Circuit Board Repair & Prototyping

If it is not fully activated (brought up to solder temperature), you should not leave any ionic residue behind. You might have to clean if...
1. Flux is being used so liberally that it all isn’t being heated along with the solder.
or
2. The non-ionic residue can impede with the function of the devise either short-term or long-term.

CircuitWorks® flux pens are available with a variety of fluxes to fit your specific shop requirements, including: no-clean, high temperature no-clean for lead-free soldering, rosin, and water soluble. Flux pens are an excellent way to dispense flux for benchtop soldering. You just hold the pen vertically and briefly depress the tip to start the liquid flow. This will saturate the tip of the pen with flux. Draw flux on the area to be soldered. Gently press the tip again when more flux is needed to keep the tip damp with flux.

A common method of repairing a broken trace is to solder on a jumper, which is basically a wire bypass around the broken trace. This can be time-consuming and visually unappealing. Chemtronics offers CircuitWorks® Conductive Pens, which contain a highly conductive material like silver or nickel suspended in a liquid polymer. These pens allow you to literally redraw the trace.

Chemtronics offers CircuitWorks Overcoat Pens in a variety of colors to match the PCB resist. Overcoat pens are basically acrylic conformal coating in convenient packaging, and the clear version can be used to coat small areas. Simply press down the pen tip and squeeze the barrel to dispense the coating material. 

Chemtronics offers the CircuitWorks® Conformal Coating Remover Pen that allows you to remove a tight area of coating around a repair area without affecting the rest of the PCB. You first saturate the tip by tapping it lightly on a surface, which opens the valve and releases solvent. Holding the tip down may oversaturate it, which could lead to solvent flowing into unintended areas.

Desoldering braid (wick) is made of fine strands of ultra-pure copper woven and coated with flux. Its geometrically precise weave design allows for maximum capillary action and solder capacity. Soder-wick optimizes heat transfer through the braid and into the solder joint, resulting in faster wicking action than any other competitive brand. Minimal flux residue on the board speeds up the cleaning process or eliminates it entirely.

1. Place the braid over unwanted solder, preferably on the greatest solder build up so that it maximizes the contact of the braid to the surface area of the solder.
2. Next, place your iron tip over the wick at 45 degrees and allow heat to transfer to the pad. Molten solder will absorb into the braid.
3. Move the solder tip and braid as needed to remove all of the solder at one time. Careful not to drag the braid over the pads, which can scratch.
4. Once the braid is full of solder, you must trim the spent portion and move to fresh braid in order to pull more solder. Remove the iron and braid simultaneously to avoid soldering the wire to the board.

Yes. As the copper is exposed to the open air, oxygen and moisture in the air interacts and oxidizes the copper. This oxidation is most apparent by the color of the wick. The brighter the copper, the fresher the wick. Oxidized copper will darken like a dirty penny. The more oxidized the copper, the slower the wicking action. We generally recommend replacing wick after 2-years, but that can be longer or shorter depending on storage conditions. Keep the wick bag or can sealed for maximum product life. If you have wick that appears to be too oxidized to use, unroll around 6”, trim off, then test the remaining wick. The outside of the spool of wick becomes oxidized first.

The temptation is to desolder an area and keep moving up the spool of braid. However, it’s best to work towards the end of the braid to isolate the heat. Once the desoldering braid is brought up to soldering temperature, the flux has been fully activated, so that part will not draw more solder. A long strand of used braid only acts as a heat sink, slowing down your process.

Desoldering braid or “wick” is a pre-fluxed copper braid that is used to remove solder, which allows components to be replaced and excess solder (e.g. bridging) to be removed. The soldering iron is applied to the wick as it sits on the solder joint, and when both are brought up to the solder's melting point, the flux is activated and, through capillary action from the braided design, solder is drawn up the wick.

White residue is generally a symptom of ineffective PCB cleaning. Common conductive flux residues from the soldering process can include various unreacted activators, binders, rheology components, and saponifiers. Among these are numerous iterations of acids (abietic, adipic, succinic among others), highly basic ingredients (amino compounds), and even constituents found in “soaps” such as phosphate and sulfate ions. When a cleaner does not fully dissolve all the constituents, or the cleaner is not allowed to flow off the PCB, the remaining solvent can evaporate off and leave behind residue that is either white or like water spots.

