Materials for PCB substrates are not brand names and have nothing to do with a “copy of last year’s bill of materials. The three conditions that the substrate must satisfy are: Electrical – Impedance and loss must be maintained; Thermal – Withstand assembly and operating environment; Mechanical – Withstand vibration, bend and warpage. Consider those three axes as the decision. Now it is not a beginning but a result being material family.
One of the issues is having the model too simple, and the assumption that “FR-4 is FR-4” when layer count is defined. The dielectric response, thermal stability and mechanical reliability are modified by resin systems, glass styles and constructions. Ignore the differences and buy field failures that have been proven to fail over the long term or a specialty laminate that was never necessary.
Electrical Performance: Dk, Df, and Signal Integrity Requirements
For many digital and RF boards, as well as for many other sections, the characteristics of the substrate (dielectric) – Dk (dielectric constant) and Df (dissipation factor) – are as critical as the width and spacing of the traces. The interconnect is still just geometry for the time being. It’s behaviour in the built board depends on the dielectric.
What Dk and Df Mean for Your Design
Dk is a measure of the ability of a substance to hold electric energy as compared to air or vacuum. The longer the same geometry, the higher its Dk, the lower its characteristic impedance and the longer its propagation delay. The opposite is achieved for lower Dk value. Thus the impedance targets cannot be specified in terms of a copper width only. These should be in writing on a construction and dielectric.
It is the percentage of the energy lost as heat in the dielectric (Df). When the frequency is low and edge rates aren’t too high, the difference between common resin systems can be minimal. Loss builds up over every millimetre of path at higher frequency, or short rise times. This is because with small loss budgets, it is essential to consider Df and Dk in addition to looking at stack-up thickness when designing a PCB.
Those values can change frequency, due to resin content, style of glass and moisture, too. Datsheet number is not a commitment to the completed board at any frequency. Try to keep the discussion at the level of the design parameters, and the normal behavior of the industry. Note that one catalogue item is not to be used as a “factory capability statement”.
When Electrical Performance Drives Substrate Choice
If any of the following applies, electrical performance should be given first consideration:
- The remainder of the product is housed on the same board as the RF/microwave path.
- Most of the loss budget is used on high-speed digital connections.
- Controlled-impedance multilayers must be based on a stable and documented dielectric.
- A very small increase in dielectric loss can be obtained with the number of channels (count) or connector transitions.
- Consumer IoT nodes, industrial I/O cards and many low speed control boards abound on the other side of the line. If the fastest edge on the board is not particularly…
A good selection of standard laminate might still be the right choice – provided the thermal/ mechanical axes are fulfilled – despite the demanding nature of the application and the impedance control requirements are limited to a few power/c clocks nets. The mistake here is to use something that appears to be the same material (hence high-speed) or a material that is similar to a standard material (hence familiar).
Thermal Performance: Operating Temperature, Heat Spreading, and Assembly Profile
Thermal performance PCB materials are not solely the domain of “high temperature” marketers. They are concerned about the stability of the construction during reflow, if heat can escape from critical components and if vias and interfaces can withstand repeated expansion and contraction.
Tg, Thermal Conductivity, and CTE: What Actually Matters
Glass transition temperature (Tg) is the temperature at which the resin system switches from solid to soft state, and the mechanical and expansion properties of the resin system change. A board which is subjected to reflow or field temperatures near and above Tg will warp more and stresses the plated vias, and will be less dimensionally stable. Tg is hence a screening value, rather than a trophy number, against the assembly profile and the operating range.
Thermal conductivity is the property which is a measure of the ease with which heat passes through the dielectric. Instead of using metal, poor conductors like the standard organic substrates are used. This doesn’t mean that they are worthless in a thermal path. It doesn’t suggest that the copper planes, thermal vias, metal-core buildings or component placement does much more than the resin. Explain this, rather than making an assumption that the laminate will act like a heatsink.
