How to successfully avoid soldering nightmares

1) What are the consequences of not keeping the soldering joints sufficiently clean?

All solder wires initially have an integral flux content, typically two to three percent. Post-soldering, some of this content will have been burnt away. However, upon completion, the soldered joints will have a light coating of flux residue.

How visible these residues are will depend on the type of flux used, temperature and chemicals used;  e.g. Rosin based residues will have a light amber colour so will be easier to identify than a more modern, no clean flux (clear residues). Most modern fluxes can be left on the PCB indefinitely without any short/long term detriment to the product.

However, there are instances when the flux residues need to be cleaned off the PCB, for example, when the assembly needs to be conformally coated or encapsulated.  Failure to clean the flux residues from the PCB may then result in the coating/encapsulation materials struggling to adhere to the surface of the soldered joints. Also, if Automatic Test equipment is to be used during manufacture, heavy flux residues can clog test pins leading to ‘false fail’ errors.

2) What is a solder bridge and why should it be avoided? What can happen as a result of a solder bridge forming?

A solder bridge is solder content that unintentionally shorts two or more adjacent leads and/or PCB pads. The presence of a solder bridge will usually compromise the intended electrical circuit leading to failure during test. In a reflow soldering process, there are numerous measures that can be taken to avoid bridging such as altering the aperture width, area ratio and thickness of the stencil, adding solder mask between the PCB pads and optimising the amount of solder paste in the screen printing process.

Too much solder paste will inevitably lead to solder bridging. In a wave soldering process ensuring that sufficient liquid flux is applied and optimising the top side PCB temperature before contact with the solder wave will both significantly help in the elimination of solder bridges. Whilst in a hand soldering process lack of solder bridges is very much due to the skill and training of the Operator. Applying too much solder, and at the wrong time, will inevitably lead to solder bridges.

This is never more clearly demonstrated than during a drag soldering process where the speed of movement of the soldering tip and the rate of feed of the solder wire are critical in producing good quality solder joints, without bridges.

3) What are solder balls and how are they formed – i.e. what are the most common causes?

As the name would suggest, solder balls are tiny spheres of solder alloy. Amongst other reasons, these can be caused by rapidly heating any moisture or flux content present. If not stored correctly, over time, moisture can be absorbed into a PCB.

Another source of moisture could be the flux present in any soldering process. In hand soldering, the application of a soldering iron at 380 °C (typically) can inevitably cause the water/flux content to boil thus producing a violent reaction which will disperse the molten solder in and around the intended solder joints. Because of the surface tension of the molten alloy these smaller amounts of solder will manifest themselves as solder balls.

4) How do overheated solder joints appear and how can these be avoided? Is it possible to completely ‘kill’ the PCB as a result of overheating a solder joint?

When too much heat is applied to a solder joint this may or may not be immediately apparent. Initially the flux residues are likely to caramelize and harden, giving a poor visual appearance. The next scenario would be to produce a joint with an intermetallic layer, which is too thick.

Whilst not apparent to the naked eye, this would produce a brittle joint which is likely to result in a field failure. If there are any concerns regarding this phenomenon, then specialist advice must be sought and cross-sectional analysis will confirm or allay any fears.

Ultimately, if a really excessive amount of heat is applied this would potentially damage or destroy both the component and PCB, e.g. Lifting of PCB pads/tracks.

5) What is a tombstoned component and how do these appear? Is it possible to avoid this phenomenon?

Tombstoned components are when one side of a device is soldered to the PCB and the other is not in contact with the PCB. It is a phenomenon more associated with a reflow soldering process and is caused by the surface tension of the molten solder lifting one end of the device. If there is any imbalance in component alignment the forces exerted on each end of the device would be unequal, thus lifting the end with the least force.

With demand ever increasing for smaller size and greater mass of components tombstoning is a very real concern because it produces a disconnection in the electrical circuit.

There are a number of factors that can cause Tombstoning including disproportionate solder paste deposits, thermal inequalities between solder joints, incorrect pad designs – either too big or too small, and ground plane circuit board design.

Fortunately, Tombstoning can be avoided by implementing a number of solutions such as revisiting the stencil design, adjusting the reflow profile to distribute heat more evenly over the components, double checking pad dimensions, component placement and routing. Understanding these causes can help control and prevent tombstoning, which is critical for successful PCB assembly.