Removing SMTs

 
This is called REWORK in the manufacturing trade, as the job has to be repeated. Reworking a job costs a company MONEY.

Therefore it’s important to get things right every time by monitoring QUALITY at all stages of assembly to REDUCE the necessity for rework. 

EXCESSIVE HEAT during rework may damage the:-

  • Components

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  • The PCB's PADS

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  • De-Laminate the Fibre Glass PCB itself i.e. a bulge appears in the PCB due to trapped gases expanding. 
Therefore priorities have to be established when reworking as to which is more important, saving the COMPONENT or the PCB’s PADS.

The PADS are usually treated MORE IMPORTANTLY than the COMPONENTS. PCB repair is always MORE DIFFICULT and TIME CONSUMINGwhen compared to the cost of purchasing new components.
 

 
Removing a 2 Legged SMT Device


Removing Multi Pinned SMT Devices

There are several options, but the CHEAPEST are discussed below:

  • Using a HEAT GUN

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    • This may cause DAMAGE to the surrounding components and PCB if TOO MUCH heat is applied. This can be a VERY EFFECTIVE method if the surrounding area is PROTECTED from the heat. This can be achieved by either using special HIGH TEMPERATURE SELOTAPE availalble or by keeping the Heat Gun IN MOTION over the PCB area to control the temperature.

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    • As long as care is taken to LOCALISE heating, the only PROBLEM will be that the solder on the ADJACENT SMALL COMPONENTS may ALSO become MOLTEN. They may even get BLOWN AWAY! However as long as the heat gun has a WIDE funnel of air, the air pressure shouldn't be enough for this to occur.

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    • Once the solder is MOLTEN the component can be either taken off with TWEEZERS or more easily by TAPPING the PCB on the bench. However BEWARE, this may also DISLODGE other adjacent components that have molten solder.

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  • CUTTING the chip's LEGS OFF

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    • This method involves sacrificing the component, to save the PCB:-

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    • Cut all the legs off the component using either a fine pair of SIDE CUTTERS or if the legs are very small then a SHARP KNIFE can be used.

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    • Great CARE is required in not DAMAGING the PCB's PADS when pressing down with the knife.

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    • Using SOLDER WICK, prepare the pads for a new component by applying the SOLDER LEVELLING TECHNIQUE described previously.
Of the 2 methods described above the HEAT GUN method has an advantage that:-
  • It's LESS DAMAGING to the PCB's pads from the SHARP KNIFE as long as the Heat Gun is kept IN MOTION as described above.
And Finally:-
    • Another popular method of rework is to use a proprietry product called LOW MELT™. This is a special LOW MELTING POINT solder but DOES REQUIRE extra equipment support so isn't discussed here in detail. Further information is available from the manufacturers at www.zephyrtronics.com.