Written by Douglas Brooks, PH.D. and Dr. Johannes Adam
Category: Features
The industry standard formula could triple the number actually required.
If a high-current trace on a board has a via going to another trace segment, the question of via size comes up. Conventional wisdom has always been that the conducting cross-sectional area of the via (or the sum of the vias) should equal or exceed the conducting cross-sectional area of the trace. The IPC standard, IPC-21521, formalizes it this way:
The cross-sectional area of a via should have at least the same cross-sectional area as the conductor or be larger than the conductor coming into it. If the via has less cross-sectional area than the conductor, then multiple vias can be used to maintain the same cross-sectional area as the conductor.
Simplify the stackups and make impedance and other measurements easier.
As many as seven different impedances are called out in various design specifications. Among these are:
33Ω for Rambus memory
50Ω for most high-speed logic
65Ω for the PCI bus
72-75Ω for analog circuits
85Ω differential for PCIExpress (42.5Ω single-ended)
95Ω differential for USB (47.5Ω single-ended)
100Ω differential impedance for most high-speed Internet links (50Ω single-ended)
Successfully designing a PCB with more than one of these impedances can be difficult, if not impossible. The main challenge is designing a PCB stackup that has enough routing room for all the traces of each impedance while maintaining a manufacturable, economical product. A design that is all one impedance would be of great value. If so, what impedance would be the best? Here we address this topic.
PCBAA's executive director lauds recent funding wins but says the heavy lifting remains.
Year-end is typically not the time when big announcements are made, but the news came fast and furious in November as TTM Technologies and Calumet Electronics both announced plans for new factories. Coupled with the opening of Schweitzer Engineering Laboratories' new fab in Idaho and the not-so-secret plant Starlink is building in Austin, one would have to return to early 2001 to see this level of PCB construction in the US.
All this new activity happily coincides with the efforts of the Printed Circuit Board Association of America. The fledgling trade group, which was founded in 2021 to advance US domestic production of PCBs and base materials, has been rallying federal legislators for attention – and funding – to ensure an onshore supply chain for domestic electronics.
We spoke with PCBAA executive director David Schild in late November on the PCB Chat podcast on the latest legislative and industry developments. Excerpts:
An overview of the design and development process.
5G radio networks provide increased bandwidth at the expense of reduced range. To compensate for the reduced range and to increase coverage, availability of cost-effective radio units is critical. In collaboration with Intel, Analog Devices, Comcores and Radisys, Whizz Systems has developed a 5G Open Radio Unit (ORU) white box solution to meet this market need. A broader overview of the 5G architecture can be found in Comcores1 and Radisys.2
Here we provide an overview of the design and development process for the various hardware components that make up the 5G ORU white box. Whizz Systems is responsible for the electrical, thermal, mechanical engineering and manufacturing aspects, as well as system validation and bring up of the turnkey white box ORU solution. This includes design of the individual PCBAs and industrial design of the enclosure.
PCB chemical manufacturing processes can violate data independence and normality.
The most critical assumption made concerning statistical process control (SPC) charts is that of data independence from one observation to the next (free from autocorrelation).1,2 The second critical assumption is that the individual observations are approximately normally distributed.1,2 The tabled constants used to calculate the SPC chart limits are constructed under the assumption of independence and normality.
Many printed circuit board chemical manufacturing processes can violate the assumption of data independence. This is because inertial elements drive reduction-oxidation (redox) chemical processes. When the interval between samples becomes small relative to the inertial elements, the sequential observations of the process will be correlated over time.
Statistical process control charts do not work well if the quality attributes charted exhibit even low levels of correlation over time. Correlated data produce too many false alarms – correlated data underestimate the upper and lower control limits.
An overview of the physical and electrical characteristics of rigid and flex laminates.
Electronic devices functioning at high frequencies are currently evolving at a dizzying pace, particularly within the field of wireless communication. When developing new products, therefore, emphasis is placed on utilization of materials suitable for high-frequency work, above 1GHz.
When selecting materials intended for high-frequency printed circuit boards, several characteristics are of importance:
Dielectric constant (Dk) – dielectric coefficient of the resin. This parameter must be low and stable within a wide range of high frequencies. High Dk values may decelerate signal transfer speed.
Dissipation factor (Df) – the parameter responsible for the signal's quality. The Df value should be low. The lower this value, the more stable the signal, and losses will be reduced.
Moisture absorption – another imperative parameter when selecting materials intended for high frequencies. This is important because the Dk of water is Dk;water = 80.4, a value so high, in fact, the absorption of very small amounts of moisture will instigate a significant increase in the overall Dk of the material.
Coefficient of thermal expansion (CTE) – thermal dimensional expansion parameter of the dielectric material. This parameter must be close to the CTE of the conductive metal; in the case of PCBs, it is copper. Working at high frequencies causes increased heating of the PCB and thus if a significant discrepancy exists between the CTE of the dielectric material and the copper, structure delamination may occur during the activation of heating/cooling cycles.
Additional important parameters such as:
Thermal resistance.
Chemical resistance.
The adhesion strength between copper and the dielectric material.
