing processes to avoid repeating errors.

In the manufacturing of circuit boards, there are also many opportunities for defects to sneak in and as with everything else, it is important to prevent them from happening. One area that can be prone to defects is when components are soldered onto the board. We’re going to take a look at some of the more common types of soldering defects here in this article and explore some design recommendations that can prevent them from happening in the first place.

Five Types of Soldering Defects to be Aware Of 

Solder defects can happen for a variety of reasons when manufacturing a printed circuit board. Here are five such reasons for PCB designers to be aware of: 

1. Sunken joints

During wave soldering, the wave of molten solder will wick up into the holes of thru-hole parts to solder the lead into the hole. The problem that can happen here is if the solder sinks back down through the hole, leaving an incomplete solder joint that can be easily broken.

2. Solder shorts

 As lead pitches become smaller and smaller, it becomes easier for the solder to short across some of these leads. Back when leads were at 0.100” pitch, it wasn’t a problem. Now that lead pitches come down through 0.050” to 0.025”, the solder has a greater chance of bridging between the leads.

3. Shadowing

 When surface mount components are waved soldered, larger components that proceed smaller parts into the wave may shadow them. This can produce an undesirable condition where the shadowed parts do not get a sufficient amount of solder on them due to the shadowing.

4. Tombstoning

Smaller discrete components going through solder reflow can sometimes stand up on one end instead of both ends being soldered to their respective pads. This is due to a thermal imbalance between the two pads causing the solder paste on one to reflow sooner than the other pulling the part up with it.

5. Incomplete joints

If the airflow in the solder reflow oven gets inhibited, it can cause some parts to not get heated enough for a good solder joint to be formed. This can be caused by larger parts blocking smaller components in a similar way to what happens with the shadowing of parts during wave soldering. 

Fortunately, there are steps that you can take while designing the circuit board that will prevent these problems from occurring.

Design Recommendations to Avoid Soldering Defects

To help your PCB design have the best chance of forming good solder joints, here are some design recommendations aimed at improving the solderability of the board:

• When developing the PCB component footprints for thru-hole parts, make sure to use the recommended pad size for the drill hole being used. Holes that are too small for the thru-hole leads can create problems when inserting the part, while holes that are too big can allow the molten solder to fall back down the hole before a good solder joint is formed.
• When creating thru-hole pads for high-density parts such as connectors that have fine-pitched leads, use an alternatively shaped pad to help with the soldering. Pads that are increased in length on the side of the pad that will exit the wave will help control the tendency for solder bridging to occur.
• Make sure to work with your PCB contract manufacturer before you design your board, to understand which way the board will enter the wave. This will allow you to locate your surface mount components in a way that reduces the chance of larger components shadowing smaller ones.
• It is important to use appropriate thermal reliefs when connecting small surface mount passives to large metal planes. A large metal plane that completely engulfs an SMT pad will act as a heat sink and cause a thermal imbalance that can lead to tombstoning.
• As with wave soldering, be careful about the placement of tall components such as heat sinks or tantalum capacitors where they might block the reflow oven’s airflow to smaller parts. This way you can ensure that all parts will be evenly heated in the reflow oven to create good solder joints.

Many other design methodologies can be used to help improve the soldering process, and your PCB contract manufacturer can help you to decide which ones will be best for your design.

Additional Information on Soldering

Not only are contract manufacturers well versed in the different industry standards for the assembly of circuit boards, but they also have plenty of practical experience to work with as well. This gives them the ability to advise you on what the best component placement strategies will be based on the empirical data that they have collected over the years.

 At VSE we have been building circuit boards for our customers now for over 30 years, and we have extensive knowledge of what is required to eliminate solder defects from a PCB design. Our engineering team routinely works with circuit board designers and they can give you the best advice on what your design needs for manufacturing success.