My father always used to say, you can fix most problems with bungee cords and duct tape. Unless you plan on duct taping your components to your PCB, your boards will have to go through a manual or automated soldering process. This is where it is important to understand the factors that affect PCB solder joint strength.
As it turns out, some of these factors are related to your layout, while others are related to the soldering process itself. The two are actually related and some simple design choices can help ensure your solder joints are strong, components do not tombstone during reflow soldering, and there is no unintended bridging. A lot of manual rework can seriously extend your lead time, but working with the right CM will help ensure your design does not experience common problems that impact PCB solder joint strength.
Solder Quality: Temperature Distribution, Wetting, and Flux
Not all solder alloys are created equal. Tin-lead solder mixtures were used universally in the past. These solder mixtures melt at approximately 180-190 °C, which is near or above the glass transition temperature for many PCB laminates. Because lead has some damaging effects on our health, the industry is moving away from lead and toward lead-free solder in accordance with the EU’s Restriction of Hazardous Substances in Electrical and Electronic Equipment (RoHS) Directive. Lead-free and silver alloy solders melt at much higher temperatures (~220 °C and ~450 °C, respectively).
No matter which type of solder you use, the temperature and its ability to wet a surface will affect the PCB solder joint strength. In addition, each type of solder can form oxides if the incorrect type of flux is used, which will also decrease PCB solder joint strength.
Flux and Oxidation
Most PCB assemblers are not chemists, but it helps to understand some basic chemistry to understand the value of solder flux. During soldering, the molten solder forms a hot eutectic, and the molten solder can easily react with oxygen to form oxides. Metal oxides tend to have low mechanical strength compared to their pure metal counterpart.
The use of flux is intended to suppress oxidation, as well as to draw solder onto the intended area during assembly. Flux provides another benefit in that it helps clean the area of any oily residues. The correct flux and a sufficient amount of flux must be matched to the correct type of solder during PCB assembly.
Temperature and Wetting
When solder is heated, the mix of metals forms a hot eutectic and wets the surface of a pad (for SMT components) or via (for through-hole components). The surface tension of the molten solder during wetting will draw the solder into a convex “fillet” shape at the edge of the component, and the solder will cool once the heat source is removed. However, if the solder temperature is too low during heating, the solder will not form a thoroughly-mixed eutectic and will solidify as a cold joint.
A cold joint has dull, pockmarked surface and low strength due to insufficient mixing of the eutectic. The lack of heat on the joint during soldering can result from one of two possible reasons:
- Insufficient iron temperature: The soldering iron was too cool for the type of solder being used.
- Excess heat dissipation: Too much heat was removed from the solder point, which decreased the temperature of the eutectic during soldering.
Cold joint on a PCB
Regarding the second point above, a cold joint can form if pads or traces are connected to a large ground plane. A pad connected directly to a plane through a via without thermal relief will lose heat from the soldering iron to the plane. If not properly addressed in the design phase, cracks can form in the joint over time, eventually leading to failure.
Should You Use Thermal Reliefs to Enhance PCB Solder Joint Strength?
Thermal relief vias are simple structures. These are standard vias that connect between large pads and plane layers, and sectors are removed from the via pad in one or both layers. Electrically, they function like any other via until you get into the 10s of GHz frequency range. However, some designers simply omit them for a variety of reasons. In addition to cold joints and other solder defects, here’s what can happen if you don’t use thermal vias in critical locations:
Tombstoning
On SMT components, uneven solder joint temperature and wetting can also lead to tombstoning. When this occurs, one end of the solder ball fully wets and begins cooling before the other end. The surface tension of the wetted solder ball pulls on the component and can separate it from the other pad. In extreme cases, this can cause a component to stand up on the wetted end like a tombstone.
This typically happens when an SMT component is being soldered to a large pad that is connected to a plane. Similarly, components with surface ground pour might be soldered directly to the ground pour. Although this is an electrically correct design choice, it is difficult or impossible to properly solder, leading to cold joints.
The standard solution is to add thermal relief vias on the end of an SMT component that connects to the large pad or plane. Rather than heat being quickly dissipated into the conductor, the thermal relief via will confine heat near the end of the SMT component, ensuring sufficient heat and wetting during soldering.
Die-Attached Paddles for Heat Dissipation
Many components that generate significant heat during operation will contain a die-attached paddle to move heat away from the component. Vias are used on these pads to provide a uniform ground plane connection for the component and to move heat into the ground plane. Thermal reliefs are counterproductive on die-attached paddles, and typical vias should be used. An important point here is the size of the via and spacing between vias affects whether the component can be properly attached with high mechanical strength and without solder wicking through vias.
Ideally, the number of vias in a thermal pad and elsewhere in a design should be minimized to reduce drill bit wear and fabrication costs. If the via is too small, then a smaller drill is required, which carries longer drilling time and higher fabrication costs. In addition, a very small via creates a challenging cleaning and plating process. An experienced CM can recommend the right via size to use to ease fabrication costs, which will ensure sufficient heat is dissipated during operation without compromising strength.
