Tips and Tricks for Successful Rework

Although soldering is one of the oldest known joining techniques, manual or hand soldering is still a process highly dependent upon individual operator skills. While soldering and rework have become increasingly automated, we nevertheless operate in an imperfect world, and defects in the mass soldering process will occur.

Despite attempts to automate processes, faulty solder joints resulting from improper soldering or rework remain a major cause of equipment failure. Thus, the importance of using techniques that provide high-quality results while exposing the solder connection to the least stress continues to be a major focus.

Many elements affect the hand-soldering process. These elements may range from the proper use of flux, wetting, controlling heat, soldering iron tip geometry, thermal mass, thermal linkage, and technique.

Sometimes, a little innovation and process tweaking are in order.

Fine-Pitch Gull Wing Soldering
The following technique is often used on gull-wing leaded components with delicate fine-pitch leads. The following procedure works best.

Once the pads have been cleaned and prepared, apply liquid flux to the corner pads.
Place the component in position and align the pads.
Placing the soldering iron tip at the junction between the pad and component lead, at one of the corners, and solder in place to secure the component.
Wait a moment for the solder to solidify before soldering the opposite corner.
Place a length of small-diameter solder along the edge of the component leads.
Place the solder iron tip against the solder in line with the tip of the first component lead to be soldered. As the solder melts, a uniform amount of solder will flow and create a consistent solder joint.
Move the solder tip down the line, one lead after another, until all the leads along that side are soldered.

Auxiliary Heat Desoldering

Figure 1: When using auxiliary heat desoldering, place a soldering iron tip against the lead on the component side of the circuit board while desoldering the joint from the solder side.

This technique is often used on through-hole solder joints with a large thermal mass. This scenario is most common on multilayer circuit boards.

Apply a small amount of liquid flux to the solder joints of the component to be removed.
Place a soldering iron tip against the lead on the component side of the circuit board. (See Figure 1).
Align the desoldering tip with a component lead end and lightly make contact with the soldering joint.
After the solder has melted, begin a rotating or oscillating motion with the desoldering tip.
Continue the rotating motion until a change in the feel of the rotating motion occurs.
At this instant, the solder in the solder joints is completely molten. Immediately activate the vacuum, extracting the solder from the solder joint.
After the solder has been extracted from the solder joint, remove the desoldering tip and the soldering iron tip from the component lead.
Desolder each of the remaining components leads individually using a skipping method to reduce thermal buildup at adjacent hole locations.
Probe each component lead to ensure they are not soldered to the side of the plated hole, and then remove the component.

BGA Dog Bone Masking

Although challenging, BGA rework can be routine. A key to ensuring a successful outcome is control of the solder volume. During the original board assembly process, a solder mask was applied between the BGA pad and the connecting via, which acts as a solder dam to prevent the solder from flowing down the connecting circuit and into the via.

This technique covers the preparation of these pads to control solder flow:

Carefully inspect each dog bone under a microscope to determine if a solder mask is needed.
Carefully scrape away any loose solder mask and the solder connecting the BGA pad to the via. The solder must be removed to ensure that solder will not flow down the tinned surface onto the connecting via.
Apply a small amount of high-strength epoxy to seal the exposed copper. See (Figure 2).
Once completed, the BGA can then be processed normally, ensuring that the proper volume of solder will remain where it should.

Several members of the Circuit Technology Center team contributed to this feature story.