Understanding material properties is critical in determining requirements for PCB manufacturing.  In this blog, we will look at the technical definitions, as well as an overview of why each property may be important to your designs.  All of these properties should be specified on the data sheet for most commonly used PCB materials.

PCB Solutions always recommends that you contact the material supplier and review their data prior to making a decision.  The data below is available to help you navigate the terms but does not serve as advice on which material to chose for your application because there are so many variables for Rigid, Rigid Flex and Flex PCB designs.

1. Dielectric Constant (Dk or Er):

Technical definition: The ratio of the capacitance of a capacitor with the given dielectric to the capacitance of a capacitor having air for its dielectric but otherwise identical.  The Dk value is calculated as the relative permittivity of a material.

Why this is important: The Dielectric Constant is a major factor in calculating and controlling impedance requirements of signals on PCBs.  All PCB materials (cores, prepregs, solder masks), have a Dk value.  The actual Dk value can vary based on resin content of materials.  Values typically range from 3.5 to 5.9.  Specific material is available for both very low Dk and Very high Dk values. A low Dk material is often used for RF applications while a high Dk is often used for High Frequency applications.

2. Glass Transition Temperature (Tg):

Technical definition: The temperature at which a polymer changes from hard and brittle to soft and pliable.

Why this is important: The Tg indicates the temperature at which the PCB base material starts yielding. It is important to avoid any yielding of PCB base materials, so the Tg is not an indicator for the operating temperature of the PCB. The Tg temperature can usually only be sustained for a very short time.  The actual minimum Tg required for your PCB will depend on many factors including surface finish and assembly process; however, the industry guideline for most ROHS applications in a minimum material Tg of 170 degrees C.

3. Decomposition Temperature (Td):

Technical definition: The temperature at which material weight changes by 5%.

Why this is important: The decomposition is the breaking of chemical bonds in the resin system. The resin in the laminate is basically burning up.  This value is widely considered to be more critical than the Tg value with regards to ROHS considerations during the assembly process.  Like Tg, the actual minimum Td required for your PCB will depend on many factors including surface finish and assembly process; however, the industry guideline for most ROHS applications in a minimum material Td of 340 degrees C.

4. Loss tangent (Dissipation Factor):

Technical definition: The ratio at any particular frequency between the real and imaginary parts of the impedance of the capacitor.

Why this is important: A large loss tangent means you have a greater amount of dielectric absorption, which can cause the value of capacitance to change with frequency.  If clean, consistent, capacitance is a requirement of your design, then look for a material with low loss tangent values. For high speed designs (greater than 1Ghz) it may be recommended to choose a material with a dissipation factor of less than .015.

5. Moisture Absorption:

Technical definition: Maximum percent of moisture absorbed by material in high-humidity conditions.

Why this is important: Absorbed moisture can raise Dk values, expand the board, and cause thermal defects such as substrate blisters, barrel cracking and delamination during assembly. If the PCBs are stored for only short times in low-humidity locations before assembly, then moisture may not be a problem. However if the PCBs are stored in high humidity for long periods of time, then they may need to be pre-baked before assembly.

6. Coefficient of Thermal Expansion (CTE):

Technical definition: A material’s fractional change in length for a given unit change of temperature.

Why this is important: Glass, copper, gold and nickel all have fixed expansion rates up to their respective melting points.  A large difference in laminate expansion rates can cause strain on the plated hole wall causing cracked barrels and lifted land patterns during the assembly process.  The common unit of measurement for CTE is ppm/°C, parts per million per degree centigrade. 1 ppm is equivalent to 0.0001% of total observed dimension. A material rated at 250 ppm/°C would change 0.025% in dimension for every degree change in temperature. On a .100” thick board over a 100°C temperature range there would be a total thickness change of 2.5% which equates to 0.0025”

7. Thermal conductivity:

Technical definition: Ability of a material to conduct heat.

Why this is important: As the power and density of components on a PCB rises, the need to dissipate heat through the base material of the PCB increases.

Materials that offer greater thermal conductivity can be utilized with designs that have high power, or high heat output devices such as LED’s, coils or relays.

9. Peel Strength

Technical definition: The strength of the bond between base material and copper cladding as measured by IPC-TM-650.

Why this is important: As components become smaller, their footprint or pad patterns also become smaller, resulting in much less area of contact between the base material and the copper land patterns.  The strength of the bond at this area will determine the ability of the pads to avoid lifting from the material surface.

