When soldering a PCB, a basic training and understanding of the soldering process is essential. With advancement in electronics, the requirement for more compact components significantly raises the chance of soldering issues.
Hand soldering skill has always been an important capability for any professional in the field of electronics. But no matter how simple it seems, soldering can be a complicated skill to master. Although it is possible for anyone to hold a soldering iron and work it around a Printed Circuit Board, whether or not you get reliable solder joints is a different matter altogether.
To build an acceptable functioning assembly you need to observe basic rules:
*Your PCBs and soldering surface should be kept clean.
*Your iron should be at the correct temperature.
*The tip should be ‘tinned’ with fresh solder for every use.
*Observe the need to transfer enough heat to prevent ‘dry’ joints
Your solder joints should effectively provide both an electrical and a mechanical connection between the board and the part being soldered. Far better than learning to rework, is to make it right first time. Statistically, reworked boards are more likely to fail in service than first-time-right assemblies. Therefore there is a cost and a reliability penalty for not getting it right first time.
If your PCB is going to be used for a life critical application, it becomes more crucial than ever to know what mistakes need to be avoided while soldering. Some assemblies for very high reliability applications such as space and military products are not permitted to be reworked at all. Here, the use of properly trained and fully equipped staff is essential
If rework is acceptable, here are some of the most common mistakes, and how to fix them;
1. A disturbed Solder Connection.
A movement of any kind while the solder is solidifying may cause a disturbed joint. It is easily identified by a rough, frosty or wavy pattern on the joint surface.
To fix this, simply flux then reheat the solder joint and allow it to cool down properly. To avoid this in future, try to find the source of movement and take necessary action to prevent it occuring again (for instance with jigs, or with procedure changes).
2. Insufficient Wetting
This happens because of a reduction in heat or through contamination of the solderable surface. It could also be an operator error, that there wasn’t enough solder fed onto the joint. Here, experience plays the key role in determining the right amount of solder to use.
A dirty soldering surface can also be the cause behind insufficient solder in joints. Unclean surfaces cause solder to poorly bond to the pad or component, which in turn results in poor solder joints. Often these joints may not fail during production, but can cause a latent failure in service.
To prevent this, make sure your work areas are free from anything not required to work on the product, thus avoiding contamination. Always clean and re-tin your tip with solder before placing it back in the holder, to prevent oxidation.
3. Excessive Solder
In general, beginners or poorly trained operators make this mistake in a bid to avoid insufficient wetting (adding extra ‘just in case’). However, when too much solder is used, the joints are deeply covered, and proper inspection is not possible: you cannot see whether or not your component and pads are connected properly. Consequently there remains a possibility that neither the component nor the pad are properly wetted. To make matters worse, it also increases the probability of forming solder bridges that could damage components or PCB if not rectified.
A solder joint with the correct amount of solder and a concave surface allows better access to inspect the wetting of the joint. In the end, experience and judgement would be used to decide the optimum amount of solder. Training is useful especially when new products with different parameters are brought to the production line.
4. Solder Bridging
A solder bridge is one of the by-products of excessive solder. An excess amount of solder flows between neighboring joints on a board and creates a solder bridge, alongside or possibly under a component. To rectify this you need to use a vacuum desoldering iron or copper braid to remove excessive solder from the joints.
5. Cold Solder Joint (dry joint)
A cold solder joint occurs when the solder and PCB are not heated to the required temperature. Cold joints are typically identified by a rough and unsmooth surface. Cold joints are problematic because they are incapable of fulfilling their intended purpose, yet may well pass electrical test and appear ready for shipment.
Cold/dry joints might provide electrical connection for a while, but the mechanical stability will be poor: cold joints lack sufficient mechanical bonding ability and tend to develop cracks over time.
To correct this, simply flux and reheat the solder to the appropriate temperature and this will rectify the joint. To prevent this error in the future, always ensure that the soldering iron is at the right temperature. Additionally, ensure that the soldering iron tip is always left coated after use, then preheated and coated with fresh solder before next use (tinning). This thin layer of solder coated around the tip will provide a much better transfer of heat from the tip to solder joints, helping prevent cold joint issues.
6. Overheated Joint
Just as too little heat will cause poor joints, too much heat will also cause some serious issues. Overheated joints often have a burnt look with typically a lumpy appearance. Also look out for PCB issues such as delamination, blistering, measles and lifted pads & tracks as indicators of overheating at the joint.
To prevent this problem in the future, make sure to heat your iron to the correct temperature to match the solder alloy and PCB.
CONTACT ART TODAY.
It requires professional training to perform these procedures repeatably and to the highest standards, Advanced Rework Technology Ltd (ART) can help you with well designed training courses. We can teach you the skills required to perform to this level. We also offer bespoke courses for specific challenges you face in your production (please enquire), and industry certified training for inspecting the acceptability of all PCBs produced.
Whether you come to ART’s own facilities in Witham, or we bring the equipment to your facility, we can teach you all you require. Please call us on 01245 237083