Editing Printed circuit board (section)

== Materials ==

=== Laminates ===
Laminates are manufactured by curing layers of cloth or paper with [[thermoset]] resin under pressure and heat to form an integral final piece of uniform thickness. They can be up to {{Convert|4|by|8|ft|m}} in width and length.  Varying cloth weaves (threads per inch or cm), cloth thickness, and [[resin]] percentage are used to achieve the desired final thickness and [[dielectric]] characteristics. Available standard laminate thickness are listed in
ANSI/IPC-D-275.<ref>{{cite book |title=IPC-D-275: Design Standard for Rigid Printed Boards and Rigid Printed Board Assemblies |publisher=IPC |date=September 1991 }}</ref>

The cloth or fiber material used, [[resin]] material, and the cloth to resin ratio determine the laminate's type designation (FR-4, [[Composite epoxy material|CEM]]-1, [[G10 (material)|G-10]], etc.) and therefore the characteristics of the laminate produced. Important characteristics are the level to which the laminate is [[fire retardant]], the [[dielectric constant]] (e<sub>r</sub>), the [[dielectric loss|loss tangent]] (tan δ), the [[tensile strength]], the [[shear strength]], the [[glass transition temperature]] (T<sub>g</sub>), and the Z-axis [[coefficient of expansion|expansion coefficient]] (how much the thickness changes with temperature).

There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit. Some of these dielectrics are [[polytetrafluoroethylene]] (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well known pre-preg materials used in the PCB industry are [[FR-2]] (phenolic cotton paper), FR-3 (cotton paper and epoxy), [[FR-4]] (woven glass and epoxy), FR-5 (woven glass and epoxy), FR-6 (matte glass and polyester), [[G10 (material)|G-10]] (woven glass and epoxy), CEM-1 (cotton paper and epoxy), CEM-2 (cotton paper and epoxy), CEM-3 (non-woven glass and epoxy), CEM-4 (woven glass and epoxy), CEM-5 (woven glass and polyester). Thermal expansion is an important consideration especially with [[ball grid array]] (BGA) and naked die technologies, and glass fiber offers the best dimensional stability.

FR-4 is by far the most common material used today. The board stock with unetched copper on it is called "copper-clad laminate".

With decreasing size of board features and increasing frequencies, small non-homogeneities like uneven distribution of fiberglass or other filler, thickness variations, and bubbles in the resin matrix, and the associated local variations in the dielectric constant, are gaining importance.

=== Key substrate parameters ===
The circuit-board substrates are usually dielectric composite materials. The composites contain a matrix (usually an [[epoxy resin]]) and a reinforcement (usually a woven, sometimes non-woven, glass fibers, sometimes even paper), and in some cases a filler is added to the resin (e.g. ceramics; titanate ceramics can be used to increase the dielectric constant).

The reinforcement type defines two major classes of materials: woven and non-woven. Woven reinforcements are cheaper, but the high dielectric constant of glass may not be favorable for many higher-frequency applications. The spatially non-homogeneous structure also introduces local variations in electrical parameters, due to different resin/glass ratio at different areas of the weave pattern. Non-woven reinforcements, or materials with low or no reinforcement, are more expensive but more suitable for some RF/analog applications.

The substrates are characterized by several key parameters, chiefly thermomechanical ([[glass transition temperature]], [[tensile strength]], [[shear strength]], [[thermal expansion]]), electrical ([[dielectric constant]], [[loss tangent]], [[breakdown voltage|dielectric breakdown voltage]], [[leakage current]], [[tracking resistance]]...), and others (e.g. moisture absorption).

At the [[glass transition temperature]] the resin in the composite softens and significantly increases thermal expansion; exceeding T<sub>g</sub> then exerts mechanical overload on the board components - e.g. the joints and the vias. Below T<sub>g</sub> the thermal expansion of the resin roughly matches copper and glass, above it gets significantly higher. As the reinforcement and copper confine the board along the plane, virtually all volume expansion projects to the thickness and stresses the plated-through holes. Repeated soldering or other exposition to higher temperatures can cause failure of the plating, especially with thicker boards; thick boards therefore require a matrix with a high T<sub>g</sub>.