10. Arc Resistance

Technical definition: Measure of electric breakdown condition along an insulating surface, caused by the formation of a conductive path on the surface.

Why this is important: Typically a consideration for high-voltage/high power PCBs.  Arc resistance is a measurement, in seconds, of the amount of time for breakdown along the surface of the material.

If you have questions regarding materials or anything else PCB- Send us an email at info@PCB-solutions.com and we will be happy to guide you in the right direction.

Please visit our home page at: www.pcb-solutions.com We are a supplier of Domestic and off-shore Rigid, Rigid-Flex and Flex Printed Circuit Boards (PCBs), Domestic Military PCBs, Domestic Tier I PCBA, Domestic Sheet Metal, Domestic Injection Molding and other Custom Fabricated Services.

As PCB designers face greater challenges with fine-pitch components, less real estate and greater need for thermal conductivity, the use of epoxy filled vias has become common.  With two very different options of non-conductive and conductive (silver filled) epoxy available, the question if which is best often leaves designers and engineers with a difficult decision to make.

Conductive (Silver Filled) Epoxy:

Conductive silver filled epoxy contains organic solvents which require storage at a temperature of less then 5 degrees C, and limit the shelf life of the material.  Because of this, many PCB factories do not stock this epoxy and purchase it in quantities needed for the order, which may cause delays in production.  The size of the silver balls in the epoxy can make filling smaller holes difficult and often lead to air pockets inside the hole, which will actually decrease the thermal conductivity of the hole.  If the air pockets are close to the surface, this can cause voids at the pad surface, leading to assembly issues.  As the CTE (Coefficient of Thermal Expansion) of Silver Filled epoxy can be very different from the CTE of high TG material used in lead free or ROHS assemblies, blistering of surface pads can be common, as well as lifted pads during high temperature soldering operations.

Silver filled epoxy reacts to process chemistry during metallization by expanding which will cause an uneven or swelled pad surface.  This creates the need to add an additional process of “planarization” of the pad surface which can be costly, time consuming and add to pad surface imperfections including dimples in the land pattern and voids that can outgas during assembly.

Non-Conductive Epoxy:

Non-Conductive epoxy is 100% solid epoxy material which usually yields good pad planarity for via-in-pad designs.  The lack of silver in the epoxy resin allows the process to be used on smaller holes, including micro-vias often down to as small as .004 inch.  The CTE (Coefficient of Thermal Expansion) of non-conductive epoxy is often very close to that of high TG material, meaning less issues with hole expansion during the assembly process.  This also allows Non-Conductive Epoxy to be used in sequential lamination processes to fill blind and buried vias.

Non-Conductive Epoxy usually exhibits very little shrinkage during the thermal curing process, which will yield good pad planarity and will not require a separate planarization process.   The flat surface of the pad after plating over the Non-Conductive Epoxy means less chance of lifting pads during the assembly process and greater rework ability.

As there are no temperature or shelf-life restrictions to Non-Conductive Epoxy fill, most PCB factories have a ready supply of the material in stack, thus eliminating production delays caused by material procurement.

Effects on Thermal Conductivity:

Improved thermal conductivity is often cites as the reason for choosing Conductive Epoxy Fill over Non-Conductive Epoxy fill.  It is true that pure silver has the highest thermal conductivity; however, when surrounded by epoxy the silver surfaces are insulated and there is no direct contact of the silver to effectively increase the thermal conductivity.  Additionally, the air pockets present when using Silver Filled Epoxy may also reduce the thermal conductivity.  A properly filled and capped plated through hole will have better thermal conductivity using Non-Conductive Epoxy fill in place of Conductive Silver fill.

Conclusion:

The development of improved formulations of Non-Conductive Epoxy fill have created an equal or superior bond to electroless copper and increased the Thermal Conduction of filled holes, while eliminating many of the manufacturing and assembly concerns with Conductive (Siler Filled) epoxy.  Due to the points raised above, PCB Solutions strongly recommends Non-Conductive Epoxy

Please visit our home page at: www.pcb-solutions.com We are a supplier of Domestic and off-shore Rigid, Rigid-Flex and Flex Printed Circuit Boards (PCBs), Domestic Military PCBs, Domestic Tier I PCBA, Domestic Sheet Metal, Domestic Injection Molding and other Custom Fabricated Services.

Through the years, the printed wiring board industry has had to evolve in both materials and processes to meet the needs of the world’s electronic challenges. At first the laminate resin systems were inadequate to survive thru multiple thermal excursions of double-sided assembly and rework processes, then the need to meet the high speed signal integrity requirements were needed and now because of the environmentally friendly initiatives of RoHS have again asked the industry to step it up.

