How Does Solder Paste Composition Affect Tombstoning?

Solder paste composition varies greatly with many compositions available for purchase. Solder pastes affect reliability and assembly capability, as well as the potential for tombstoning on small SMD passives. When you are determining assembly requirements, mechanical reliability is an important factor in the PCBA, which type of solder paste can best balance high reliability with high yield?

As it turns out, mechanical reliability, in terms of fracture strength and modulus, and potential for tombstoning are linked through the mechanical properties of solder pastes. In this article, we look closely at how the solder paste composition will affect the strength of the final assembly as well as tombstoning during reflow.

Solder Paste Composition and Joint Strength

The composition of solder paste is relatively simple; small beads of an alloy are mixed with a flux, and the mixture is deposited on component landing pads in preparation for reflow soldering. Deposition is normally through a stencil, although it can be done with an automated solder paste dispenser.

Solder paste is classified into two groups based on its chemical composition:

• Leaded vs. non-leaded - Solder paste can contain lead, which will impact environmental compliance (REACH and RoHS) for certain products.
• Eutectic vs. non-eutectic - The alloy formed by the solder during reflow could form a eutectic.

Not all alloys form eutectics, including solder paste alloys. The composition of an alloy containing two metals can form a eutectic at just the right fractional composition of each metal. The image below shows a eutectic diagram for a 2-metal alloy, and the regions where the various constituents are solid or liquid are marked.

The eutectic point is the specific temperature where both metal components can immediately transfer from liquid to solid as the mixture cools. For a given alloy with two metals (A and B), the alloy can exist as a eutectic only if it contains a specific fractional composition of metal B. All other compositions of those two metals will not exist as eutectic at any temperature.

Why the Eutectic is Important

The table below shows a range of solder pastes and their compositions. The table is divided into leaded and lead-free groups, and eutectic-forming solders are marked in light blue. The two temperatures are important for determining the tension on the component during cooling: the boundary dividing the (solid + liquid) and liquid regions in the eutectic diagram is called the liquidus, and the horizontal line marking the melting point is called the liquidus.

If a solder paste is designed such that it has a eutectic alloy, the solder paste will very rapidly change from a liquid to a solid right at the eutectic point during cooling. The result is a very strong pulling force on an SMD component during the cooling phase. This results in higher chances of skew or tombstoning. To compensate for this, the assembler can extend the cooling time in the reflow temperature profile.

In general, for a given reflow profile, a non-eutectic solder paste will have lower chances of tombstoning than a eutectic solder paste for the same reflow profile. If there is a larger difference between the solidus and liquidus, one could expect the tension exerted on components during cooling to set in gradually, which also reduces the risk of tombstoning.

What Are the Tradeoffs?

Clearly, when selecting different types of solder paste, there will be tradeoffs when optimizing for reliability vs. optimizing for assembly. The different solders listed above will have some tensile strength that will determine the probability of solder ball fracture due to vibration or fatigue. Therefore, compare the above options to tensile strength values in your solder paste datasheet when selecting a solder for volume assembly.

If mechanical reliability is most important in the assembled PCB, and you end up opting for a eutectic alloy, then the assembler might need to take extra care to ensure high yield assembly, such as:

• Clean stencils regularly and inspect them for damage
• Review SMD footprints for symmetric paste masks on component pads
• Inspect the reflow profile and temperature distribution

When you need to specify your assembly requirements for your manufacturer, you can add assembly notes into your PCB design file and export this for use in a drafting program. You can then complete your assembly drawing with required 2D and 3D views, assembly diagram, and acceptability requirements before starting an assembly run. Fab and assembly notes are commonly added to the native PCB design files in a mechanical layer and included in a Gerber export, or they can be accessed in a DXF export.

For footprint creation and PCB layout, use the CAD tools in OrCAD from Cadence to build your PCB for volume assembly. OrCAD is the industry’s best PCB design and analysis software with utilities covering schematic capture, PCB layout and routing, and manufacturing. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.

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