PCB Substrates: Knowing Your Dielectric Material’s Properties

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A dielectric product conducts minimal power, and also gives a shielding layer between two conducting copper layers. One of the most common dielectric material is FR-4, however prior to selecting it for your board, you have to carefully consider its homes.

Here is a review of the most essential properties to take into consideration for any type of dielectric material:

1. Thermal homes
2. Electrical properties
3. Chemical homes
4. Mechanical buildings

Once you prepare to pick a product, utilize the Sierra Circuits Product Selector Tool to input requirements and also get an easy-to-compare checklist of dielectrics with the right residential properties.

Thermal Qualities

PCB Material

Glass Transition Temperature (Tg).

Glass transition temperature level, or Tg, is the temperature array in which a PCB substratum shifts from a glassy, rigid state to a softened, deformable state as polymer chains end up being extra mobile. When the product cools pull back, its residential or commercial properties go back to their original states. Tg is shared in systems of degrees Celsius ( ºC)

Decay Temperature (Td).

Disintegration temperature level, or Td, is the temperature at which a PCB material chemically breaks down (the product loses a minimum of 5% of mass). Like Tg, Td is expressed in units of levels Celsius ( ºC)

A material’s Td is a vital ceiling when putting together PCBs, because when a product gets to or exceeds its Td, changes to its buildings are not reversible. Contrast this to Tg, glass change temperature level, where residential or commercial properties will certainly return to their initial states once the material cools down below the Tg array.

Pick a material where you can work in a temperature array that’s more than Tg however well below Td. A lot of solder temperature levels throughout PCB assembly remain in the 200 ºC to 250 ºC array, so see to it Td is greater than this (fortunately, the majority of products have a Td above 320 ºC).

Coefficient of Thermal Growth (CTE).

The coefficient of thermal growth, or CTE, is the rate of development of a PCB material as it heats up. CTE is shared in parts per million (ppm) expanded for every single degree Celsius that it is warmed.

As a material’s temperature level rises past Tg, the CTE will certainly climb also.

The CTE of a substratum is typically a lot greater than copper, which can cause affiliation concerns as a PCB is heated.

CTE along the X and also Y axes are usually reduced– around 10 to 20 ppm per degree Celsius. This is typically thanks to the woven glass that constricts the material in the X as well as Y instructions, and also the CTE does not change a lot even as the material’s temperature boosts above Tg.

So the material should increase in the Z instructions. The CTE along the Z axis should be as low as feasible; aim for less than 70 ppm per degree Celsius, and also this will enhance as a product goes beyond Tg.

PCB Product; CTE (Coefficient of thermal expansion).

PCB Material; CTE (Coefficient of thermal expansion)
CTE is likewise valuable to find the Tg of a material by utilizing a CTE contour. Plot a material’s temperature versus displacement, then discover the intercept of both curves:.

Thermal Conductivity (k).

Thermal conductivity, or k, is the home of a material to carry out warmth; reduced thermal conductivity means reduced warm transfer while high conductivity means high warmth transfer. The action of the rate of warm transfer is expressed in watts per meter per degree Celsius (W/M – ºC)

. The majority of PCB dielectric products have a thermal conductivity in the series of 0.3 to 0.6 W/M- ºC, which is quite reduced contrasted to copper, whose k is 386 W/M- ºC. Therefore, more heat will certainly be lugged away rapidly by copper aircraft layers in a PCB than by the dielectric product.

Electrical Residences.

Dielectric Continuous or Family Member Permittivity (Emergency Room or Dk).
Considering the dielectric constant of a material is necessary for signal honesty and also insusceptibility factors to consider, which are essential factors for high-frequency electrical performance. The Er for the majority of PCB products is in the series of 2.5 to 4.5.

The dielectric consistent varies with frequency and generally reduces as regularity boosts; some products have much less of an adjustment in relative permittivity than others. Materials suitable for high regularity applications are those whose dielectric continuous remains reasonably the exact same over a broad regularity variety– from a few 100MHz to a number of GHz.

Dielectric Loss Tangent or Dissipation Aspect (Tan δ or Df).

A material’s loss tangent offers an action of power shed because of the material. The reduced a material’s loss tangent, the less power shed. The Tan δ of many PCB materials vary from 0.02 for most frequently used products to 0.001 for extremely low-loss high-end materials. It additionally differs with regularity, enhancing as regularity increases.

Loss tangent isn’t usually a critical consideration for electronic wiring, other than at extremely high regularities over 1Ghz. Nonetheless, it is a really essential parameter for analog signals, as it establishes the degree of signal attenuation and hence impacts the signal to sound ratio at different points along signal traces.

Volume Resistivity (ρ).

Quantity resistivity, or electrical resistivity (ρ), is one of the procedures of the electric or insulation resistance of a PCB product. The greater a product’s resistivity, the much less easily it allows the motion of electrical fee, and vice versa. Resistivity is revealed in ohm-meters (Ω-m) or ohm-centimeters (Ω-cm).