The coefficient of thermal expansion (CTE) is especially significant in the z-axis, and measures how much the board expands as the temperature goes up. The diameter of copper plated vias increases at various rates. This mismatch is one of the causes of cracking under thermal cycling of plated holes. If selecting a material to be used with either Tg or CTE, it is important to note that these two properties should be considered in tandem; a higher Tg may be accompanied by a more stable expansion profile from below the transition, thus a thick multilayer and high reliability build would take into consideration both properties.
Think these are parameters to be used in design. Don’t claim to give a guaranteed capability set of Tg, CTE or conductivity values for any specific factory.
When Thermal Performance Becomes the Limiting Factor
If product/process is close to a limit, then use thermal performance:
- The surrounding temperature around the enclosure is high, or heat cannot be released from the enclosure.
- There are multiple reflow cycles of lead-free soldering in the assembly profile and it might require rework.
- High density of power in a compact size.
- The stack up is tall and deep and contains many plated vias to be cycled.
- The cracking, pad cratering, or delamination were all seen in previous versions.
Where they are that case, the substrate is not just an insulator between the copper layers but it is a part of the thermal system. Identify how the heat is going to exit the package, number of thermal cycles the board will experience and if the board is able to accommodate z-axis strain. All materials that appear as a good signal-integrity material might not be suitable for the oven and field.
Mechanical Performance: Stiffness, Warpage, Vibration, and Bending
The behavior of the board in a chassis can’t be explained by electrical or thermal numbers. Mechanical properties of PCB Substrate: The resistance to stiffness, flatness and toughness of the completed PCB construction when being mounted, shaken, vibrated and bent.
Rigid, Flex, and Rigid-Flex as Different Mechanical Strategies
Rigid boards stay planar and have the majority of the interconnect stiffness. The thickness, glass style, copper weight and number of layers are all factors that affect the degree of flex and bows that the panel will experience after lamination and reflow.
Flex constructions are available because the interconnect needs to bend, either when it is being put together, or as part of its operation. The next mechanical parts to watch out for are bend radius, coverlay, copper type and static / dynamic flex. Thin FR-4 is not a proper characterization of a flex circuit.
Rigid-flex: A combination of stiff regions for the components/connectors, flexible regions for the packaging or motion. It’s a construction decision, first; and a material-family decision, second. Thin versus thick is not the only thing that matters when it comes to mechanical performance. It’s a combination of construction, material and bend or support strategy.
Warpage, vibration and mounting of real world constraints are talked about.Real-World Constraints – Warpage, Vibration and Mounting are discussed.
Warpage, Vibration, and Mounting as Real-World Constraints
Do not lock a substrate before thinking about the questions it will be asked of by the factory and enclosure at some time:
- What are the points of attachment of the board and what is the structure that joins them?
- Will reflow warpage cause connectors or BGAs to be out of coplanarity?
- Are there continuous vibrations on the product or vibration is part of a vehicle sub-system?
- Has the board ever asked for anyone’s support?
The vibration or warpage limit may be the ultimate failure mode of a thin high layer count design that meets electrical requirements. With a thick, stiff board a low z-height product may not always be a bad choice of chassis. Add those restrictions to the same page as Dk, Tg and loss. They must go before the same decision.
Balancing Electrical, Thermal, and Mechanical Requirements
In reality, the optimal board is a very special case in each of the three dimensions. The work done is useful when it comes to ordering the axes, writing down the minimum values that the axes must attain, and involving the fabricator before the stack-up is frozen.
When Electrical and Thermal Requirements Conflict
The selection of low-loss material for use in high-speed or RF channels can be different from the typical FR-4 in Tg, CTE, moisture behaviour and in drilling, plating, and lamination. Materials and components selected for their high stability to heat and for use in hot environments may not have the lowest losses in a long channel assembly.
Those tensions come up, list the needs instead of collect “best” datasheet lines. If a tight loss budget communications board is desired, it can be treated with a more severe process window. In the event of experiencing a large temperature swing to the power or industrial controller, it can be used with a slightly higher Df to allow desired via reliability and warpage. If it’s the initial stack-up with the fabricator, or the limits on the three axes, you’ll want to communicate this prior to sending Gerbers.
