Printed circuit structures, the evolution of printed circuit boards

Abstract

The Printed Circuit Board (PCB) is the backbone of electronics and a large number of consumer devices. The challenge to put more function in a smaller space requires more components utilizing smaller bond pads, smaller lines and tighter pitch. The electronic packaging industry has aggressively pursued novel ways to shrink and stack multilayer boards inside smaller volumes. Industry is approaching serious obstacles in the continued size reduction requirements with the need for wires, epoxy, vias, solder and sometimes bolts and screws to mount the boards. The next logical step is to move beyond 2D stacking, which is 2.5D to make 3D packages and to utilize the 3 rd dimension directly. Eliminate the traditional 2D FR4 board and the wires, epoxies, vias and solder and make the next generation packages utilizing the 3 rd dimension; the Printed Circuit Structure (PCS). The PCS concept will allow passives, actives and even antennas to move out of the XY plane and into the XZ and YZ planes. This new dimension will appear to be very complex and next generation circuit optimization will be required, but the end result will net a significant improvement in volume utilization. In addition, if new materials are developed and utilized properly, the PCS will be the box or the package thus eliminating all the bolts and screws necessary to mount a PCB in a traditional box or package, thus again saving space and reducing weight. nScrypt and the University of Texas at El Paso will present 3D Printing of Printed Circuit Structures. A demonstration of true 3D electronic structures will be demonstrated and shown as well novel approaches which utilize Computer Aided Design (CAD) to 3D Printing which will include the electronics portion. Introduction Printed Circuit Boards are a critical component in almost every electronic device. Electronics come in a variety of shapes and sizes which is determined by function, environment and physical shape; at least the physical device comes in a variety of shapes. The electronics portion of the device is limited to a standard 2.5 D approach. This implies building a multilayer board with dimensions in X and Y and then mounting that board to a structure of specific shape and volume. But data transfer speeds are seeing their limitations in this conventional PCB approach. New materials are being used to overcome these limits, but the actual structure will need to see improvements too. 1,2 Wires, epoxy, vias, solder and connectors all contribute parasitic harmonic effects to standard PCBs due to impedance mismatching and/or sharp turns that create electromagnetic (EM) reflections. 3,4 Using 3D printing, there will no longer be a need for wires, epoxy, vias, solder or bolts and screws. The 3D printing process can move from building a circuit which is flexible enough to be rolled into a cylinder, to building a cylinder with a curved circuit within it. These structures will be solid or even porous depending on the applications of the PCS. 3D printing has been around since the 1980’s and starting as a novel demonstration, but is now becoming more ubiquitous. With the introduction of table top fused deposition manufacturing (FDM) machines even home users have the opportunity begin exploring possible 3D prints. These printers allow users to convert their CAD into physical structures for prototyping or even small part replacements. 5 3D Printing 3D printing or Additive Manufacturing (AM) is an efficient and green form of manufacturing which fabricates products by building successive layers of material, thus creating little to no waste. Traditional subtractive techniques start with bulk materials and machine away unwanted excess.The first concept of AM known as Selective Laser Sintering (SLS) places a thin layer of the powdered material onto a work surface and a laser beam patterns metal thin shapes by sintering the powder particles together. The work surface is lowered and a second layer of powder is spreadon top of the existing metal shape. Through multiple lowering’s, powder spreading and sintering cycles, a 3D structure can be built with features and voidsthat subtractive processes cannot achieve; resolution of lines are around 0.005in and layer thickness of 0.004in. StereolithographyApparatus (SLA) is another 3D printing approach which is a similar SLS but instead of sintering powder, it hardens photosensitive resin (liquid). There are some builds that require support structure materials if there are large gaps in the 3D structure during the build. Support structure materials are temporary and typically dissolved with water. SLA, like SLS, is an expensive process given the time it takes to build a part (minutes to hours) and the photosensitive resin is very expensive. The features created are solid and the surface finishes can be smooth; feature sizes can be as small as 0.001”per inch for commercial grade tools, but research tools have achieved 0.0001” features. Figure 1 are photos of commercial tools for 3D printing and Direct Printing.