1. The Reflow Process
Standard SMT solder paste reflow is achieved by transporting the PCB or substrate through a series of temperature controlled zones which gradually increase the temperature of the PCB and solder through a programmed temperature profile to achieve optimum solder paste melting and joint formation. This reflow profile is determined by the thermal mass of the PCB and by the specific chemistry of the solder paste or adhesive used.
Other processes using reflow ovens are curing of adhesives and component and PCB baking to remove moisture.
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2. Reflow Applications
The primary function of standard pick and place systems is to populate PCB's with components prior to reflow. However a number of special applications are being required of the systems as technology advances, these include semiconductor applications such as die placement, package on package etc aswell as placement of membrane switch assemblies. To enable these applications, engineers are having to design very specific feeder options thus increasing the potential uses for the equipment.
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3. Reflow Oven Types
The majority of SMT reflow ovens now make use of forced convection technology to achieve the best results. Forced convection reflow ovens use a fan to circulate the hot air within the specific zone in question. This ensures that the heat from the element is evenly distributed and produces even heating for the PCB.
Previously IR heating was often employed, this is being superceded now due to limitations of the IR process with regard to heat variations depending on colours of components and PCB's reacting differently to IR radiation.
Reflow ovens come in many different zone configurations. Ovens with a large number of zones are normally used for higher volume manufacture as you can get more product through the oven for a given profile.
Flux extraction systems are now often employed in reflow ovens. These systems use a number of techniques to remove flux volatiles from within the oven which are generated as the flux in the solder paste is evaporated. This reduces the cleaning required as less flux condenses on the oven interior surfaces.
Nitrogen is also sometimes incorporated in the reflow oven to aid in the soldering process.
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4. Troubleshooting Reflow Problems
A number of factors are in play during the reflow process, all of which must be correct to ensure that the end result is as expected. Some of the common problems you may come across are listed below:
Tombstoning..
This is when components leave the reflow oven standing on their end or billboarded. This defect is caused by uneven wetting of the component terminals, resulting in one end soldering before the other.This is commonly caused by poor profile, bad pad design, excess solder and even bad component wettability.
Solder Balls..
Small beads of reflowed solder around or at the edge of a component are referred to as solder balls. This defect is caused by excess solder coalescing and squeezing out during reflow.Therefore you must find out the reason why solder is inder the component, this is usually due to problems with printing, pad design or placement. Although PCB's which have previously been poorly washed down are another cause of this defect.
Failure to reflow..
The most obvious cause of a non-reflow of paste is that the temperature of the oven has been incorrectly specified. Solder paste needs to be raised above its liquid temperature to reflow and then form a joint by wetting both surfaces to be joined. If the temperature is not reached or not held for sufficient time the paste will not completely flow. This is often caused by the wrong profile being selected.
Other reasons for non reflow are, putting PCBs which have been left for a long time before reflow which causes the paste to dry out and lose its flux strength and the use of the wrong type of solder paste ie Pb free.
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5. Expert Advice..
Please visit individual supplier websites for further detailed information.Browse manufacturers reflow ovens here
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