The materials used determine the substrate's [[dielectric constant]]. This constant is also dependent on frequency, usually decreasing with frequency. As this constant determines the [[phase velocity|signal propagation speed]], frequency dependence introduces phase distortion in wideband applications; as flat a dielectric constant vs frequency characteristics as is achievable is important here. The impedance of transmission lines decreases with frequency, therefore faster edges of signals reflect more than slower ones.

Dielectric breakdown voltage determines the maximum voltage gradient the material can be subjected to before suffering a breakdown (conduction, or arcing, through the dielectric).

Tracking resistance determines how the material resists high voltage electrical discharges creeping over the board surface.

Loss tangent determines how much of the electromagnetic energy from the signals in the conductors is absorbed in the board material. This factor is important for high frequencies. Low-loss materials are more expensive. Choosing unnecessarily low-loss material is a common engineering error in high-frequency digital design; it increases the cost of the boards without a corresponding benefit. Signal degradation by loss tangent and dielectric constant can be easily assessed by an [[eye pattern]].

Moisture absorption occurs when the material is exposed to high humidity or water. Both the resin and the reinforcement may absorb water; water also may be soaked by capillary forces through voids in the materials and along the reinforcement. Epoxies of the FR-4 materials are not too susceptible, with absorption of only 0.15%. [[Teflon]] has very low absorption of 0.01%. [[Polyimides]] and cyanate esters, on the other side, suffer from high water absorption. Absorbed water can lead to significant degradation of key parameters; it impairs tracking resistance, breakdown voltage, and dielectric parameters. Relative dielectric constant of water is about 73, compared to about 4 for common circuit board materials. Absorbed moisture can also vaporize on heating, as during [[soldering]], and cause cracking and [[delamination]],<ref>Sood, B. and Pecht, M. 2011. Printed Circuit Board Laminates. Wiley Encyclopedia of Composites. 1–11.</ref> the same effect responsible for "popcorning" damage on wet packaging of electronic parts. Careful baking of the substrates may be required to dry them prior to soldering.<ref>{{Cite journal |last=Lee W. Ritchey, Speeding Edge |date=November 1999 |title=A Survey and Tutorial of DIELECTRIC MATERIALS USED in the Manufacture of Printed Circuit Boards |url=http://speedingedge.com/PDF-Files/tutorial.pdf|journal=Circuitree Magazine}}</ref>

=== Common substrates ===
Often encountered materials:
* [[FR-2]], [[phenolic paper]] or phenolic cotton paper, paper impregnated with a [[phenol formaldehyde resin]]. Common in consumer electronics with single-sided boards. Electrical properties inferior to FR-4. Poor arc resistance. Generally rated to 105&nbsp;°C. 
* [[FR-4]], a woven [[fiberglass]] cloth impregnated with an [[epoxy resin]]. Low water absorption (up to about 0.15%), good insulation properties, good arc resistance.  Very common. Several grades with somewhat different properties are available. Typically rated to 130&nbsp;°C. 
* [[Aluminum]], or ''metal core board'' or [[insulated metal substrate]] (IMS), clad with thermally conductive thin dielectric - used for parts requiring significant cooling - power switches, LEDs. Consists of usually single, sometimes double layer thin circuit board based on e.g. FR-4, laminated on aluminum sheet metal, commonly 0.8, 1, 1.5, 2 or 3&nbsp;mm thick. The thicker laminates sometimes also come with thicker copper metalization.<ref>{{cite web |last=Fjelstad |first=Joseph |url=https://www.circuitinsight.com/pdf/manufacture_aluminum_substrate_pcbs_ipc.pdf |title=Method for the Manufacture of an Aluminum Substrate PCB and its Advantages |website=CircuitInsight.com |access-date=January 17, 2024 }}</ref><ref>{{cite journal |last=Yung |first=Winco K. C. |date=2007 |url=https://www.bestpcbs.com/public/pdf/Metal-Core-PCB-design-guide.pdf |title=Using Metal Core Printed Circuit Board (MCPCB) as a Solution for Thermal Management |journal=Journal of the HKPCA |issue=24 |pages=12–16}}</ref>
* [[Flexible electronics|Flexible substrates]] - can be a standalone copper-clad foil or can be laminated to a thin stiffener, e.g. 50–130&nbsp;μm
** [[Kapton]] or [[UPILEX]],<ref>{{cite web |url=http://www.upilex.jp/en/application.html|title= Applications {{!}} UBE Heat Resistant Polyimide Materials |publisher=UBE |website=Upilex.jp }}</ref> a [[polyimide]] foil. Used for [[Flexible electronics|flexible printed circuits]], in this form common in small form-factor consumer electronics or for flexible interconnects. Resistant to high temperatures.
** [[Pyralux]], a polyimide-fluoropolymer composite foil.<ref>{{cite web |url=http://www.dupont.com/products-and-services/electronic-electrical-materials/flexible-rigidflex-circuit-materials/brands/pyralux-flexible-circuit.html |title=Pyralux Flexible Circuit Materials |publisher=DuPont }}</ref> Copper layer can delaminate during soldering.

