Hand soldering has always been considered as an iconic skill of every electronics manufacturer. Manual soldering is an important link in daily PCB assembling. Many customers’ products do not fully consider SMT components and SMD components process during the design stage. Like the different temperature between the reflow oven and wave soldering, which results in the different sensitivity of the DIP components and the SMT components to the temperature, or when the material is complete, some special-shaped parts also require manual soldering.

As the components become smaller and smaller, the layout becomes more compact, and the probability of hand soldering problems increases. Moreover, a good solder joint appearance of the PCB in important applications will be more important than ever. The following can help you identify the quality of hand soldering and avoid these problems in PCB assembly production.

Ideal Solder Joints For PCB Hand Soldering

Ideal Solder Joints For PCB Hand SolderingThe ideal solder joints for DIP components are “concave corners”, which have a smooth and shiny concave surface at an angle of 40 to 70 degrees from the horizontal plane. Ideal solder joints for SMT components are SMD solder joints with smooth, concave rounded corners.

Therefore, the general characteristics of a good solder joint are:
1. Has good wettability.
2.With concave rounded corners.
3. Glossy and clean.

The following figure is some simulation diagrams of poor soldering, for your reference:

simulation diagrams of poor soldering

Common problems and improvement methods of manual soldering

1. Solder Bridge

Of the many problems caused by smaller and smaller components, solder bridging tops the list. Inadvertently connect two or more solder joints can infom solder bridge. Excessive soldering between solder joints or the use a too wide soldering iron tip will cause this problem too. Solder bridges size sometimes is too small, so identifying solder bridges can sometimes be difficult. If not detected, it may cause a short circuit and burn out the components.

Solder Bridge

Improve methods For Solder Bridge:

The solder bridge can be secured by fixing a soldering iron in the middle of the bridge to melt the solder and then pulling it out to destroy the bridge. If the solder bridge is too large, the excess solder can be removed with a suction cup.

2. Too much Solder

If pin apply too much solder, it will cause too much accumulation, which is characterized by its rounded shape. Beginners generally think that more solder is better, although more solder will increase the amount of material, it is difficult to know what excessive solder will affect the component, and it also increases the risk of forming a solder bridge. In general, enough solder is sufficient to fully wet the pins and pads, and the concave surface still maintains the best shape because it allows us to better wet the joints.

Improve Methods For Much Solder:

Use an electric soldering iron to remove excess solder until the amount of solder reaches a proper level.

3. Solder Ball

Solder balls are also one of the most common soldering defects. It looks like a small piece of solder ball that adheres itself to the laminate, resist or conductor surface.

Solder Ball

Improve Methods for Solder Ball:

Use an electric soldering iron to suck out the appeared solder balls, use a soldering wire to pad the remaining solder residue, and then heat the soldering wire with an electric soldering iron to stick out the excess tin.

4. Cold Soldering

The surface of the cold seam appears dull, lumpy and numb, this is usually caused by insufficient heat to completely melt, some different reason can cause insufficient heat.

For example, not give the soldering iron or the solder joint sufficient time to fully heat up, the temperature of the soldering iron may not be set high enough to melt the specific solder type used – for example, lead-free solder has a higher melting temperature, or it may be a solder pad and the design of the trace – for example, directly connect a pad to the ground plane without considering heat dissipation will cause the heat of the soldering iron to be dissipated to the ground plane.

If can not liquefy stubborn solder joints, it may indicate a problem with the design. If not properly corrected, cracks may form over time and eventually cause failure.

Cold Soldering

Improve Methods for Cold Soldering

It is recommended to use an electric soldering iron to remove the cold-soldering solder, and then add solder wire to re-solder. Note that the temperature of the electric soldering iron must meet the soldering requirements.

5. Soldering Iron is Overheated On PCB

Soldering Iron is Overheated On PCB

The temperature of the soldering iron set too high may cause Overheating of the solder joint, or the solder pad is not flowing – It may be because a layer of oxide has formed on the surface of the pad or the lead, preventing sufficient heat transfer and heating the solder point, causing the pad to lift completely.

To avoid this, choose the correct soldering iron temperature and use a flux to clean the dirty connectors and pads.

6. Insufficient Tin

Insufficient Tin

Not fully melted contacts are weak and no firm connection to the printed circuit board. Ideally solder should be 100% melted using pads and pins without leaving any gaps or spaces. Inadequate wetting of the leads and pads is due to the inability to apply heat to the leads and pads at the same time and to not give solder enough flow time. Dirty circuit boards sometimes will cause this too.

The technique to fix this is to clean the PCB board thoroughly and heat the pads and pins evenly.

7. Solder Splash

Solder Splash

The solder spatters irregularly on the solder mask, forming a spider web-like appearance. Inadequate use of flux during the soldering process or the PCB board has contaminant will cause the irregularly shaped threads, which can sometimes cause short circuits.For the defect of solder spatter, we can use the way of heating the solder wire to stick out the excess tin as much as possible.

Soldering Iron is important for Hand Soldering

Soldering IronFor hand soldering, a good electric iron is the mainstay of the PCB soldering operation, it will make our soldering work easier and less prone to problems. Therefore, maintaining the electric iron is very important and necessary. Poor tip maintenance is one of the main reasons for poor manual soldering. Oxidation of the soldering iron tip or any contamination on the tip of the soldering iron will reduce the thermal conductivity of the soldering iron, thereby reducing the quality of the solder joint.

Before starting soldering, clean the tip with a cleaning pad. For severely oxidized tip, you can use a tip activation agent to repair it. After completion, the tip should also be tinned. Tinning the soldering iron tip means covering the soldering iron tip with a layer of solder to protect the soldering iron tip from oxidation and improve its thermal conductivity. After soldering two or three joints, clean the tip and then clean it again after each soldering. This can prolong the service life of the soldering iron and improve the quality of the solder joint!

Manual soldering is a skill, the more you practice the better! Before working on a project that is too valuable to break, you can do as many exercises as possible on an old circuit board or soldering practice board. Finding the most suitable way to hold the soldering iron will control the solder material and use the tip of the soldering iron skillfully to avoid mistakes.