High-Reliability PCB Design Tips for Harsh Environments

Harsh-environment and high-reliability engineering often go hand-in-hand.

Whether a sensor package is placed on an airplane, a spacecraft, or down the borehole of a wellhead, the cost of failure can often be measured in hundreds of lives or millions of dollars lost.

For these situations, engineers have to pay particular attention to design choices. The following article presents solder and via considerations for your next design.

Quality & Reliablity

When a Printed Circuit Board is manufactured by an accredited facility, an employee is assigned the task of ensuring the board meets or exceeds expectations in all aspects of the design: plating thickness, surface finish, planarity, etc.… These are measurable, quantifiable metrics, and the board either meets them, or it does not. The PCB manufacturer’s responsibility for quality usually ends the moment the PCB is delivered to and accepted by the customer. At delivery, 100% of the boards are expected to function as designed.

Reliability covers the lifecycle of the product from delivery to dumpster -- and it is a much harder metric to quantify because board failure dates are unknown and variable. Several factors affect reliability, including PCB design, manufacturing process control, and material variability. Even if the design follows all best practices and the processes are carried out flawlessly, there is always a bit of material variability that introduces slight variations to manufactured printed circuit boards. This variability, along with poor design choices and poor process control, can hide latent defects inside a design.

Quality is easy to determine -- either the board meets specifications, or it does not. Reliability is hard -- because a board might fail 1 year, 5 years, or 20 years into its service life.

Unless your printed circuit board operates unprotected in an environment of salt spray and hydrofluoric acid, the factors that most affect the lifespan of your product are usually mean operating temperature and thermal cycling.

Maintaining Connectivity

Electronic components must be connected to each other to function. Silicon dies are connected to metal lead frames with bonding wires, lead frames are connected to copper pads with solder and copper pads are connected to each other with traces, copper pours, and vias. To maintain a high-reliability board, all of these connections must remain intact over the lifespan of the device.

Temperature Issue

Thermal Expansion

When the temperature of a printed circuit board or its components increases, the inter-atomic distance increases as well -- you might remember this is called the Coefficient of Thermal Expansion (CTE). Unfortunately, the relationship between separation distance and temperature varies with material composition. And to make matters worse, for PCBs, the CTE is often anisotropic -- which means the value depends on the direction.

Take FR-4, the basic building block of most boards -- it is made of a two-dimensional weave of fiberglass strands. The weft-and-warp fibers help to minimize in-plane expansion (x-axis, y-axis). But they do little for out-of-plane (z-axis) thermal expansion.

You should also be aware that not all weft-and-warp fibers are identical, some dielectrics purposely use different glass fibers which will result in slight CTE variability by axis. Check your dielectric’s slash sheet to find out more about your laminate materials.

PCB prepreg and core material are often made of a biaxial weave of fiberglass encased in epoxy resin as shown in the picture above.

Via Failures

The CTE for copper is low compared to the CTE for PCB dielectric and base materials. When a via is formed in a circuit board, and the board is heated, differential z-axis expansion leads to shear stress between the via wall and the dielectric layer. This force will eventually cause a failure in either the via or the copper foil.

The via aspect ratio, the amount of temperature fluctuation, and the quality of the PCB plating process will determine whether the via will fail on the tenth thermal cycle or the ten-thousandth thermal cycle.

Vias and Pads

Vias are holes in your printed circuit board that are electroplated with copper. The cheapest design option is to leave those holes open to the air.

Unfortunately, this is the least reliable design option for several reasons. For example, open vias can capture contaminants such as acidic flux that will slowly eat away at the copper over time.

But one often overlooked problem associated with open vias is solder thieving. During the reflow process, solder paste will liquify and follow exposed copper to a hole where it will enter and fill the hole. This leaves the copper pad with an insufficient amount of solder metal to form a proper bond between the part and the PCB.

Adding unfilled vias to a thermal pad can cause issues as well. Not only does the via allow solder thieving, but it can also allow outgassing into the space between the part and the PCB, forcing the solder away from the thermal pad.

This x-ray image shows an open via placed too close to a BGA part. The solder left one pad and filled the via hole, leaving an insufficient amount of solder on the pad.

This image of a 76-pin TQFP part shows multiple open vias beneath the thermal pad in this design. Every via beneath the pad exhibits evidence of either solder thieving or outgassing. This part would be acceptable as long as there is less than 50% of air between the part and the PCB.

The solution to this problem is to not leave open vias beneath or near solder pads.

For vias near pads, use your solder-resist layer to provide a barrier between the solder pad and the via. Via-tenting is an inexpensive, albeit unreliable option, but it only works on the smallest of vias -- a more reliable option is to fill the vias with LPI solder-resist.

For vias beneath pads, the most reliable option is to use filled and capped vias where a layer of copper is electrodeposited atop an epoxy fill material. The second best, albeit somewhat riskier option, is to use small diameter vias (<12 mil) that are then filled during electroplating with 1-oz or thicker copper -- there will still be holes or voids present that can allow outgassing, but the volume of the remaining hole should be small enough to minimize solder thieving.

Solder

Before the RoHS act, the majority of components were attached to circuit boards with a Lead/Tin eutectic solder. Eutectics are combinations of metals, that when mixed, have a low transition temperature that allows a direct solid-to-liquid phase transition. This is important to electronics manufacturers because high temperatures will destroy, or severely shorten the lifespan of printed circuit boards and their components.

The above diagram shows the various phases of matter based on temperature and the percentage mixture of the metals Tin and Lead, by weight. At approximately 60% Tin and 40% Lead, the metals undergo a single-phase transition from solid to liquid, without passing through an intermediate phase of matter -- this is the Eutectic point, and it occurs at a temperature that is lower than the melting temperature of either metal in pure form.

Engineers were concerned that they would never find material as ductile or wet table as their beloved Sn-63 Pb-37 mixture, so metallurgists got to work mixing various compositions of Aluminum, Antimony, Bismuth, Cadmium, Chromium, Copper, Gold, Indium, Lead, Molybdenum, Nickel, Palladium, Platinum, Silver, Tin, Tungsten, and Zinc to better understand the phase diagrams and material properties.

This periodic table shows some of the elements that were tested for their viability in solder compounds.

After the initial engineering uproar settled, engineers found out that there were dozens of alloys suitable to act as solders in a RoHS application. And, since these solders all had different material properties, they could be used for different applications.

For example -- Transient Liquid Phase Sintering Paste (TLPSP) is a type of solder paste that liquefies once. Once the inter-metallics have formed, the phase diagram changes and the liquid phase disappears. Once melted, TLPSP cannot be re-melted until the temperature is so high it will destroy both the integrated circuit and the underlying Printed Circuit Board. This makes the paste useful for multiple-reflow cycles and harsh environments.

Summary

Designing for reliability entails a wide array of design choices, material selections, and carefully controlled manufacturing processes. Focusing on thermal management will help improve your board’s lifespan and reliability. But it is just one part of the equation. You also have to make decisions that limit the temperature rise of your board by either limiting heat input or increasing heat-sinking. Be sure to look at our other papers on incorporating heavy copper into your designs.