Thermal management of printed circuit board components

Abstract

A first thermal management approach involves an air flow through cooling mechanism with multiple airflow channels for dissipating heat generated in a PCA. The air flow direction through at least one of the channels is different from the air flow direction through at least another of the channels. Alternatively or additionally, the airflow inlet of at least one channel is off-axis with respect to the airflow outlet. A second thermal management approach involves the fabrication of a PCB with enhanced durability by mitigating via cracking or PTH fatigue. At least one PCB layer is composed of a base material formed from a 3D woven fiberglass fabric, and conductive material deposited onto the base material surface. A conductive PTH extends through the base material of multiple PCB layers, where the CTE of the base material along the z-axis direction substantially matches the CTE of the conductive material along the x-axis direction.

Description

FIELD OF THE INVENTION[0001]The present invention relates to printed circuit boards (PCBs) and printed circuit assemblies (PCAs) in general, and to the management of thermal issues associated with a PCB or PCA, in particular.BACKGROUND OF THE INVENTION[0002]Printed circuit boards (PCBs) are used to mechanically support and electrically connect different electronic components using conductive tracks, pads and other features etched from conductive sheets, typically copper. These copper sheets are typically laminated onto a non-conductive substrate. PCBs, also called printed wiring boards (PWBs), can be single sided (with one conductive layer), double sided (two conductive layers), or even multi-layer.[0003]When choosing PCB substrate materials, the mechanical, electrical, chemical, and thermal properties of the material should be taken into consideration. A commonly used resin for commercial applications is FR-4, which is a designation given to a composite material of woven fiberglass...

AI Technical Summary

Benefits of technology

The present invention relates to a printed circuit board (PCB) with enhanced durability. This has been achieved by using a three-dimensional (3D) woven fiberglass fabric impregnated with resin as the base material for at least one PCB layer. The 3D woven fiberglass fabric includes a first group of fibers arranged in a plurality of parallel layers, and a second group of fibers interlaced with the first group of fibers. The coinage of the first and second groups of fibers along the first and second directions matches the expansion coefficient of the conductive material used in the PCB. The use of this 3D woven fiberglass fabric as the base material for PCB layers results in a more durable PCB. The method of fabricating the PCB involves depositing a layer of conductive material onto the base material and forming at least one conductive plated through hole (PTH) extending through the base material of multiple PCB layers.

Problems solved by technology

In the recent past, heat loads on printed circuit assemblies (PCAs) have increased significantly, in some cases rising from approximately 30 W to 130 W. Standard VME (Versa Module Europa) conduction cooling designs (such as VITA 46 / 48.2) struggle to provide adequate heat dissipation for such a power level, and are strongly dependent on the amount of ECS (Environmental Control System) cooling available.

Designers use the mezzanine boards to locate high power central processing units (CPUs) or graphics processing units (GPUs), which can generate high power dissipation of around 50 W. For fan cooling platforms, power limitations are also restricted due to the increase of the components operational temperatures.

These factors can cause a reduction in reliability and service life of the boards.

However, liquid cooling solutions significantly increase the dimensions of the cooling system since they require additional items (such as: a liquid to air heat exchanger, a pump, a volume compensator, and the like).

While such an AFTC configuration suggests a solution to overcome the actual heat power requirements for many PCAs, there remain several limitations which restrict board design, such as the location of high power components, e.g., central processing units (CPUs), graphics processing units (GPUs) and field-programmable gate arrays (FPGAs).

This restriction can force the designer to add several high power components in the same board, which can result in poor thermal distribution on the board.

Furthermore, it can affect the design of the heat exchanger which can cause pressure drop.

The increase in pressure drop induces several limitations on the cooling system, aircraft environmental control system (ECS), or fan cooling.

One well known failure mode associated with PCB reliability is the phenomenon of via cracking and PTH fatigue.

This material property mismatch leads to differential expansion during temperature variations and the formation of cracks in the via barrel and inner layers as a result of mechanical fatigue.

When exposed to thermal cycling, the initiated via cracks may propagate along the copper plated barrel and gradually expand, leading to degradation and eventual PTH failure.

In particular, via cracks continually affect electrical discontinuities in the PCB, which can ultimately result in catastrophic failure of the entire PCB-based device.

Such a failure is particularly problematic in certain technical fields, such as various military, aerospace, automotive, and medical device applications.

Large aspect ratio vias tend to have greater plating thickness at each end as compared to the center of the barrel, which increases the likelihood of cracked via barrels due to z-axis expansion when soldering.

However, decreasing the PTH aspect ratio is extremely difficult (if not impossible) in the current era of High Density Interconnect (HDI) PCB designs.

A standard copper plating thickness is approximately 25 μm, and increasing the thickness beyond around 35 μm is extremely difficult from a manufacturing standpoint.

This option may be theoretically possible, but remains unproven.

Yet another approach would be to utilize a Cu material with higher ductility and lower strength, although such a material has not yet been identified commercially.

These options are not feasible since the board processing requires several necessary thermal processes.

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