Less-often encountered materials:

* FR-1, like FR-2, typically specified to 105&nbsp;°C, some grades rated to 130&nbsp;°C. Room-temperature punchable. Similar to cardboard. Poor moisture resistance. Low arc resistance.
* FR-3, cotton paper impregnated with epoxy. Typically rated to 105&nbsp;°C.
* FR-5, woven fiberglass and epoxy, high strength at higher temperatures, typically specified to 170&nbsp;°C.
* FR-6, matte glass and polyester
* [[G10 (material)|G-10]], woven glass and epoxy - high insulation resistance, low moisture absorption, very high bond strength. Typically rated to 130&nbsp;°C.
* G-11, woven glass and epoxy - high resistance to solvents, high flexural strength retention at high temperatures.<ref>{{cite book|url=https://books.google.com/books?id=nnCNsjpicJIC&q=printed+circuit+board+materials+%22fr-1%22&pg=PA475|title=Op Amps for Everyone|first=Bruce|last=Carter|date=March 19, 2009|publisher=Newnes|via=Google Books|isbn=9780080949482}}</ref> Typically rated to 170&nbsp;°C.
* CEM-1, cotton paper and epoxy
* CEM-2, cotton paper and epoxy
* CEM-3, non-woven glass and epoxy
* CEM-4, woven glass and epoxy
* CEM-5, woven glass and polyester
* [[PTFE]], ("Teflon") - expensive, low dielectric loss, for high frequency applications, very low moisture absorption (0.01%), mechanically soft. Difficult to laminate, rarely used in multilayer applications.
* PTFE, ceramic filled - expensive, low dielectric loss, for high frequency applications. Varying ceramics/PTFE ratio allows adjusting dielectric constant and thermal expansion.
* RF-35, fiberglass-reinforced ceramics-filled PTFE. Relatively less expensive, good mechanical properties, good high-frequency properties.<ref>{{cite web |date=September 1, 1998 |url=http://www.microwavejournal.com/articles/2426-a-high-performance-economical-rf-microwave-substrate |title=A High Performance, Economical RF/Microwave Substrate |publisher=MicrowaveJournal.com |access-date=November 4, 2024 }}</ref><ref>{{cite web |url=http://www.multi-circuit-boards.eu/fileadmin/user_upload/downloads/e_taconic_rf35-hf_www.multi-circuit-boards.eu.pdf |title=RF-35 datasheet |publisher=Taconic |via=Multi-CB }}</ref>
* [[Alumina]], a ceramic. Hard, brittle, very expensive, very high performance, good thermal conductivity.
* [[Polyimide]], a high-temperature polymer. Expensive, high-performance. Higher water absorption (0.4%). Can be used from cryogenic temperatures to over 260&nbsp;°C.

