Material selection provides the basis for functionality – the best-designed boards without supporting materials may fail to meet performance benchmarks. Electronics are feature-rich and fulfill many roles, so what should be a simple choice can quickly compound in difficulty. Additionally, the physical design, such as the substrate’s or copper’s thickness, plays an integral role. Deciding upon the correct PCB material will require designers and manufacturers to weigh several viable options.
An Overview of Common PCB Materials
|The most common rigid PCB material consists of a fiberglass weave and epoxy resin.
|High industry familiarity, low water absorption, good insulation, inexpensive.
|As the de facto standard, it may be unsuitable for special application boards.
|Cotton paper impregnated with phenol formaldehyde resin.
|Cheaper than FR-4.
|Limited to single-side boards and temperatures of ~100 ℃.
|Metal core board material.
|Supports cooling of high-power boards.
|Substrate limitations may restrict copper placement to one side.
|A ceramic material.
|Cheaper than aluminum with better thermal conductivity than FR-4.
|Its thermal conductivity is something of a tweener, and it is brittle.
|Found in flex/rigid-flex circuits.
|Allows for flex printed circuits for compact assemblies, miniaturization, and replacement of wire harnesses.
|Difficult to work with, material attributes generally restrict circuit size and manufacturing complexity.
The Attributes of PCB Materials And Their Effects on Performance
The two primary considerations are the dielectric constant (Dk) and dissipation factor (Df). The dielectric constant indicates the insulative ability of a material, which, in strict electrical terms, governs the polarizability of a material – how easy/difficult it is to polarize the material using an external field. Providing an insulative layer between conductive layers and storing energy in an internal electric field is essential to multi-layer board designs that require isolation for signal integrity. Moreover, the dielectric constant impacts efficiency due to the work needed to orient polarized molecules according to dipoles, sapping available energy while heating their surroundings and thermally aging materials. This effect also contributes to higher thermal resistance, which can attenuate signals (especially with increasing frequencies).
The Df, also known as the loss tangent (tan δ), is a more general measure of energy loss due to thermal dissipation within the material. This attribute varies with the signal frequency: at DC, the current distributes equally throughout the conductor thickness. As the frequency increases, most of the current cannot penetrate as deep within the conductor and becomes increasingly localized at the imperfect, rough surface of the laminate. Since the current no longer flows most shortly through the material but instead along a longer surface path, the resistance and corresponding loss increase. Material vendors will indicate the surface roughness of the copper to indicate its susceptibility to loss at high frequencies – high-speed designs will need lower-profile copper teeth.
Most common substrate materials are a particular ratio of epoxy resin to fiberglass weave, although more exotic materials like ceramics are available. For the former, the weave itself plays a significant role in the signal integrity of the board. A looser weave contains gaps that produce an inhomogenous dielectric background in a random signal pathway: two signals may not encounter the same average dielectric constant during travel. At the extremes, the fiber weave effect can cause significant differences in long-running traces. One pathway sees maximum travel over the gap, and another sees maximum travel over the weave for a differential pair.
In this case, despite the length-matching of the signals, the arrival time will be asynchronous, perhaps to the point of disrupting the timing. For high-speed designs, vendors offer materials with tighter weaves to minimize the dielectric constant homogeneity and minimize time-related errors. For cases where timing issues present after purchasing the board materials, a few ad-hoc solutions are available: designers can increase trace width to average the difference in the dielectrics or route in observation of the weave. Both methods are less than ideal as they restrict the layout, a primary concern during HDI routing.
For high-power circuits, thermal routing constraints may become an issue. Note that thermal conductivity is several magnitudes worse for most common substrates than copper. Intuitively, this makes sense: substrate materials function as electrical insulators first, and insulators are poor electrical and thermal conductors. However, designers may want to incrementally improve the conductivity of the substrate material to aid thermal dissipation through the board materials alongside more conductive channels (copper pours, vias, and heat sinks). Designers can also incorporate ceramics with considerably higher thermal conductivity than epoxy-based resin/fiberglass materials or metal core PCBs with a metal layer for even greater dissipative ability.
Inertness is important for the long-term reliability of the circuit board. Depending on the end-use environment of the product, designers will need to address the hygroscopicity of general board materials. While the most pressing issue related to moisture is electric shorts that could damage or destroy the board and its components, delamination or outgassing can also be an issue in board materials due to excess moisture during fabrication. For these reasons, printed circuit manufacturing needs to control the shipping/storage environment of the material and include a pre-bake step to eliminate any moisture before lamination. Additionally, most boards utilize some level of flame retardancy as a precaution in the event of catastrophic failure to prevent further damage; these materials are so common as to have a shorthand designation of FR. Designers can specify higher fire retardance materials in applications where flammability is either more likely or the outcome more unacceptable.
UL 94 Flammability Classifications
|Slow burning rate on a horizontal test specimen: <76 mm/min for board thickness <3 mm or cessation before burn length of 100 mm.
|Burning ceases within 30 seconds on a vertical specimen; flaming particle drip is allowable.
|Burning ceases within 30 seconds on a vertical specimen; non-flaming particle drip is allowable.
|Burning ceases within 10 seconds on a vertical specimen; non-flaming particle drip is allowable.
|Burning ceases within 60 seconds on a vertical specimen; no drip is allowable, but hole development is acceptable.
|Burning ceases within 60 seconds on a vertical specimen; no drip or hole development is acceptable.
Material Matters, And So Does Your Choice of Contract Manufacturer
A circuit board is the sum of the parts, and selecting the PCB materials most appropriate for the board’s functionality and performance is no exception. Designers can talk to manufacturers about suitable materials and perform cost comparisons quantitatively with prototype boards to discern the differing impacts. Most manufacturers work directly with materials vendors and can act as middlemen to gauge any performance tradeoffs. If you’re new to board manufacturing, VSE is a team of engineers committed to building electronics for our customers, focusing on quality control, reliability, and cost-effective productions. Alongside our valued partners, we’ve been realizing life-saving and life-changing electronic assemblies over the past forty years.