As dielectric insulators, PCB materials are called for to have extremely high values of resistivity, in the order of 10 ⁶– 10 ¹⁰ Megaohm-centimeters. Resistivity is somewhat influenced by moisture and temperature.

Surface Resistivity (ρS).

Surface resistivity (ρS) is the action of the electric or insulation resistance of the surface of a PCB product. Like quantity resistivity, PCB products are needed to have very high values of surface resistivity, in the order of 10 ⁶– 10 ¹⁰ Megaohms per square. It is additionally somewhat influenced by both moisture and also temperature level.

Electric Strength.

Electrical strength measures a PCB product’s capability to resist electrical break down in the PCB’s Z direction (perpendicular to the PCB’s aircraft). It is revealed in Volts/mil. Normal electric strength values for PCB dielectrics are in the series of 800 V/mil to 1500 V/mil.

Electric toughness is established by subjecting the PCB product to short high voltage pulses at common AC power regularities.

Chemical Qualities.

Flammability Specifications (UL94).

UL94, or the Requirement for Security of Flammability of Plastic Products for Components in Gadgets and also Equipments screening, is a plastics flammability criterion that identifies plastics from lowest (least flame-retardant) to highest (most flame-retardant).

The criteria are specified by Underwriters Laboratories (UL). The majority of PCB products comply with UL94 V-0; below are its requirements.

1. The samplings might not melt with flaming burning for greater than 10 seconds after either application of the test fire.
2. The total flaming burning time may not surpass 50 seconds for the 10 flame applications for each collection of 5 specimens.
3. The samplings might not melt with flaming or radiant burning up to the holding clamp.
4. The samplings might not drip flaming fragments that fire up the completely dry absorptive surgical cotton situated 300 mm below the test specimen.
5. The samplings may not have glowing burning that continues for more than 30 secs after the second elimination of the test flame.

Wetness Absorption.

Moisture absorption is the ability of a PCB product to resist water absorption when submersed in water. It is offered by portion rise in weight of a PCB product because of water absorption under controlled problems according to standard test methods. Many products have moisture absorption values in the range of 0.01% to 0.20%.

Moisture absorption influences the thermal as well as electric residential properties of the product, as well as the capacity of the material to withstand conductive anode filament (CAF) formation when a PCB circuit is powered.

Methylene Chloride Resistance.

Methylene chloride resistance is a measure of a material’s chemical resistance; specifically, the ability of a PCB material to stand up to methylene chloride absorption.

Similar to dampness absorption, it is expressed by percentage rise in weight of a PCB product due to exposure to or taking in methylene chloride under controlled problems. The majority of PCB products have methylene chloride resistance values in the range of 0.01% to 0.20%.

Mechanical Residences.

Peel Toughness.
Peel toughness is a measure of the bond stamina in between the copper conductor and the dielectric product. It is revealed in pounds of pressure per straight inch (PLI, or ordinary load per conductor width) needed to divide bonded products where the angle of splitting up is 180 degrees.

Peel toughness examinations are done on examples of copper traces of 1 OZ thick and ~ 32 to 125 millimeters vast after common PCB production processes. It is finished under 3 conditions:.
– After thermal stress: after an example is floated on solder at 288 ºC for 10 secs.
– At raised temperatures: after a sample is revealed to hot air or liquid at 125 ºC.
– After direct exposure to procedure chemicals: after an example is exposed to a defined series of steps in a chemical or thermal process.

Flexural Stamina.

Flexural toughness is the step of a material’s capacity to withstand mechanical tension without fracturing. It is shared in either Kg per square meter or extra pounds per square inch (KPSI).

Flexural strength is usually examined by supporting a PCB at its ends and packing it in the facility. IPC-4101 is the Spec for Base Products for Rigid as well as Multilayer Printed Boards, and also it provides the minimum flexural toughness of different PCB products.

Youthful’s Modulus.

Young’s modulus, or tensile modulus, additionally gauges the stamina of a PCB product. It gauges the stress/strain proportion in a certain instructions, and some PCB laminate producers provide strength in regards to Young’s modulus as opposed to flexural toughness. Like flexural stamina, it is revealed active each area.

Some PCB laminate makers provide the PCB materials strength not in regards to Flexural Strength however in regards to Youthful’s modulus which is an action of Stress/strain proportion in a certain instructions.

Density.

The thickness of a PCB material is shared in grams per cubic centimeter (g/cc) or pounds per cubic inch (lb/in ^ 3).

Time to Delamination.

Time to delamination procedures for how long a product will certainly stand up to delamination– the material’s splitting up from the laminate, aluminum foil, or fiberglass– at a specified temperature. Delamination can be triggered by thermal shock, the wrong Tg in the product, moisture, a bad lamination procedure.

PCB Delamination.

Delamination is the splitting up of the resin from copper or support, and time to delamination actions how much time this takes to happen at a defined temperature level. You can also check out FR4 Material

For even more assistance selecting the ideal PCB product, make use of the Sierra Circuits Product Selector Tool. Input a certain product building or homes, and also the tool will certainly provide you a list of materials that fulfill them. A simple comparison table lets you compare the detailed residential properties of up to 5 products.

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