When Mechanical Needs Override Pure Electrical or Thermal Optimization
A few products are physically limited first. Repeated flexing, severe vibration, connector strain, hard thickness and flatness limits can cause a construction without regard to cleanliness of electrical and thermal pick.
This is a system decision. A cracked via will not be “repaired” by subsequently changing the value of the resistor.A broken via in a vibrating enclosure or a bent neck (a bent as a rigid board is bent) cannot be fixed by changing the value of the resistor later. If mechanical axis is the one that sucks the life out of the product, then allow it to have veto and once again, restore the electrical/thermal performance through layout, copper and assembly design, not by ignoring the substrate.
A Practical Selection Workflow for PCB Substrates
The PCB substrate selection workflow should be short, so that it can be used again for the next revision. When designing, the following sequence should be included, not a materials-science notebook.
Start From Application Requirements, Not From a Material Family
Before anyone names a laminate write the requirements:
- Frequency content or edge rate and/or control of impedance.
- Not only the ambient temp. rating on the enclosure drawing but also the operating temperature range including hot spots.
- Assemble using regular or mixed lead parts, rework/reflow.
- Power density and the proper exit of the heat from the board.
- Mechanical constraints: bending, vibration, mounting span, thickness and warpage limit.
- Failure definition and number of thermal cycles and expected life.
- This starts with understanding how to select a PCB laminate to meet reliability criteria.
It’s a response to that list that the family name standard FR-4, high-Tg, RF, flex, metal-core is the name. The family-name comes first in the list.
Use a Simple Scoring Approach Across the Three Axes
If it doesn’t pass a minimum, get rid of it. If there are survivors, choose the one that is at risk, but only meets the other two axes. The electrical requirements for the communication module may be in the “exceed” category, while the electrical requirements for the thermal & mechanical may be in the “meet” category. Thermal/ mechanical and meet if electrical are the two ratings that can be used for industrial drive.
Record the scores with the revision. The next engineer should be able to understand the reasons for acceptance or rejection of a material without having to recreate the reasons from a vendor part number.
Common Pitfalls When Selecting PCB Substrate Materials
The vast majority of changes to the material, made at the last minute, are not due to the lack of a datasheet. They are the consequence of a choice which did not test all 3 axes.
Choosing by Brand or Habit Instead of Requirements
If the electrical load, thermal and mechanical loads on the new board are different from those on the last used board, then the material should not be copied. The “old reliable” laminate may not be the best option because of a faster interface, a cooler enclosure, a thicker stack-up or longer unsupported span.
Symptoms will not be seen until after the first prototype: eye diagrams will be on the margins; warpage will be unexpected after lead-free reflow; failures will be seen after thermal cycling. There are no changes to the datasheet. The application did.
Optimizing One Axis and Ignoring the Other Two
Lowest Df is not even enough. This is not a full specification. Not all copper is thickest.Thickest is not a full specification for copper.
Bench test is not enough for electrically beautiful and thermally delicate board. A board with insufficient thermal strength and an insufficent link margin or compliance will fail. Label the three axes. Make a conscious veto, not a last person speaking veto.
Locking the Substrate Before Consulting the Fabricator
Availability; Glass Style; Usable Dielectric Thickness; Impedance Process Window; Fabrication Sensitivity are all things to be discovered after signing the stack-up. Choosing a single substrate results in late substitutions, additional NRE and non-conforming impedance coupons.
Share the requirement list and a preliminary stack-up prior to the material freezing. The question for the fabricator is, “can you build this?” So it is “which construction complies with these three axis limits and has a process window that I can live with?”
Summary Guidance – Treat Substrate Selection as a System Decision
The “right” substrate is the one that is suitable for the electrical, thermal and mechanical needs of the system. It is not the data sheet that is the most perfect datasheet line.
First write limits for the three axis. Mark candidates to the mark these. Maintain the option that is in the middle of the minimum and exceeds. Then test construction and process window with the fabricator prior to locking the stack-up and before production files exit the design team.