Rework and repair.

1. The number one rule of rework is patience. Whether reworking through-hole or surface-mount devices, take time to perform the task properly. An impatient operator can turn a simple rework operation into a major repair or scrap. Observe complete solder melt before trying to remove the component.

2. Before starting rework, make sure all of the tools and equipment needed are available at your facility. Do not invite failure; even the most highly trained and experienced personnel are unable to perform acceptable rework without the proper tools.

3. Do not put any downward pressure on the pad surface. Having the soldering iron temperature set too low will cause the connection to heat slowly. The operator then bears down on the iron tip to help the connection heat faster. Typically, the soldering iron tip temperature should be set between 600 and 700[degrees]F. An oxidized iron tip is another culprit; it causes the tip to transfer heat inconsistently and in some cases not at all. Again, the operator's tendency is to bear down on the tip. Maintaining a clean, well-tinned tip will help improve the transfer of heat. Remember, nothing solders like solder.

--Training Department, Soldering Technology International, [e-mail]

4. Verify the operational temperature of the rework unit. Many rework units' temperature indicators will show the operational temperature to be within the manufacturing specifications. In fact, most are running much hotter than indicated, which may cause chip damage that cannot be detected immediately. When the product experiences temperature and/or humidity fluctuations, the repair will fail. Take a syringe with eutectic solder and place a spot about the size of a small pea on a scrap circuit board. Put the board and paste under the rework tool with the temperature set at 300[degrees]F. See if the solder melts. If not, keep increasing the temperature of the unit in five-degree increments until the solder melts. When it does melt, note the indicated temperature and adjust the unit to reflect the true temperature.

--Bill Scheu, A.P.E. South, [e-mail]

5. A reflow profile for a ball grid array (BGA) rework procedure may be developed without drilling the PCB. After selecting a specific BGA component to be removed from the circuit board, place fine 36AWG thermocouples at the solder ball level by feeding them underneath the BGA. This practice negates drilling the circuit board for underside thermocouple attachments. Several thermocouples at various spots underneath the component should be used.

6. To ensure that a BGA or chip-scale package (CSP) is reflowed properly on a consistent basis, the following must be observed. First, the nozzle should be positioned around the reworked component so that an even clearance exists from all four sides of the component body to the nozzle wall. Second, a reflow profile must be optimized for a specific component package at a specific location on the printed circuit board (PCB). Finally, rework is time consuming; always follow proven step-by-step rework procedures.

--Steven Pitkis, Advanced Techniques US Inc., [e-mail]

7. When reworking odd-shaped components or radio frequency (RF)/electromagnetic interference (EMI) shields, consider using a convection rework system. Make sure to consult the applications engineering department on the availability of custom reflow nozzles. A convection system and properly designed reflow nozzles will be safer for the PCB during rework.

8. Remember to use gel or tacky flux during the removal cycle. This practice will help protect the solder that is left on the board and makes reattaching shields after the repair is completed much easier.

9. Do not forget the importance of bottom-side heating. With the increased mass and shields' ability to dissipate heat, a subzone heater can help reduce dwell times and eliminate board damage.

--Ed Zamborsky, Metcal, [e-mail]

10. Use a soda can to "prove the process" when experimenting with fixtures for the rework of odd-shaped RF shields. Consider this situation: The problem a manufacturer faced was applying heat from the underside in a local area without heating the entire circuit board to extreme temperatures and causing damage to other devices. At the same time the board temperature had to be at an acceptable level to prevent the board from delaminating. Due to the odd shape of the shield and the complex thermal properties of the circuit board, determining exactly what configuration of bottom heater was best for this application was difficult. A solution was to remove the bottom of a soda can with cutting shears, and an appropriate size hole was cut from the top of the can. By experimenting with the size and shape of this inexpensive tool, a more defined fixture could be designed for production work (see photo below).

--Larry Sirois, ViTechnology [e-mail]

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