March, 2013
www.us-tech.com Reduced Cost Micro Array Component Rework Continued from previous page
full array of solder joints to achieve reflow simultaneously, yet in such a way that does not sacrifice the in- tegrity of the component, the board or any adjacent components. This is only achievable with a rework sys- tem that provides even heating on both the top and bottom sides of the board, with the heat flow contained within the space occupied by the de- vice and applied in such a way that it minimizes any temperature gradi- ents across the component. This calls for low-pressure convection airflow that heats through the body of the component. This should result in the simultaneous reflow of the joints that, combined with automatic vacu- um part removal, reduces the proba- bility of lifting pads off the board. For each combination of PCB
and array component, an appropri- ate reflow profile must be estab- lished. Careful profile setup is the key to successful reflow, and the proper placement of thermocouples during testing is critical to ensure uniform temperature across the component and the PCB assembly. Board profiling must be done on a completely populated PCB with new thermocouples, a calibrated record- ing device and placement of the thermocouples in both high and low thermal demand areas of the board. The rework equipment should allow the operator to monitor tempera- tures beneath the component body as well as the output temperatures of the system’s heaters.
Site preparation: After component removal, the pads need to be prepared for the replacement component. The residual solder should be removed with a solder wick saturated with paste flux, used in conjunction with a direct power soldering iron equipped with a hot blade tip. The operator must be careful, keeping the wick be- tween the tip and the board. The old solder can be removed quickly without risking pad damage that is likely to re- sult from direct contact between a hot tip and the board. Subsequently, flux residue on the board should be re- moved with an approved solvent, leav- ing the land area clean and ready for the solder paste or flux application.
Solder paste or flux deposition: In the case of LGAs, it is necessary to deposit solder paste on the pads in preparation for reflow of the replace- ment component. The deposition of the paste or flux can be achieved by using a single-component stencil plate. This allows for deposition on the bottom of the component without worry of adjacent components; such a product can be used with standard assembly solder paste.
Placement: The placement of BGAs and LGAs in the rework process re- quires the simultaneous viewing of pads and leads at an adequately high magnification (i.e., 30-50X). Once the images of the pads and leads have been aligned, the component is placed onto the board. For most LGAs, view- ing at 25X magnification should be sufficient; however, 50X magnifica- tion is required with µBGAs because of the smaller size of these packages to ensure that the balls and pads are properly registered and aligned.
Reflow: As previously mentioned, it is critical to apply a uniform, repeat- able reflow profile. Profile setup is the key to successful reflow, and the
proper placement of thermocouples during testing ensures uniform tem- perature across the component and the PCB assembly.
Process Control The rework equipment must
provide all the necessary functions described in the rework process (see above). The level of process control is especially critical in the reflow proce- dure, whether in the removal or re- placement mode; although improper site cleaning, solder paste deposition and component placement can be cor- rected by repeating the pro cess. Poor process control in reflow can destroy the component, adjacent components or the board.
The equipment should offer the
appropriate balance between the lev- el of process control provided and the ease and simplicity of equipment op- eration. A graphical interface en- ables the operator to visualize the temperature parameters during re- work and save the actual process graph for verification and future analysis. Additionally, it is impor- tant that operators and technicians be properly trained and have the equipment available to properly identify any defective components.
Conclusion All the factors that make the µB-
GA package more attractive than pe- ripheral-leaded devices in the assem-
bly environment carry through to re- work. For example, the ease of solder or flux deposition on a µBGA pad pat- tern and the ease of alignment and placement of µBGAs with image over- lay technology makes µBGA rework easier in comparison to fine-pitch QF- Ps. With the introduction of high- quality, lower-priced convection array rework systems, users have the bene- fits of an easy-to-use system combined with the technology and process con- trol to ensure repeatable results. Contact: OK International, Inc.,
12151 Monarch Street, Garden Grove, CA 92841, % 714-799-9910 fax 1-714-799-9533 E-mail
info@okinternational.com Web:
www.okinternational.com r
Page 33
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