Material coefficient of thermal expansion (CTE) is one physical property of a material. When temperature changes, the part will change shape and will have stress. We cannot stop that. The core of making any electronic product is to put parts that meet requirements onto a printed circuit board (PCB). We join parts to the board by soldering. For through-hole technology (THT), parts have leads. When the board bends or expands, the lead bends and soaks up some stress. The part body sees less stress then. So longer leads mean less stress goes into the part body. Solder stress from temperature change then mainly affects the solder joint and the pad. That affects solder reliability.
When we use surface mount technology (SMT), we can put more parts on the board. But parts have no long leads to soak up stress. Stress goes straight into the part body. That can break the part or the solder joint if the board and part expand at different rates.
CTE Characteristics of FR4 Base Material
Common glass fiber reinforced base material (fr4) has different CTE in the z axis (through the board thickness) and the x-y plane (along the board). The board will expand more in z when the temperature rises. When the material is below its glass transition temperature Tg, the material is in a glass state and its CTE is small. We call this CTE value a1. When the material is above Tg, the material is in a rubber-like state and its CTE is larger. We call that CTE value a2. Usually a2 is about three times a1.
For example, when temperature is below Tg, glass fiber epoxy CTE in the x and y directions is not the same either. It differs by about 1 to 5 ppm/°C. In the z direction it is about 20 ppm/°C. When a large surface-mount device has a CTE that matches the board, this difference matters a lot. It helps increase solder joint life.
Impact of CTE on PTH and Microvias
CTE is an important factor for PWA mounted through-holes and microvias. Aspect ratio (that is board thickness divided by hole diameter) for these vias is usually larger than the aspect ratio of a just-finished drilled hole in a bare board. In processes like SMT reflow, part removal, reflow for repair, board manufacture, ball attachment, hot-air leveling, and wave solder, a large z-direction CTE will cause too much PTH tensile stress. That can make failures.
So when you pick a board base material, you must first think about whether its CTE matches the CTE of the parts to be mounted. If they do not match, you must take compensation measures.
Basic Definition and Significance of CTE for PCBs
Many people use the word CTE when they talk about PCBs. But how many really know what CTE means and how CTE starts to affect a board?
CTE stands for coefficient of thermal expansion. It shows the percent change in size when a PCB is heated or cooled. Every material in the world expands or shrinks a little with temperature. For example, a house is slightly bigger in summer than in winter.
Some materials shrink when heated, but most expand a little when heated. Expansion is shown in parts per million per degree Celsius (ppm/°C). If a PCB expands 14 ppm/°C laterally, that means if a PCB were one million inches long, it would grow 14 inches for every 1°C rise.
CTE Mismatch Between PCB and Silicon Chips
A typical fr4 laminate has a CTE of 14 to 17 ppm/°C. That looks ok until we compare with a large silicon chip. Many silicon chips have a CTE near 6 ppm/°C. The difference in expansion is big enough—especially for large BGA packages—that when the PCB and chip heat, the PCB will expand more than the chip. That extra movement can pull solder joints off the chip.
So from the PCB point of view, manufacturers often use low-CTE materials. But how does CTE affect the board and how we design and make it?
Typical CTE Values of PCB Materials and Components
| Material / Component | CTE (X-Y Direction) ppm/°C | CTE (Z Direction) ppm/°C | Notes |
|---|---|---|---|
| Standard FR-4 Laminate | 14 – 17 | 60 – 70 (above Tg can be >200) | Most common PCB material |
| High Tg FR-4 | 12 – 16 | 50 – 60 | Better thermal stability |
| Polyimide PCB Material | 12 – 14 | 40 – 50 | Used for high-temperature applications |
| Copper | 16.5 – 17 | 16.5 – 17 | Reference conductor material |
| Silicon Chip | 2.6 – 3 (bulk silicon) | — | Package effective ~6 ppm/°C |
| BGA Package (Typical) | 6 – 10 | — | Depends on substrate type |
| Ceramic Package | 6 – 8 | — | Good CTE matching with silicon |
| Aluminum | 22 – 24 | 22 – 24 | Used in metal core PCB |
| Copper-Invar-Copper (CIC) Core | 8 – 10 | 8 – 10 | Low CTE metal core |
| Copper-Molybdenum-Copper (CMC) Core | 6 – 8 | 6 – 8 | Very low CTE metal core |
| Aramid / Kevlar Core | 7 – 8 | 7 – 8 | Low CTE composite material |
Low-CTE PCB Core Solutions
When we pick a laminate we look at specs like Tg, dielectric constant Dk, and dissipation factor Df, among others. These all matter and they affect each other. If we pick a laminate to lower CTE, we will find that all fr4 types have similar CTE values. Most are still high (about 14 ppm/°C). That means for very large silicon packages we need another approach to control CTE. One way is to add a core made of metal, Kevlar, or aramid.
These low-CTE cores are often used under fr4 outer layers to make low-CTE boards. Metal cores can be copper-invar-copper (CIC) or copper-molybdenum-copper (CMC). They are usually about 6 mil thick. The copper on the metal core outer surface lets us laminate normal fr4 prepregs and cores over the metal.
Two widely used metal cores are CIC and CMC. Their core CTE values are about 8 ppm/°C and 6 ppm/°C, respectively. When you anchor fr4 outer layers to a metal core, the overall board CTE becomes about 12 ppm/°C for CIC and about 9 ppm/°C for CMC.
We can also use a Kevlar Thermount or an aramid laminate as a core. Their low core CTE is about 7 to 8 ppm/°C. With standard fr4 outer layers, the whole board CTE becomes about 12 ppm/°C. In multilayer production, a low-CTE core replaces a typical fr4 core. It is interesting that Kevlar fiber actually has a negative thermal expansion itself. When we bind the fibers with epoxy, the result is a small positive CTE.
Cost, Process and Additional Advantages of Low-CTE Cores
Using a low-CTE laminate, the most expensive option is usually the metal core. Kevlar and aramid laminates cost less. In the past, Arlon Kevlar Thermount was hard to get, but new production increases supply now. All low-CTE cores are harder to drill and to process. But they are the only way to meet the 6–9 ppm/°C needs of very large silicon packages.
Besides controlling CTE, metal core PCBs can also improve high-power heat transfer. Remember metal expands more than fr4 when heated. A metal core gives more control of board expansion than Kevlar. A metal core can change the expansion of the whole board more than Kevlar can.
Practical Measures to Reduce CTE Mismatch Risk
In practice, to reduce CTE mismatch risk you can do these basic things:
Try to match board CTE to the large package CTE when you can.
Use low-CTE cores like CIC, CMC, Kevlar, or aramid for big packages.
Watch Tg and use materials with a Tg above your highest process temperature.
For SMT parts, design pads and land patterns to reduce stress on solder joints.
In manufacture, control peak reflow temperature and cooling rates.
For through-hole parts, use leads that can flex to take up stress.
Know that low-CTE cores cost more and need different drilling tools and processes.
Summary of CTE Importance
CTE is a simple number, but it affects many parts of a PCB project. If you do not think about it early, you can see solder joint failures, cracked parts, and low reliability. If you plan for CTE from the start, you will get better life from your board and parts.
