1. The Selective Soldering Process
Selective soldering equipment is an alternative to masking off or using selective pallets when wave soldering only certain areas of a PCB.
The same 3 steps as wave soldering are still recommended when selective soldering equipment is employed, namely fluxing, preheating and soldering.
Instead of the full width solder wave used in wave soldering, selective soldering uses dedicated nozzle technology to pump small mini waves or fountains of solder, either through a single point to point setup or multi nozzle dedicated tooling.
Other methods for "selective soldering" are robotic selective soldering using various methods, including laser, hot flame, induction etc
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2. Selective Soldering Applications
Selective soldering machines are used on mixed technology PCB's where the amount of or positioning of through hole components makes standard wave soldering uneconomical or impractical. Although generally slower than wave soldering, the flexibility offered by selective soldering is resulting in a growing number of suitable applications. The technique is often used for soldering of connectors and high pin count through hole devices.
Robotic soldering is used for in situ applications where access to the joint is limited or if there is a requirement for a specific soldering temperature which requires alternate techniques.
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3. Selective Soldering Machine Types
There is a large number of selective soldering machine types available for specific applications and volume requirements.
Small table top mini wave systems are often used for rework of through hole components, allowing easy removal and repair. Other uses for this type of system are component tinning etc.
Batch selective soldering machines offer the ability to automate the soldering of the product and usually employ a single point to point solder nozzle technique. These systems also have local fluxing and preheat options to make the process easier. Batch selective soldering systems are best for low volume applications.
Inline selelctive solderingequipment is aimed at higher volume applications and often have external preheat and fluxing. To increase the throughput of these machines, the multi-nozzle single dip technique often replaces a point to point method for soldering.
Robotic soldering involves the use of a soldering iron/station on programmable X-Y gantry with computer control of soldering parameters.
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4. Troubleshooting Selective Soldering Problems
Selective soldering machines normally produce excellent results however they often face the same process problems as in wave soldering, and a lot of the skills required are the same. Some problems are descibed below:
Stringing/Webbing..
This symptom manifests itself by leaving a fine string of solder around soldered areas, often in the shape of the relvant solder nozzle. This is caused by insufficient fluxing of the area to be immersed in solder. Areas of solder resist around the joints which have not been fluxed will cause stringing.
Fillet tearing..
Fillet tearing or pad lifting is more prominent in selective soldering processes and is difficult to remedy on susceptable PCB's. The cause is due to larger thermal expansion properties and reduced preheat capabilities. When the solder makes contact with the joint, it results in large increase in local temperature inducing thermal shock issues. This problem can be overcome by careful use of any preheat caoability the selective soldering machine has ensuring that you are not shocking the pads and inducing lifting.
Copper Dissolution..
Copper dissolution is where the copper content of the pads dissolves into the molten solder. This problem can be more obvious in selective soldering and when using lead free solders, the reason being the potential for excessive joint dwell times together with higher molten temperatures of Pb free alloys. Therefore you must ensure that your soldering parameters are tightly controlled avoiding long contact times and excessive pot temperatures. A thicjer copper layer can help to minimise the problem on difficult devices.
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5. Expert Advice..
Please visit individual supplier websites for further detailed information. Browse manufacturers selective soldering machines here
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