Feb 2022

Embedding cooperative thought in your new PCB design materials selection

Embedding cooperative thought in your new PCB design materials selection may provide more significant benefits than you may think!

Because collaboration is a good thing.

It increases knowledge exponentially, reduces time to market, and ensures organizations access the latest thinking, building resilience and maximizing profit.

As it was known during the early 1970s, collaboration or participatory design originated in Scandinavia?

Its roots stemmed from the work of The Norwegian Iron and Metal Workers Union. Which created a project aimed at improving cooperation between management and union as a means of improving worker conditions, and streamlining decision making!

In true Scandinavian style, at the PCB Connect Group, collaboration is at the heart of everything we do. Working with our customers during the materials selection phase of a PCB design is one example of where cooperation can drive cost right from the earliest stage.

The correct specification of material, when considering high frequency, HDI, and products requiring multiple lamination cycles, is essential. An incorrect decision at the concept stage can lead to a lifecycle of poor yield, increased costs, and potentially becoming trapped in a less than optimal supply chain.

So how can collaboration help?

Play the long game.

When considering your new prototype, your domestic fabricator may prefer to use a premium material set ideal for low volume prototyping but may not be cost-effective in volume production due to availability.

Collaborating with potential volume producers at the outset ensures that they can check the lifecycle of the proposed material, the short to mid-term availability, and ensure that possible factories are certified to manufacture and release your PCB correctly.

If you specify a brand of material that is not readily available in your primary manufacturing location, you immediately lock yourself into longer lead times, higher costs through MOV’s and potential certification issues.

What steps do I need to take before determining the most appropriate material for my application?

Temperature matters!

When considering suitable material systems, the TG of the material is essential. The TG or glass transition temperature indicates when the material becomes unstable, effectively moving from a solid-state to a semi-solid state. It is necessary at this stage that you consider not only the operating temperature of the PCB in its final native state but also the type of components to be placed on the PCB and the required temperatures the board will be subject to during the assembly process.

TG values for standard PCB materials increment from TG130, TG150 to TG170 degrees celsius, with each TG type having a higher cost attribute dependant on the value of the material specified.

Comparative Tracking Index (CTI) What is it?

CTI, or the comparative tracking index, measures the electrical breakdown properties of an insulator (PCB substrate), determining how close a pair of conductors or tracks can be positioned without the risk of energy leaking from one conductor to another when power is applied to the substrate.

The higher the CTI rating of the material, the more energy the PCB will withstand before energy leakage starts to occur, ultimately leading to short circuits between the pair tracks.

Tracking Index (V)                     PLC
600 and greater                     0
400 through 599                     1
250 through 399                     2
175 through 2493
100 through 1744

Having considered the thermal regime that the PCB could be subject to over its lifetime, we turn our attention to the electrical properties of the laminate and, specifically, what is known as the Dk value or dielectric constant.

The Dk value of PCB laminates is essential when considering high-speed designs and especially products with impedance requirements. The dielectric constant measures the energy between a pair of tracks within the laminate compared to that pair of tracks if they were contained within a vacuum.

The Dk value for a vacuum is 1.0, with all other materials having a .higherDk value. A laminate with higher Dk value stores more energy than a laminate with a lower Dk value. At higher Dk values, energy flows slower through the tracks, making higher Dk laminates less suitable for higher speed applications.

So I have a new PCB design; what should my significant considerations be?

Ensure that you have selected an appropriate laminate system that meets the operating temperature requirements of your product in its final native form and understand the temperatures that your assembly facility operates at in assembling the product correctly.

Check that you have correctly modelled any impedance characteristics of the PCB relative to the Dk value of the material you are considering.

From a production standpoint, the primary consideration is to ensure that the fabricator you want to use has the correct underwriter laboratories approval to certify and use the PCB laminate you have under consideration. Manufacturing a production batch and finding that your factory cannot support UL approval can be a very costly exercise.

Check the lifecycle of the material either with your likely fabricator or directly with the material producer. Suppose your application has a requirement to provide ongoing products over an extended time frame; ensuring continuity of supply directly from the material producer may save you time and money if the laminate system becomes obsolete mid-way through a project.

Check that your fabricator has the correct UL approvals to certify and release the product in each territory that you expect to sell and distribute.

Understanding the critical aspects of a material’s thermal and mechanical properties and collaborating with your PCB fabricator will ensure the lowest total cost for your new design and ensure that your PCB has multiple sourcing options at any point in its lifecycle.

For more information on materials selection and how the PCB Connect Group can support your materials decision-making process, why not contact us for more details.