Why touch-up should be used sparingly, if at all, and other helpful hints. 

We rework and repair printed circuit boards every day. From our experience, here are the 10 most common rework mistakes – and their solutions:

1. Mistake: Trying to fix what is not broken.
Fix: Do not rework a good solder joint that just “looks funny.” Know the IPC-A-610 workmanship standards and the class being inspected to. Do not try to fix a solder joint that passes those standards. Adding another heat cycle to make it look prettier is not good for long-term reliability.

2. Mistake: Excessive pressure or pressing down with the solder iron.
Fix: Use light pressure only. You cannot drive more heat in by pushing the tip into the pad. Doing so only increases the chance of lifting the pad or measling the board.

3. Mistake: Using a higher temperature to solder faster.
Fix: Cranking up the heat does not necessarily make the solder flow better. In fact, if you crank it too high, it will reduce the flow because the tip oxidizes too quickly, and the higher temperature burns off the flux too soon. The best way to get maximum flow and hole fill is to combine the right temperature (generally 700°-770°F) and dwell time (generally 2-5 sec.).

4. Mistake: Lack of proper ESD protection.
Fix: Maintaining the same electrical potential between the operator, work surface and ESDS device is critical. Simply wear a wrist strap and use an ESD table mat that is properly connected to a common point ground. What good is it to fix a defect perfectly but cause a latent ESD failure that will manifest itself in the field? Proper ESD protection is simple and inexpensive, so be sure to get grounded.

5. Mistake: Improper use of flux.
Fix: Before soldering, ensure appropriate flux is being used. This will eliminate oxidation, which harms solderability. It also will add wetting and improve your technique. Additionally, remove the flux with an appropriate cleaner to eliminate the possibility of dendritic growth.

6. Mistake: Improper tip size.
Fix: Select the largest tip that will not extend over the annular ring. It takes a few extra seconds to change the tip, but it pays off in better workmanship.

7. Mistake: Not cleaning and shocking the tip.
Fix: Every solder station should have a wet, sulfur-free sponge to clean, and be sure to thermal shock the tip immediately prior to soldering. This will help the solder flow, aid in heat transfer and prevent FOD.

8. Mistake: Not cleaning after rework or touchup.
Fix: Always clean the flux from hand-soldered PCBs, even if no-clean flux is used. No-clean flux only can be left on when it is fully cured. In hand soldering, unlike in the reflow oven, all the flux is not cured, so ionic contamination or dendrite growth will occur.

9. Mistake: Collateral damage.
Fix: With today’s densely packed PCBs, operators must be extra careful not to displace or reflow adjacent components. It even may be necessary to tape off adjacent components to protect them.

10. Mistake: Improper training.
Fix: Get the proper training for the job. Owning a solder station does not make you a rework technician. A one-week IPC-7711 class provides the skills to fix almost any PCB rework problem.

Bob Doetzer is president of Circuit Technology (circuittechnology.com); bob@circuittechnology.com.

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