Like Gulliver in Lilliput: reworking tiny components: rework of 0402 and 0201 chip components is not impossible.
Consider 0402 and 0201 component types--the name designation relates directly to the part's physical dimensions. For example, 0402 translates to 40 thousandths of an inch long by 20 thousandths of an inch wide. 0201 components are half that size! When component size is measured in thousandths, one can well imagine the possible complications associated with rework and replacement.
Traditional Removal Processes
Larger, more traditional chip components are removed in various ways familiar to most rework technicians: 1) the use of focused hot air and 2) the use of hot tweezer tools that grasp components with heated tips--after the solder is reflowed, the chip is easily removed. One may also use a special fork-shaped soldering iron tip. Each method has existed for quite some time, but extremely small components are difficult to remove with tweezer tools, due to the small size. Available hot tweezer tools simply cannot grasp such tiny, grain-sized components.
When working with the smallest of components, familiar and proven techniques--usually employed for handling larger chip components--must be discarded. For removal, hot air may still be used occasionally, although the problem of grasping and removing the component remains. I find that a soldering iron, with a very fine-point, curved tip, works well. Such tips are available from soldering tool manufacturers. The tips tend to oxidize quickly, due to their small shape, and need careful maintenance. The tips also require more frequent replacement than tips used in ordinary applications.
For removal, a small amount of flux is added to the component. The tip of the soldering iron is then applied. Wipe the iron tip across the top, or side, of the component--since reflow is almost instantaneous, the tip's tinning will bridge both leads and gently scoop up the component (Figure 1). Solder surface tension holds the component to the iron tip. The technique is simple, and, if many small chip components need to be removed from the same board, an operator can remove five to 10 components (they remain on the tip) before cleaning of the tip is needed.
[FIGURE 1 OMITTED]
Replacement is accomplished using a point-to-point soldering method. Prepare the site by wicking away excess solder from the pads. Then, add liquid flux to one of the two pads, while prefilling the other pad with solder. Next, place the component in position and hold steady with the fine-tip probe--doing so prevents the soldering iron from pushing the component out of alignment. Now, place the soldering iron tip at the junction between the prefilled pad and the component lead. The solder should flow instantly, allowing the component to drop into position. Remove the tip and wait a moment for the solder to solidify before soldering the other side of the component. Clean, if required, and inspect.
Reworking such small components cannot be performed with the naked eye. The procedure requires the use of a microscope that will allow the operator sufficient magnification for precision, comfort and clarity.
A bit of creativity is involved when working with 0402 and 0201 devices. Experienced operators suggest that a shift in thinking is required when learning the process of rework and replacement. Inexperienced operators, using heated tweezers, will struggle and may damage pads and do other harm to the board. Successful rework requires a less direct approach--an operator should allow the solder surface tension to pick up the component instead of grasping the component directly. Patience, gentle movement and a steady hand are most helpful tools any operator can possess.
Jeff Ferry is president of Circuit Technology Center, Hayer hill, MA; (978) 374-5000; www.circuittechctr.com.
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|Title Annotation:||Rework and Repair Depot|
|Date:||Jun 1, 2002|
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