pcb solder

What is Solder Thieving?

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

Conclusion

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.

pcb solder

Role of Solder & Paste Masks in PCBs

For improving the quality of soldering on printed circuit boards (PCBs), the electronic industry uses two types of masks—solder and paste masks. As the names are very similar, it is easy for newcomers to the industry to be confused about the usefulness and functioning of the two. In reality, the industry uses the two essential masks for entirely different purposes. In this article, Rush PCB explains their individual functioning, and the advantages of using them.

Solder Mask for PCBs

As the name implies, the solder mask creates the soldermask on the PCB, allowing soldering on selected areas of a PCB, while masking the others. Most commonly, the soldermask appears on the PCB as a green layer on its two outer surfaces. The green layer covers most of the PCB surface, except for copper pads that will accept component leads for soldering. Therefore, if a copper trace or pad is under the green soldermask, it will not be possible to make solder adhere to it.

Copper traces and pads on a PCB, if left uncovered, readily tarnish when they encounter air. The tarnish is due to the formation of copper oxide, a compound that does not conduct electricity, and does not allow soldering. Moreover, the presence of Sulphur in air causes the formation of copper sulphate, a corrosive substance. The net result, for a PCB with exposed copper on it, is a degradation of its quality when stored for some time.

To get over this problem, Rush PCB takes recourse to two processes. The first is to cover those parts of the PCB that will not undergo soldering with a green soldermask, and the other, to apply surface treatment to the exposed copper. The two processes ensure no copper surface on the PCB encounters air, while making it is easier to solder components on the PCB during assembly. With a soldermask present and surface treatment on the PCB, Rush PCB ensures the possibility of storing bare boards for a longer time, without any damage, until it is time to assemble them.

The common implement to apply the green soldermask on a PCB is the solder mask. The actual mask is a negative or complement of the green soldermask, and therefore, covers all the copper areas that will undergo soldering. When applying, the green color penetrates the open areas of the mask, and deposits on the PCB, covering the areas the mask exposes.

Paste Mask for PCBs

This mask or stencil is applicable to the use of solder paste, controlling the amount of paste the assembler dispenses to each solder pad on a PCB. Applying the proper amount of solder paste using a paste mask is an important process during assembly, since the presence of excess solder paste may cause solder to overflow, thereby creating shorts during reflow soldering, while inadequate solder paste may create dry soldering on some components.

Rush PCB prefers making paste masks or stencils of stainless steel, although it is possible to make them with other materials as well. For accuracy, the openings in the stencil must be laser cut and the surface finished with chemical etching. This creates a smooth inner surface in the openings, allowing for a smooth deposition and no sticking of paste on the stencil walls.

The thickness of the paste mask determines the volume of solder paste the process deposits on the copper pads. For dense PCBs, it is essential to have a thin stencil to adequately control the amount of solder paste deposition. For good quality of soldering, it is essential to accurately register the stencil on the PCB.

Advantages of Solder Mask and Paste Mask

Rush PCB uses both solder and paste masks on PCBs to ensure the quality of soldering. We need the solder mask while fabricating the board, and use the paste mask while assembling it. Both are indispensable for ensuring high quality. For instance, in HDI boards, we use the solder mask to create soldermask dams in between closely spaced pads, thereby preventing molten solder from overflowing and bridging neighboring pads during reflow soldering.

For PCBs using fine-pitch components, such as BGAs and gull-wing ICs, the pitch between neighboring pads may not be enough to allow placing a soldermask dam. For such PCBs, Rush PCB recommends using soldermask defined pads. While the pads are of a larger dimension, a smaller soldermask opening defines the opening for the solder paste.

Designing a paste mask or a stencil is an involved subject, and eminent PCB manufacturers such as Rush PCB have evolved their own special techniques for fabricating them.

Please consult Rush PCB with your design today, and get expert advice on the type of solder and paste masks necessary. Visit our website, or give us a call for a free quote.