Flux applied outside of the contact area escapes the high temperatures necessary to render it benign.
There are several options for performing selective soldering for plated thru-hole (PTH) components, and each presents a risk of ionic contamination. Primary types of selective soldering include:
- Selective pallets in a traditional wave-solder process
- Automated point-to-point systems
- Manual solder with a fountain
The one common risk with all three of these methods is overspray of the flux into unintended areas of the PCBA. Manufacturers typically assume that any flux coming into direct contact with the molten solder will be rendered near benign; and this is true in most cases when using a no-clean, spray flux, as the activators are outgassed when they reach a certain temperature for an adequate amount of time.* The issue lies in any spray flux that gets outside of the contact area.
*This can be proven with localized extractions and IC analysis over the PTH areas.
When using a selective pallet process, the full board is normally sprayed from the bottom side with an indexing spray flux system. The flux has a low surface tension and can wick into the keep-out areas defined by the pallet where it is subsequently shielded from the full thermal exposure required to render the residue benign.
Pallets themselves can also be a source of trouble, as pallet maintenance is often overlooked. Over time the hold-down springs can lose their required force, which sometimes leads to a bowing effect that lifts one side of the board up inside the pallet. This creates a large gap that spray flux can easily make its way into. If the pallets are not cleaned on a normal basis, the keep-out areas of the pallet will collect flux activator as the carrier is evaporated. These small pockets of activator can be extremely conductive, and any transfer to the components seated in these wells is at an increased risk of electrical leakage.
When using point-to-point systems, problems can occur in areas that have direct contact with the solder nozzle. The jet drop dispensers used by most point-to-point systems release a small blast of liquid flux up into the PTHs to be soldered, and excessive pressure can cause flux to be transferred to the top side of the assembly. There are few systems with sufficient topside heaters to render these flux residues benign.
When using point-to-point, the thermal mass of larger components like connectors can be more difficult to properly solder, and these components often see insufficient hole fill. To combat this issue, many companies will increase the amount of flux to aid in hole fill. While this may solve one problem, it often has unintended consequences. Connector bodies collect the excess flux inside the body as it wicks up the leads and deposits on the surface between the pins. In this situation, an electrical leakage path can propagate with as little as 40% relative humidity in the operating atmosphere.
Manual selective solder with a fountain typically refers to an operator with a squeeze bottle or other dispenser applying the liquid flux. Excess flux that flows or travels away from the primary point of heat application faces the same risk of not being fully activated and leaving active residues.
When using manual solder methods, the uncontrolled application of flux can be more difficult to optimize. Because of this, the use of automated, and measurable, flux application is an important parameter to consider. Operator training is key when using this type of selective soldering for assembly.
Takeaway: watch where you’re putting that flux
Selective soldering can be performed with a variety of methods, but all require care to ensure appropriate amounts of flux are applied in the right areas. Otherwise, unactivated flux residue can lead to field failures.