Soldering is a key process in the electronic industry, where it enables electrical connectivity and mechanical stability. Solder bonds the leads of electronic components to copper pads on the printed circuit board (PCB), thereby not only helping them to stay in place, but also providing the components the necessary electrical connection to the rest of the circuit. Rush PCB recognizes solder thieving as a major problem during the soldering process.
However, solder thieving is not to be confused with copper thieving. Although both are related to PCB design, solder thieving occurs only during the soldering process, while copper thieving is a process to allow uniform copper plating in via holes. In copper thieving, designers add copper dots to the outer layers of the PCB, creating a uniform distribution of copper across the board surface. A non-uniform copper distribution can cause heavy plating in areas with little exposed copper, while areas with connector pins or BGA pads will not plate properly.
The presence of copper dots helps by stealing part of the plating current, preventing it from concentrating on sparse features, and spreading it more uniformly in areas with dense features. Rush PCB uses copper thieving on all types of PCBs, including FR-4, flex, and rigid-flex.
Mechanism of Solder Thieving
During soldering, it is very essential that the amount of solder at the joint is adequate. If the amount of solder is too little, the joint will have inadequate mechanical strength, and the component’s thermal and electrical connectivity to the PCB will suffer. Via-in-pad designs suffer from this problem of solder thieving occasionally, as the molten solder wicks down the untented via leaving very little solder for the joint itself. Unprotected copper areas and via-near-pads may also cause problems of solder thieving.
Preventing Solder Thieving
Presence of plated holes within or near pads along with missing solder-mask boundaries allow molten solder to flow away from the pad. While solder fills the via cavity, very little of it remains on the pad to form the inter-metallic bond between the part and the pad. Rush PCB suggests design options to prevent solder thieving in PCBs by:
- Filling the via with Conductive materials
- Filling the via with Non-Conductive materials
- Plugging the via
- Tenting the via
Conductive Via-filling: Conductive via-fill materials generally use silver coated copper particles in epoxy to fill the via hole. After the fill cures, the material provides some electrical and thermal conductivity. Conductive via-fill materials may vary in finished coefficient of thermal expansion (CTE) because of the overall particulate size they contain.
Non-Conductive Via-filling: Non-Conductive via-fills also use epoxy, but do not have any conductive material. They provide a better CTE match with standard PCB laminates.
Via-Plugging: In this process, the manufacturer uses solder mask or any other non-conductive media to plug the via hole. Next, they apply LPI mask over the plug, to guarantee covering all vias on the PCB.
Via-Tenting: While tenting vias, the mask data layer has no apertures, and the solder mask therefore covers the via pads and covers the hole. However, Rush PCB does not recommend this procedure, as it may create entrapment and lead to lowering the board reliability.
Solder Thieving as a Solution
Just as solder thieving can cause problems of inadequate solder for joints, it can also be useful for removing excess solder. While soldering, molten solder tends to accumulate in certain locations on the PCB, where it may cause solder bridging. For reflow soldering processes, this may be because of the congregation of several surface mount components on a single copper land that has inadequate solder masking. For wave soldering processes, solder accumulation is usually at joints with several component leads close by.
The wave soldering process causes molten solder to flow across the surface of the circuit board. Although this allows soldering the board with speed, solder bridges may form between pads of ICs. Typically, the bridge forms between the last two pads of a sequence of a row of pads. Earlier, the distance between adjacent pads was large enough to prevent bridges during soldering.
However, with increasing circuit densities, the pin spacing in ICs is becoming smaller, and the potential for bridge formation is also increasing. In such cases, designers are using solder thieving is a solution.
By providing the circuit board surface with additional pads, designers reduce the tendency of solder bridges to form. The additional pads act as solder thieves that draw off the excess solder preventing the formation of any bridge.
The designer places an approximately rectangular shaped pad touching the edge of the last pad of a sequence of pads, orienting the length of the pad in the direction of the flow of solder. The dimensions of the solder thief are such as to enable the pad to spontaneously wick the excess solder away from the nearest two pads, thereby preventing a solder bridge from forming between them.
Designers use the same process for preventing formation of solder bridging on SMT pads that undergo wave soldering. To prevent solder from forming clumps on the extra pads, the design of solder thieves includes two pads separated from each other but adjacent to the last pad such that they wick solder away from the last pad. By distributing the excess solder between two thieving pads, designers prevent clumps of solder from interfering with automated test equipment probes.
Read About: Role of Solder & Paste Masks in PCBs
Although solder thieving can be a problem for the soldering process, design methods are available to counter it. On a similar note, designers use solder thieving techniques to prevent formation of solder bridges.