| Posted by Bob Willis on 10 January 2007 at 14:10
|
For many years my preferred method of component removal for passive and small active rework has been a hot air pencil, and it can still be very effective. Having one tool to remove and replace surface mount components is simpler and cost effective, even for small passives like 0201 packages. Many people say you can’t use hot air; you blow the parts off the board. The simple solution is to turn down the air pressure--it’s set too high!
As we move to lead-free assembly and rework, the use of contact tools proves faster in production but are best suited for removal only. Modified soldering iron tips and contact tweezers are two types of equipment often used in production. Each rework technique is covered on wall charts and interactive CD-ROMs available from the Global SMT & Packaging web site. The following procedures outline the normal steps taken during hands-on operator training and certification for removal of SOICs with tweezers and with a modified soldering tip.
1. Removal of Small Outline Integrated Circuits (SOICs) components using conductive tweezers
Flux joints prior to component removal; this helps the solder reflow on the pad and reduces spikes on the surface of the pads when the component is lifted from the surface of the board. It also leaves solder on the pads, making replacement with another component simpler.
Select the proper size removal tips for the component. It’s very important on multi leaded parts that the tool tips selected contact each of the terminations. Clean and re-tin the tips--this will provide better contact with the terminations and speed up both the heat transfer and the complete rework process. Any contact tool is the fastest method of removing components with lead-free alloy but requires more skill and experience on the part of the operator.
Gently grasp the component terminations, ensuring good contact until solder reflow is observed. This can take 5 to 15 seconds, depending on the component type and, of course, the thermal demand of the board construction. Remove the component using the tweezers--be careful not to lift or twist the component until the solder has reflowed or the pads may be lifted.
Note: If the component is held in place using adhesive for wave soldering a slight twisting action may be required to break the adhesive bond. This must be applied only after reflow of the solder.
Inspect the pads on the surface of the board for any damage or lifting.
Clean the rework area if necessary.
2. Removal of SOIC, Quad Flat Pack (QFP) using modified soldering iron tip
Flux joints prior to component removal; this helps the solder reflow on the pad and reduces spikes on the surface of the pads when the component is lifted from the surface of the board. It also leaves solder on the pads, making replacement with another component simpler.
Select the proper size removal box tip for the component and tin the tip. This allows good contact with all the leads and faster reflow of the joints. It is very important that the tinned tip makes contact with all the terminations; otherwise pads will be damaged. Any contact tool is the fastest method of removing components with lead-free alloy but requires more skill and experience on the part of the operator.
Contact the terminations with the tinned tip and wait for reflow to take place. Do not apply any pressure to the component. It is more difficult to see reflow with a box tip; hence this method is very dependent on operator skills.
Remove the component using the special tip. The solder on the tip will hold many components in place due to surface tension. It is good practice to have tweezers ready to assist lifting the part off the board surface. As the components increase in size there is more likelihood of parts dropping.
Inspect the pads on the surface of the board for any damage or lifting.
Clean the rework area if necessary.
Bob Willis will be running the “Process Advice & Defect Clinic” a special feature at APEX 2007 in California sponsored by IPC and Global SMT magazine. Bob will be providing FREE advice on process and production issues including lead-free at APEX 2007, visitors will be able to bring process problems and printed boards for examination and discussion during the show. Engineers intending to visit the show can also email questions to be answered by Bob at APEX by visiting the web site www.goipcshows.org |
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