Yes, flux should be cleaned off of a printed circuit board (PCB) after soldering is completed. The following are the reasons to remove flux residues:

• Improve Aesthetic Appearance of PCB - If you are a contract manufacturer of PCB’s, the visual appearance of the board reflects on your work. A clear, greasy-looking residue around a solder joint may raise flags for your customer’s incoming QC inspectors. If the flux residue chars and forms spots on the solder joints, it may look like a true defect like a solder joint void or “blow hole”. If the flux residue is from a rework process, it acts as a fault tag in the rework area, calling attention to the work even if there shouldn’t be a concern.
• Improve Reliability of PCB - Reliability requirements are generally driven by the nature of the final product. For a disposable product like a computer keyboard, nobody loses their life if it stops working. In that case, an EMS supplier may use no-clean flux and forgo the cleaning process. On the other end of the scale, requirements for pacemaker electronics, where board failure could directly lead to death, are going to be much stricter. In that example, cleaning will be required after assembly and any subsequent rework, and the process will be thoroughly tested for effectiveness and repeatability. Long-life durable goods may fall somewhere in-between, with cleaning a requirement, but without the rigid testing and controls.
• Prevent Corrosion on Components and PCB - Flux residues left on electronic circuit boards are acidic. If they aren’t removed with a cleaning process, the residues can draw in ambient moisture from the air and lead to corrosion of component leads, and PCB contacts.
• Avoid Adhesion Problems with Conformal Coating - Most people understand that when painting something, the surface must be prepared so it is absolutely clean. Otherwise, the paint will quickly lift off the surface and peel off. The same logic applies to conformal coating, even when the contamination is from no-clean flux. “No-clean” refers to the amount of ionic material left after soldering. It has nothing to do with whether or not coating can stick to it. When there are flux residues left on the PCB before the coating process, it is common to see the coating lift or delaminate from the surface of the board. This is evident when the pockets are isolated around solder joints rather than the overall surface (the exception being the bottom of a wave soldered PCB). To make matters worse, coatings are generally semi-permeable, so breathe to a certain extent. Moisture can enter and soak into the flux residue, and potentially lead to corrosion.
• Prevent Dendritic Growth from Ionic Contamination - Polar or Ionic particles left from flux residue and other sources, when exposed to moisture from the ambient air and when current is applied, can link into a chain or branch called a dendrite. These dendrites are conductive, so form an unintended trace that cause current leakage or, over a longer period of time, even a short circuit. This is not as much of a concern for no-clean flux. No-clean flux contains minimal ionic material that is fully consumed when the flux is activated, or in other words, brought to soldering temperature. If all of the flux isn’t activated, like when you apply a lot of flux but only solder a small area, you still need to clean the PCB.

Most contact cleaners are safe to use on printed circuit boards (PCBs) in electronic devices. Make sure the contact cleaner solvent is compatible with all components, connectors, and packaging by testing on a scrap part or inconspicuous area.

You may be required to remove a conformal coating from the PCB to replace damaged components or other types of rework. The basic methods as cited by IPC are:

• Solvent Removal – While most conformal coatings can be dissolved in solvent, you should make sure the solvent won’t damage parts or components. Acrylic  is the fastest and easiest coating to remove with solvent. Silicone and urethane coatings will take more soak-time and will probably require brushing to fully remove the coating. Chemtronics offer Electro-Wash Two Step [https://www.chemtronics.com/electro-wash-two-step], which when heated, quickly removes acrylic, silicone and urethane coatings. CircuitWorks Conformal Coating Remover Pen [https://www.chemtronics.com/circuitworks-conformal-coating-remover-pen] is available to dissolve small areas of coating.
• Peeling – Some conformal coatings can be peeled from the circuit board. This is mainly a characteristic of some silicone conformal coatings and some flexible conformal coatings.
• Thermal/Burn‐through – A common technique of coating removal is to simply burn through the coating with a soldering iron as the board is reworked. This method works well with most forms of conformal coatings.
• Microblasting – Micro blasting removes the conformal coating by using a concentrated mix of soft abrasive and compressed air to abrade the coating. The process can be used to remove small areas of the conformal coating. It is most commonly used when removing Parylene and epoxy coatings.
• Grinding/Scraping – In this method, the conformal coating is removed by abrading the circuit board. This method is more effective with harder conformal coatings, such as parylene, epoxy and polyurethane. This method is
  only used as a method of last resort, as serious damage can be incurred.