PCB soldering is an essential part of the entire printed circuit board assembly process. Unfortunately, the soldering process can face many problems, making it challenging to achieve top-quality final PCB products. As PCB components become smaller and more compact, the chances of issues occurring when soldering PCB becomes higher.
But what exactly are these PCB soldering problems? More importantly, what can you do to avoid them, so you save time and improve efficiency?
That is what this comprehensive guide covers. We will outline 15 of the top PCB soldering problems and how to steer clear of them.
- I.Solder Bridging Bad solder joints
- II.Excessive Solder
- III.Solder Balling
- IV.Cold Joint
- V.Overheated Joint
- VII.Insufficient Wetting
- VIII.Solder Skips
- IX.Lifted Pads
- X.Solder-Starved Joints
- XI.Solder Splashes
- XII.Pin Holes and Blow Holes
- XIII.Solder Flags
- XIV.Solder Balling
- XV.Solder Discoloration
I.Solder Bridging Bad solder joints
The top problem that smaller and more compact components cause when it comes to PCB board soldering is solder bridging. The problem happens due to the inadvertent connection between two or more joints. This connection is typically the result of excessive solder between bones.
Identifying a solder bridge is often challenging, given their microscopic size. It can be a massive problem as solder bridges can cause short circuits and lead to a component's burning up.
So how do you fix this problem? Once you identify the bridge, hold a soldering iron in its middle to melt the solder. Draw through the deck to break it. In case the bridge is too large, remove the excess solder using a solder sucker.
（PCB short circuit after PCB soldering）
This problem occurs when you apply more solder onto a pin than you should, leading to excess buildup. The issue is common among beginners who assume that the more solder they ask, the better.
However, the main problem is that it is difficult to know what is happening underneath after doing so much welding.
Excessive solder on the pin may deter proper wetting of both the pin and the pad. Moreover, too much solder could cause solder bridging, as we mentioned earlier.
The best way to avoid this problem is always to apply enough solder to wet the pin and the pads correctly.
（Removing a component during PCB soldering）
Another common PCB board soldering defect is solder balling. Typically, this issue occurs with reflow soldering. The problem has the appearance of small spheres of solder adhering to a laminate or conductor surface.
Several factors can cause solder balls, including:
- Poor solder paste printing
- Rough PCB design
- Poor reflow temperature
- Using oxidized components
Adopting proper PCB soldering techniques is the best way to avoid this problem.
（PCB soldering to fix solder balling issues)
You have probably noticed the surface of PCB joints looking dull and lumpy before. This problem results from too low PCB soldering temperature reaching the bones; hence the joint does not melt.
So why does insufficient heat reach the joint? Well, there is a variety of potential reasons. It could be that you have not allowed the soldering iron or the collective to heat up adequately to melt the solder. Sometimes, the defect is the result of the improper design of the traces and pads.
Cold joints need immediate rectification; otherwise, cracks may form and lead to the entire component's eventual failure.
(Visually inspecting a PCB for faults before PCB soldering）
This issue is the opposite of cold joints. The problem may be the result of your setting PCB soldering temperature too high on the soldering iron. Sometimes, the problem is that the solder may not flow, which may be due to the pad surface's nature. The weld may also fail to flow when lead already has an oxide surface that prevents enough heat transfer.
As a result of the issues above, you end up heating a joint for too long, sometimes causing severe damage. Even when the cost is not critical, it may still cause pads to lift and lead to expensive repairs.
The solution here is to set the right solder iron temperature. Also, always clean dirty pads and joints using flux.
（Burnt PCB board due to overheating during PCB soldering）
Ideally, the solder attaches to both pads and begins the wetting process. In some instances, however, the solder on one pad does not complete the wetting process, leading to the tilting of one side of the component. This tilt looks like a tombstone, hence the name.
In reflow soldering, anything that causes solder paste on one pad to melt before the other may cause tombstoning. Examples include the absence of relief design and where the traces connecting to the pad have unequal thickness.
In wave soldering, tombstoning can occur when the incoming solder wave physically pushes components with large bodies. To avoid this issue, layout engineers must carefully consider the stream's direction as they design boards that will undergo wave soldering.
（PCB soldering to remove faults）
Without sufficient wetting, joints are weak. They cannot form a connection with the board that is loud enough. That is why soldiers must achieve 100% wetting with both the pin and the pad. They should never be any gaps or spaces.
