Drag soldering is a simple method of soldering fine pitch surface mount components without expensive equipment. This video demonstrates the process:

Technique

High quality surface mount soldering without solder bridges requires two key ingredients:

• Correct amount of flux
• Correct amount of solder

Flux improves solderability. Without flux, metal surfaces quickly oxidize and cannot be soldered. In traditional soldering, parts are first heated with an iron and solder is applied to the parts. Flux from the solder core surrounds metal surfaces and removes oxide. Insufficient flux causes solder to appear dull, fail to flow easily, and exhibit poor wetting. With the proper use of flux, solder will flow smoothly and harden with a shiny surface.

The following video explains the wetting forces and their effects on the quality of solder joints:

Flux in the core of common electronic solder works well when applied to parent metals. When solder is melted on an iron, the flux is quickly lost. For this reason, flux core solder by itself is inadequate for drag soldering. Flux must be manually applied using liquid applicators, flux paste, or flux pens.

Excess solder can cause bridges on fine pitch components. This excess solder can be removed using solder wick, but the wick might remove too much solder leaving the joint weak and unreliable. With experience, the correct amount of solder will become apparent. If a solder bridge does occur, it can be removed cleanly with a brushing motion of a soldering iron tip. If the solder bridge is not easily removed, more flux may be required to improve flow.

Flux selection

Several classes of flux exist. A highly active flux is very effective at removing oxide, which is a desirable characteristic. Flux can leave residues which damage components or introduce undesirable leakage currents. Correct selection of flux is important to high quality solder joints.

• Inorganic acid flux is used in plumbing and metal work, but rarely used for electronics. Inorganic acids are very active, but can damage sensitive electronic components.
• Rosin flux is widely used for electronics and further categorized into three classes:
  • Rosin Activated (RA) is most active of rosin fluxes, but requires thorough cleaning
  • Rosin Mildly Activated (RMA) is not as active as RA flux, but cleaning is not always required. It may attract dust and other contaminants. Cleaning is recommended in sensitive applications. Kester 186 is an example of RMA flux.
  • Rosin (R)
• Organic acid flux is more active than rosin, but not as active as inorganic acids. Organic acids are water soluble and require cleaning to remove undesirable residues. Kester 2331-ZX is organic acid flux.
• No clean flux tends to be inert and does not attract dust or other contaminants. These fluxes tend to be the least active of all and contain fewer solids. Solid content can be helpful, so this type of flux is not always desirable. Kester 951 is an example of no-clean flux.

Overall, RMA flux is a good tradeoff in terms of activity and cleaning requirements.

Procedure

1. Apply adequate flux to completely cover component leads and pads. Flux pens are suitable dispensers and available at low cost.
2. Tin soldering iron tip and clean thoroughly to remove contaminants.
3. Apply a modest amount of solder to the iron tip. Less solder avoids solder bridging. Experience helps in determining the proper amount.
4. Apply light pressure to the toes of surface mount component leads and drag at a modest pace. Solder should flow smoothly and leave a shiny surface.

If too much solder is used, bridging of pins might occur. Bridges can be removed with more flux and a brushing motion of the soldering iron tip. If insufficient flux is used, solder will flow poorly and tend to bridge pins.