Sometimes, instead of molten solder remaining on the SMD pad, it may flow away to a connected copper element, typically a copper area or pad that has been left uncovered by soldermask. However, this effect is most pronounced with a nearby via hole that does not have a soldermask web covering or a dam in between. Especially so, if the via hole is very close or within the SMD pad itself. The reason for this is the capillary action and meniscus effect that causes molten solder to flow towards and into the hole, thereby starving the SMD pad of its share of solder. The net result of the above phenomenon, also known as solder escape or solder wicking, is either a dry or a poor quality of solder joint. In this article, PCB Trace Technologies Inc looks at the various reasons of solder escape happening, and how to prevent it.
Increasing the volume of solder paste on the pad may sound like an obvious solution to the problem. However, that may give rise to additional undesirable situations:
- Neighboring SMD pads also get too much solder paste.
- The stencil may require a larger solder paste aperture.
- The affected pad may have to be made larger.
- The stencil may have to be of the stepped type.
Solder escape, also known as solder drainage or solder wicking, is a well-known issue in many processes for PCB assembly, especially during the reflow soldering process. As stated above, molten solder may flow away from the pad on the PCB, resulting in the joint being starved of solder and thereby, turning into a dry joint. Solder wicking is a serious problem where the issue is of molten solder flowing into a via hole or climbing up the lead of a component. Here are some common reasons for solder escape, or solder wicking:
Thermal Dissipation: During reflow, a larger copper area with a higher thermal capacity is usually hotter than the small pad to which it is linked. As a result, it draws away the heat from the smaller pad, causing molten solder on the pad to move towards the larger area. This typically happens if a soldermask dam is not present to separate the two and prevent the solder flow. This is mostly true for copper tracks and vias that do not have a protection layer of soldermask on them, and this makes them more vulnerable to the problem of solder wicking. In severe cases, solder may even reach the other side of the board through the via and form bumps on that side.
Melting Point: Typically, there is solder present on the component terminals also. However, this solder usually has a lower melting point as compared to that of the solder paste present on the pad to which the component will attach itself. Therefore, the solder on the component terminal is likely to melt before the solder present on the pad can. This allows the molten solder to flow away from the pad and accumulate in unwanted places. Typically, a component lies on the solder paste present on the pad. Once the soldering process is underway, solder melts and flows towards the surrounding areas and the via hole.
Variable Reflow Profile: The reflow profile of a board is usually suitable for a specific constitution of solder paste. The reflow profile may change even for small changes in the constituents of the solder alloy, as it may happen due to changing over from one solder paste manufacturer to another.
Solder Melting Before the Pad Heats Up: The solder paste may melt and heat up faster than the copper pad beneath it. This prevents the solder from completely wetting the pad, instead flowing away towards the hotter area nearby.
Solder on Leads Melting Before the Solder on Pad: If the leads of the component heat up faster than the pad, molten solder is likely to rise up the leads, leaving the pad starved of solder.
Non-Wetting: Solder wicking may also happen if the copper pad on the PCB is contaminated, oxidized, has insufficient plating thickness, or has a problem with surface treatment. In these cases, the solder cannot wet the copper pad adequately, resulting in solder wicking away to a more suitable location.
It is possible to reduce the chances of solder wicking occurrences by being attentive to certain process parameters like:
Pad and Component Lead Temperature Difference — If the component lead is hotter than the pad, there are higher chances of solder flowing towards the component lead, thereby causing wicking. This is more evident in soldering of leaded components, as they have longer leads. Reducing the temperature difference between the component lead and the pad reduces the chances of solder wicking by improving wettability of the pad and improving the solder joint.
PCB Cleanliness — Cleanliness of the PCB pad is a crucial parameter in preventing solder wicking. Molten solder easily wets a clean surface and this helps in keeping the solder in the intended location.
Pad Hole Diameter to Lead Diameter Ratio — The gap between the component lead and the pad to which it will solder is important and affects solder wicking. A bigger gap will increase the chances of solder wicking.
Solder Melting Temperature — The melting temperature of the solder is also an important factor. If the solder melts before the pad has reached a proper temperature, the molten solder may flow towards a larger copper area rather than wet the pad. A slower melting solder may be of help here.
Masked Vias — It is possible to use Kapton tape to cover an unmasked via on the board to prevent it from wicking away solder. Once the soldering process is completed, and the PCB has cooled, the tape may be removed. Kapton tape has the advantage of not degrading under exposure to reflow temperatures. Although it is a user-friendly and cost-effective, it is a temporary solution.
Filled Vias — Some vias may be positioned too close to the SMD pad to mask them effectively. One solution to this problem is to fill the via with a suitable resin. It is also possible to cover the via hole fully with soldermask. This prevents the via hole from wicking solder away from the joint.
Filling the via with resin is also a solution for vias that are present inside the pad and cannot be covered by masking or soldermask. This method is also suitable for vias present under the body of SMD components for connecting it to a heat sink.
Chip manufacturers often use the body of the component to dissipate its heat. For this, they recommend using a copper area under the main body of the component and use a matrix of via holes to attach it to another copper area on the other side of the board.
However, if the via holes are left open, they wick the solder away from the copper area and prevent the component from being soldered properly. It is possible to reduce solder wicking by covering the holes with soldermask, but this effectively reduces the area for heat transfer and may affect the performance of the board. Filling the vias with resin is a better option, as it allows the entire surface area to be used for heat transfer, while the resin prevents solder from wicking down the via hole.
While attending to process parameters to prevent solder wicking may be practical and useful, it is still better to prevent any chances of solder wicking by taking certain care during the design of the board. Some of the design solution available are:
Use of SolderMask — Use soldermask to cover up any large copper area or via connected to the pad. This prevents solder from wetting the larger area or via and therefore solder will remain on the pad. If it is not possible to cover the entire larger area or via with soldermask, placing a soldermask dam between the pad and the large area or via will also be effective.
Increase the distance — Increase the distance between the pad and the larger area or via to which it is connected. The increased track reduces the chances of solder wicking. Covering the interconnecting track with soldermask further prevents solder wicking.
Plating — Via in pad technology has proven to be useful in saving real estate while making PCB layouts, allowing denser boards while conforming to standard design rule parameters. However, these via in pads can be a major problem because of solder wicking.
In such cases it is typical to build up copper thickness by electroplating with copper. The holes are selectively plated by using a dry film resist on the surface. This prevents too much copper plating on the PCB surface. Alternately, the via holes may be filled up with epoxy. In either case, the hole is no longer open to allow solder wicking.
If your circuit board requires filled vias for preventing solder wicking, it is important that you communicate this to the PCB fabricator. They must know the specific areas that require cap plating or epoxy filling, and whether the filling should be with conductive or with non-conductive material. This is because conductive filling is more expensive.
PCB Trace Technologies Inc offers several solutions for via plugging, including vias flooded with mask, selective plugging as for BGAs, conductive and non-conductive epoxy filling, or fully plugged via in pads.