Unfortunately, sometimes full wetting does not happen. There are several reasons for this. For instance, the engineer may not apply enough heat to the pen and pad. Another reason for this problem is the failure to allow enough time for the solder to flow. It could also be that the board is dirty.
How can you avoid this issue? Well, you can start by cleaning the board thoroughly. Next, make sure that you apply even PCB soldering temperature to both the pad and pin.
（Testing and repairing faulty PCB during PCB soldering)
A solder skip refers to a solder joint where there is no wetting with solder. The defect results from welding skipping over a surface mount pad, leading to an open circuit.
A common cause of solder skips is the combination of slips during the design or manufacturing stages. The designer may have put down an uneven pad size. Alternatively, the manufacturer used the incorrect wave height between the soldering wave and the board.
(Surface mount device of a PCB. Pads need to be even before PCB soldering)
As the name suggests, a raised pad is a pad that's detached from the PCB surface. The cause may be too much PCB soldering temperature or excessive force on one of the joints. Lifted pads are tough to work with because of their fragile nature. These pads can easily tear from the trace.
Once you discover this problem, please make efforts to adhere to the pad onto the board before doing any soldering on it.
(PCB soldering in progress. Ensure pads are even before soldering.)
This problem occurs when joints lack enough solder to form reliable electrical connections. The issue is the result of applying insufficient heat to the lead, which leads to poor connection. Sometimes, the joint will still work because there’s still electrical contact. However, the joint will eventually fail due to the development of cracks that further weaken the connection.
The solution to this problem, fortunately, is quite easy. All you need to do is reheat the joint while adding more solder.
(PCB soldering. Joints need to have enough solder)
In some cases, solder bits can stick onto the solder mask in splatters, making it look like a spider web. The threads typically have an irregular shape and are often the result of not using sufficient fluxing agents. Pollutants on the surface of the boards during wave soldering might also cause this problem.
To avoid this problem, make sure the surface of the board is clean before PCB board soldering.
(PCB soldering. Sometimes solder splashes happen)
XII.Pin Holes and Blow Holes
It’s easy to recognize pinhole and blowhole problems because they appear as holes in solder joints. Pinholes are much smaller than blowholes. The defect usually occurs during wave soldering. Typically, the moisture within the holes heats up into gas during the PCB board soldering process and escapes through the molten solder, causing voids.
The best way to avoid this problem is to bake or preheat the boards so that the moisture comes out. You could also opt to use copper plating that’s at least 25um thick in the through-holes.
( PCB joints. Pinholes and blowholes can occur on these joints during PCB soldering )
Solder flags indicate reduced flux application and issues with solder drainage. They happen when the solder drains too slowly from the wave soldering machine, causing an excessive height of the board's solder. Another cause is the inconsistent application of flux, in which case you’ll see trails of welding on the board that looks like whiskers.
(PCB soldering. Solder flags on a PCB indicate reduced flux application )
This defect occurs when bits of solder stick to the PCB board's surface while you're soldering the board. It happens due to too high temperatures in the wave soldering machine, or when solder falls back into the wave during separation. Welding balling could also occur due to flux heating, which causes the solder liquid to splashback onto the board.
(PCB soldering. Bits sticking to the surface of the board cause solder balling)
Unlike the other soldering defects we mentioned before, solder discoloration is purely a cosmetic issue. However, it’s still essential that you take the time to find out its root cause. Usually, the problem is that your PCB manufacturer used different flux materials. It could also be the result of using higher PCB soldering temperatures between wave soldering runs of a single board.
To avoid this problem, it's essential to maintain consistency in flux materials, soldering temperatures, and thicknesses throughout the soldering cycle.
(Photo of a perfectly colored PCB. Faulty PCB soldering can cause discoloration)
As you can see, many problems can occur during the PCB Soldering process. Any of these problems can negatively affect your efficiency, increase lead times, and lower your PCBs' overall quality. Often, repairing these problems can consume precious time and money. Unfortunately, there’s no single foolproof PCB soldering technique that can completely prevent all soldering defects from happening. However, there are things you can do to avoid the mistakes from happening in the first place.
Working with a reputable PCB manufacturer like OurPCB is the most effective way of avoiding issues with PCB soldering. At OurPCB, we have a large team of highly experienced technicians familiar with all the potential pitfalls of PCB assembly. With our extensive industry experience, we can rapidly source the necessary components and correctly assemble a complete PCB board for you. Contact us today to check out our services.