Common SMT Process Defects to Avoid During Soldering
Soldering robot for PCB assembly
As more designs are using smaller components with surface mount pads, Surface-mount Technology (SMT) process defects can mount and impact yields due to various design and manufacturing problems. These problems may have been minor and had negligible effect on yields in the past, but they can cause rework costs to mount quickly and should be avoided. Here are some basic SMT process defects you can avoid during design and assembly.
Bridging Between Pads
Bridging is common in low viscosity solders and causes a short between adjacent pads. This can also occur when the temperature is outside the ideal soldering range, which results in poor wetting, or excessive wicking. The key to solving this problem is placing a solder mask relief around your pads (i.e., non-solder mask defined SMD pads, or NSMD). This relief provides a space for into which excess solder can wick, effectively blocking the solder from flowing between two adjacent pads. This is similar to a solder mask dam between a ball grid array (or BGA) component and its via in a dogbone fanout.
Side view of an NSMD pad and its solder mask relief for preventing bridging. The gap between the NSDM pad and the solder mask provides a space for excess solder.
De-wetting is a problem involving selection of solder paste. This problem rarely occurs with water-soluble lead-free solder pastes, although it can occur with halide-based pastes when soldering onto HASL finishes. This can also occur if the conductor surface is heavily oxidized or if the solder paste has expired (i.e., the flux is inactive). Using a highly activated solder paste will allow your solder to form a strong bond to your pad during assembly. You should also ensure that the metals to be joined have been cleaned of all oxides to the greatest extent possible. This will prevent surface tension from pulling the solder across the pad and into a ball during solidification.
Another part of the process that helps prevent dewetting is to flow nitrogen through the reflow oven during soldering. This helps prevent oxide formation in the high temperature oven. You should also check that your plating thickness is sufficient (at least 5 microns). Both measures help prevent oxide formation and diffusion into the plating during soldering.
Poor Wetting With Lead-free Solder
Lead-free tin-silver-copper solders are important for remaining RoHS compliant, but they can exhibit poor wettability when soldering onto bare copper. This is one of many reasons that surface finishes are used on exposed conductors. Tin, silver, and ENIG surface finishes are known to provide better wettability.
The peak temperature during soldering should also be within the right range. Lead-free tin-silver-copper solders operate best at approximately 240 °C, and soldering outside of this range can lead to problems with wettability; an example with BGA balls is shown in the image below. These lead-free solders on a BGA may require larger standoff distance due to their higher surface tension, and the standoff/temperature profile should be checked with a test coupon prior to full-scale manufacturing.
SMT process defects due to poor temperature control [Source].
Component Shifting and Tombstoning
Tombstoning is also a problem related to wettability. In an ideal solder process, the molten solder will wet all pads for an SMT component simultaneously. In the case where pads on one side wet (i.e., reach sufficiently high temperature) before pads on the other side of the component, the solder will pull on the component as it solidifies. This mismatch in forces on each side of the component can cause one side of the component to lift slightly from the pad or shift away from the ideal location on the pad. In some cases, the component will remain attached to the pad through the solder, although the contact resistance might be high and the bond will be weak.
In a case of extreme temperature mismatch, such as where one end of the component does not wet at all, this can cause the component to stand on one end, known as tombstoning. This is a common problem in SMT resistors and capacitors. Tombstoning during reflow soldering corresponds to a variety of possible causes. The most common cause is uneven temperature in the reflow oven, which can cause solder in different areas of the PCB to wet earlier than in other areas. Nonuniform application of solder paste during assembly can also lead to variations in wetting throughout the board.
On the design side, groups of components with unevenly aligned pads are susceptible to poor wetting and tombstoning during reflow soldering. The sizes of pads will also affect their temperature difference during reflow soldering; larger pads require more heat to reach a defined temperature, thus equal sized pads should be used on each side of a component. When defining pad sizes, it can be easy to make a pad too large, and the extra copper will dissipate heat during soldering.
Shifting of a component due to poor wetting and low temperature on component pads. [Source].
Be sure to check your pad sizes and clearances when building your layout to ensure solderability during assembly. The use of thermal reliefs on the cooler side of a component is known to help prevent tombstoning during reflow soldering. Note that pads and vias with thermal reliefs have a complicated impedance structure that resembles a real capacitor, which can create some signal integrity problems in high speed/high frequency designs. I’ll dig into this point in greater detail in an upcoming article.
The design points mentioned above should be examined during a DFM check and should even be encoded in your design rules. The powerful PCB design tools in Altium Designer are built on top of a unified rules-driven design engine, allowing you to perform important DRCs throughout the design process. You’ll also have access to a complete set of fabrication planning, simulation, and documentation features in a single platform.