=== Copper thickness ===
{{MOS|section|[[MOS:CONVERSIONS]]|date=May 2024}}
Copper thickness of PCBs can be specified directly or as the weight of copper per area (in ounce per square foot) which is easier to measure. One [[ounce]] per [[square foot]] is 1.344 mils or 34 micrometers thickness (0.001344 inches). ''Heavy copper'' is a layer exceeding three ounces of copper per ft<sup>2</sup>, or approximately 4.2&nbsp;mils (105&nbsp;μm) (0.0042&nbsp;inches) thick.  Heavy copper layers are used for high current or  to help dissipate heat.{{Citation needed|date=January 2025}}

On the common FR-4 substrates, 1 [[ounce|oz]] copper per ft<sup>2</sup> (35&nbsp;μm) is the most common thickness; 2 oz (70&nbsp;μm) and 0.5 oz (17.5&nbsp;μm) thickness is often an option. Less common are 12 and 105&nbsp;μm, 9&nbsp;μm is sometimes available on some substrates. Flexible substrates typically have thinner metalization. Metal-core boards for high power devices commonly use thicker copper; 35&nbsp;μm is usual but also 140 and 400&nbsp;μm can be encountered.

In the US, copper foil thickness is specified in units of [[ounce]]s per square foot (oz/ft<sup>2</sup>), commonly referred to simply as ''ounce''. Common thicknesses are 1/2&nbsp;oz/ft<sup>2</sup> (150&nbsp;g/m{{sup|2}}), 1&nbsp;oz/ft<sup>2</sup> (300&nbsp;g/m{{sup|2}}), 2&nbsp;oz/ft<sup>2</sup> (600&nbsp;g/m{{sup|2}}), and 3&nbsp;oz/ft<sup>2</sup> (900&nbsp;g/m{{sup|2}}). These work out to thicknesses of 17.05&nbsp;μm (0.67 [[thou (length)|thou]]), 34.1&nbsp;μm (1.34 [[thou (length)|thou]]), 68.2&nbsp;μm (2.68 thou), and 102.3&nbsp;μm (4.02 thou), respectively.

{| class="wikitable"
|-
! oz/ft<sup>2</sup>
! g/m{{sup|2}}
! μm
! [[thou (length)|thou]]
|-
| 1/2&nbsp;oz/ft<sup>2</sup>
| 150&nbsp;g/m{{sup|2}}
| 17.05&nbsp;μm
| 0.67 thou
|-
| 1&nbsp;oz/ft<sup>2</sup>
| 300&nbsp;g/m{{sup|2}}
| 34.1&nbsp;μm
| 1.34 thou
|-
| 2&nbsp;oz/ft<sup>2</sup>
| 600&nbsp;g/m{{sup|2}}
| 68.2&nbsp;μm
| 2.68 thou
|-
| 3&nbsp;oz/ft<sup>2</sup>
| 900&nbsp;g/m{{sup|2}}
| 102.3&nbsp;μm
| 4.02 thou
|}

1/2&nbsp;oz/ft<sup>2</sup> foil is not widely used as a finished copper weight, but is used for outer layers when plating for through holes will increase the finished copper weight Some PCB manufacturers refer to 1&nbsp;oz/ft<sup>2</sup> copper foil as having a thickness of 35&nbsp;μm (may also be referred to as 35&nbsp;μ, 35 [[micron]], or 35 mic).
* 1/0 – denotes 1&nbsp;oz/ft<sup>2</sup> copper one side, with no copper on the other side.
* 1/1 – denotes 1&nbsp;oz/ft<sup>2</sup> copper on both sides.
* H/0 or H/H – denotes 0.5&nbsp;oz/ft<sup>2</sup> copper on one or both sides, respectively.
* 2/0 or 2/2 – denotes 2&nbsp;oz/ft<sup>2</sup> copper on one or both sides, respectively.