Currently most printed wiring boards can meet RoHS (Restriction of Hazardous Materials) directive requirements as long as the boards’ surface finish does not contain lead. Most if not all laminate manufacturers have already removed or reduced mercury, cadmium, hexavalent chromium, polybrominated biphenyls and polybrominated diphenyl ether levels to meet the directive. The challenge has been how to reliably attach components to the PWB without the use of lead, which have higher temperature and longer oven dwell times due to lead free metals being used. For this you need to look at the laminate materials Td rating (Time to Decomposition).

That’s right, even though material Tg has been the main focus of a laminates ability to survive temperature for years, it is not as important as the material’s Td.

Td is the measure of how long a material can handle the higher temperatures of the assembly process. Materials are more thermally resistant as evidenced by their higher Td rating and their ability to achieve a T260 or T288. A T260/288 (Time to Delamination) is the amount of time the material can withstand exposure to 260 / 288°C.

However, if you have a low cost double sided board that requires the use of lead free solder, it may not necessarily mean that you need the higher Tg/Td laminate. Talk to your assembler; you may be spending more than you need to on raw material costs, and being that laminate costs are the single highest material cost of the printed wiring board, it may be worth running some tests by subjecting boards to multiple Pb free reflow cycles to verify it’s resistance to delamination and blistering.

It is true that lead free solder does require higher reflow temperatures and longer dwell times but if the overall density of board is low, it will reflow faster because the entire structure heats up faster and does not require as long of a time in the assembly ovens to get up to the necessary reflow temperature to make a good solder joint, the lower Tg/Td material can reliably withstand the assembly process. Keep in mind that even standard 130dC Tg laminates made today are superior materials that meet RoHS requirements and work well for many different applications and in many cases can take a the thermal excursion of lead free assembly.

On the other hand if you have an expensive multilayer board that has a lot of copper planes, high density, it takes the structure much longer to heat up in order to reflow all the solder joints, in this case you need the extra assurance of high Td rated laminate systems.

If you have questions regarding materials or anything else PCB- Send us an email at info@pcb-solutions.com and we will be happy to guide you in the right direction. For more detailed information on Surface Finishes, visit our Surface Finish presentation in the Tech Zone at http://www.pcb-solutions.com/files/TECHZone-09-02-surface-finish.pdf

Please visit our home page at: www.pcb-solutions.com We are a supplier of Domestic and off-shore Rigid, Rigid-Flex and Flex Printed Circuit Boards (PCBs), Domestic Military PCBs, Domestic Tier I PCBA, Domestic Sheet Metal, Domestic Injection Molding and other Custom Fabricated Services.

http://www.pcb-solutions.com/pcb.html

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Bob Neisis

Quality Manager

PCB Solutions, LLC

bobn@pcb-solutions.com

There is an old saying “Nothing Solders Like Solder” but because of shrinking component footprints, environmental initiatives, cost, reworkability, mixed technology boards, ease of assembly and of course reliability finding the right finish for your product has made the selection of PWB surface finishes a little more complex.

There has been various attempts to solve the many issues, there was high hopes for the many finishes including Organic solderability preservatives, immersion silver, immersion tin, non leaded solder, electroless nickel immersion gold, flash gold, electroless nickel immersion paladium immersion gold however some are not available at all PWB shops, some designs require multiple surface finishes that are not compatible and all of them have there pros and cons.

Other than the tried and true leaded solder, the one surface finish that seems to have lasted the longest and has gained the most acceptance is ENIG (Electroless Nickel Immersion Gold) because it has a long shelf life, a flat surface and is still relatively inexpensive due to the gold’s ultra thin thickness even though gold prices are now reaching all time highs.

The selection of surface finish needs to done with your Electronic Manufacturing Service partner because when assembled boards have solderability issues, they are the ones that have to deal with the domino effect and headaches of costly rework and fleeting schedules.

If your surface finish is the cheapest but often requires assembly rework, the extra cost of a surface finish from the PWB shop is a fraction of the cost when schedules slip, rework starts and blame is in abundant supply.

If you have questions regarding surface finishes or anything else PCB- Send us an email at info@pcb-solutions.com and we will be happy to guide you in the right direction. For more detailed information on Surface Finishes, visit our Surface Finish presentation in the Tech Zone at http://www.pcb-solutions.com/files/TECHZone-09-02-surface-finish.pdf