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A Closer Look at the coefficient of CTE PCB (Thermal Expansion)

The rate at which a PCB substrate expands when the material heats up is known as CTE or Coefficient of Thermo Expansion. Parts on each million or ppm expanded for each degree Celsius of heating is how CTE is stated. The CTE increases when the temperature of the material climbs past Tg. When a Circuit is heated, the substrate’s CTE, often significantly greater than copper, can lead to connectivity problems.

CTE is typically low in the axes of Y and X, ranging from 10-20 ppm for each degree Celsius. The reason for this is often because of the woven glass. It restricts the substrate in the directions of Y and X. The CTE also remains relatively constant as the temperature of the material rises over Tg. As a result, the material has to grow in the direction of Z. Aim for below 70 ppm/C for the CTE all along Z direction; when a material exceeds Tg, this value will rise.

All of the substrates in a Board have certain temperature-related material characteristics. The value of CTE, or the rate at which volume changes with temperature, is a significant thermomechanical characteristic of PCB composites. A designer must show concern about mechanical dependability since the system may flex excessively with high-temperature fluctuations.

A designer must choose materials to ensure that any CTE misfit among materials must minimize. It is to reduce potential mechanical issues which thermal excursions may cause. Although it will not be completely eliminated, the difference between CTE rates can be reduced to some degree. Continue reading to find out more regarding CTE values and the kinds of materials that fit better for thermal dependability.

What Are CTE PCB Values?

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All substances possess significant thermal characteristics often referred to as CTE or the thermal expansion Coefficient. This describes the rate at which a material’s volume will expand in response to a specific temperature change. It is commonly expressed in units of parts by million by degree. This means that the substance will contract or expand by 1/10,000th time for every time one °C rises. Temperature also affects other material characteristics like tensile strength and dielectric strength.

What Happens When There Is A CTE PCB Mismatch?

You want a PCB with high dependability when designing it. But a CTE disparity can lead to a number of reliability issues. Stress will develop and focus in the area between two unsuited materials as a result of variations in the value of CTE in PCB substrates. A single heat excursion seldom results in issues unless it is really severe. Yet, frequent heat cycling might result in volumetric expansion-related mechanical problems.

· Solder Fatigue

With high-reliability electrical devices that may endure extreme temperature or vibration variations, solder fatigue becomes a major problem. Solder fatigue happens due to the mismatch between the value of CTE of the substance or the copper for soldering. Vibration is another primary mechanical component causing solder fatigue. These two elements together can cause mechanical wear in welded joints.

· Solder Bridging 

Changes in CTE mismatches and volume will have an impact on several PCB production processes. One issue that might occur while soldering BGA modules is solder bridging. Due to differences in CTE among various package materials, wire-bond-molded BGA components have a tendency to expand while reflow soldering at each corner. This results in the heated solder ball deforming, which may create bridging between neighboring balls, leading to a short circuit.

· Thermal Stress In High Aspect-Ratio Vias

The coating of copper along the walls of the via may become thinner when its aspect ratio gets higher. This makes the center extra susceptible to thermal or high-temperature stress cracking. As a result, thicker plating is needed to lessen the stress concentration during temperature changes on the board. Thermal cycling, or the repetitive change in temperature from low to high vv, is known for producing cracks via necks. Moreover, it leads to interfaces in layers in stacked buried-buried or blind-buried vias on HDI circuit cards.

· Delamination And PCB Warping

A significant temperature rise may produce enough stress which triggers layer delamination and starts the deformation of the PCB. Only if somehow the CTE discrepancy between the laminate and copper is too high. Circuit boards consisting of FR4 and copper are particularly prone to this kind of damage. This results from rapid temperature swings and CTE imbalance. Higher resin density laminates could have more CTE discrepancies with copper. Moreover, for a certain temperature change, thicker copper surfaces provide higher stress.

Be sure you utilize Cadence’ OrCAD to establish the design specifications. Generate your PCB design after choosing the components you have to construct a dependable PCBA. The greatest PCB layout and evaluation software comes with OrCAD. Users of OrCAD have access to a comprehensive collection of schematic tools, mixed-signal modifications in PSpice, and CAD tools.

Importance Of Laminate CTE PCB 

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If any of the components are susceptible to the extension of the Board, the x-y extension of the plane has detrimental effects. Components like large silicon packages or LBGAs, which extend at a rate of 6 ppm/°C while the Circuit board extends at a rate of 18 ppm/°C, can harm the board solder joints. After a significant number of thermal evaluation cycles, usually between -65°C and +125°C, the recurrent mismatch in expansions will result in shear pressures on the board solder joints. This, over time, creates stress and small cracking, and finally, work stiffening the solder with the breaking of the weld joint themselves. When it comes to demanding high dependability temperature circumstances like military weapons or medical equipment, the ensuing intermittent device operation is undesirable.

Extreme overheating (i.e., surpassing the temperature at which glass transitions, or Tg) across a series of heating sessions, like having numerous soldering cycles throughout the assembly, can also increase temperature variations. As an illustration, the pattern solders the Circuit with one thermally over the cycle and solders the chip with another thermal over the cycle. Then solder the big capacitors with a third thermal over the cycle. Limiting thermal cycles above Tg is crucial for the production and assembling of the Circuit since it has an impact on the total number of operational thermal cycles. According to tests, three thermal assembling cycles over Tg are equivalent to more than 1000 additional thermal cycles at 80 degrees Celsius.

A lower x-y value of CTE laminates is available that can limit PCB expansion and lessen the risk of weld joint breaking. The PCB can also be subjected to fewer thermal low-high temperature cycles and at a lower temperature by making better cooling and cabinet selections.

Thermal Stress Cracking In CTE PCB Assembly

Thermal stress fracture of the through copper plating occurs with repeated heat cycles. It is another area where the CTE of the Circuit board can impair the dependability of the Circuit assembly. PCB materials expand volumetrically as a result of temperature increases. However, because of the laminate architecture, the expansion of the x-y and z-axis behave quite differently. Since the laminate’s restraint glass fabric restricts the epoxy from extending isotropically (equally in all directions), the x- and y-axes will see much less expansion than the z-axis.

The high young’s modulus or the expansion force intensity of the tougher glass laminated inside the circuit layers of x-y regulates the epoxy volume expansion. This simply implies that because of the laminate’s slower expansion rate. The resin cannot move in the x-y direction and must expand in the direction of z. However, this will result in much more resin expansion in the unrestricted z-axis. This will put stress on the copper that has been plated on the vias.

Whenever the temperature gets near to a Tg, the z-axis CTE dramatically increases from 4 to 14 times the value of the x-y axis. This means that in a classic PCB laminate, the axis of z is expanding at Tg at a rate of 50-200 ppm/°C as opposed to 15 ppm/°C in the axis of x-y.

CTE is typically 16–18 ppm/°C for multilayer PCBs. The resin/fiber system with the lowest CTE of Circuit boards is likely to be the one with the highest resin concentration. It is feasible to create laminates with extremely low CTEs. Choosing prepregs and laminate carefully so they won’t suffer from epoxy famine. Starvation is indeed the absence of resin flow and incomplete filling of the copper pattern voids in the internal layer. Several laminate technologies manage the CTE of the z-axis, but only a few have been effective. The laminate systems that have been successful, like Kevlar, are highly costly and have limited availability.