When selecting PCB technology for your board, you should know the PCB factories have different lamination options to meet the required PCB technology of your board. The used lamination method is based upon your board type and additionally upon the type of PCB-technology that you want to apply to the board during the PCB process. This article will give you a quick insight into the different lamination options at your disposal.
The PCB Lamination process
Before understanding the lamination process in a PCB factory, you must first understand the materials from which you can build your lamination. A PCB typically contains different components, such as copper layers, prepreg(resin/glass fiber) and innerlayers. These are key-components and can be delivered in many thicknesses and material variations.
This is what an Innerlayer looks like:
An innerlayer is typically pre-laminated by the material supplier, who will deliver this to the PCB factory in various thicknesses, sizes and types. The material supplier laminates 2 horizontal copper sheets together by separating them with a dielectric. Here are some thickness examples: 0.1 mm, 0.2 mm, 0.3 mm, 0.51 mm, 0.76mm, 1.0 mm, 1.2 mm and 1.55 mm.
The dielectric in such 2-sided lamination (inner layer) contains typically Resin(glue) and weaved Glass fiber. This laminated inner layer can become an outer layer, if it is processed as a double side PCB, where it will become etched, drilled and plated. The dielectric and the copper sheets used in the innerlayer, can also be supplied directly to the PCB factory as individual sheets, which can be used for customized lamination, when making a multilayer board.
This is what a copper sheet looks like: Thicknesses are typically 18 µm, 35 µm, 70 µm, 105 µm etc.
This is what a Prepreg sheet looks like:
The thicknesses and their characteristics are many, but here are some of the most common types: 106=0.05 mm, 1067= 0.066 mm, 1080=0.075 mm, 2113=0.1066 mm, 2116=0.12 mm, 7628=0.19 mm.
Prepreg sheets are layers of glue and is a typically a combination of Glass fiber weave and a resin. Prepregs varies in many thicknesses and type of weave. Prepreg sheets are used in the lamination process to glue Inner layers together or to glue individual sheets of copper and an inner layer together.
The material supplier can also supply a laminated dielectric, with only one sheet of copper. This is known as a single sided lamination and this is what it looks like:
If you require more than 2 layers of copper on your PCB, the PCB supplier will use all the above-mentioned sheet types as components to build a multilayer PCB.
A 4-layer PCB, is laminated by the PCB factory using 2 innerlayers, which are separated by one or multiple prepreg. Adding multiple prepregs, will help you meet the overall thickness. This is the most commonly used 4-layer PCB and is also the less expensive option as each innerlayer will be etched and processed simultaneously before it is laminated. This type is usually not relevant for HDI.
The alternative 4-layer PCB is in fact HDI friendly, but also more expensive, as the PCB factory must use more components and more time for processing. It can be laminated using individual copper sheets for top and bottom layers, with an innerlayer in the middle. The innerlayer and the top and bottom copper sheets are separated by prepreg. First the innerlayer will be processed(etched) and then it will be laminated. After lamination the top and bottom copper sheets will become etched.
Building even higher number of layers will require the factory to apply additional innerlayers or prepregs and copper sheets to above 4 layers examples. If considering HDI, sequential lamination can be required. For sequential lamination, the board will go through at least two lamination processes, but depending on the interconnection of via’s between layers, it might need to go through additional lamination processes. Sequential lamination has generally been driven by the computer chips packaging, mobile phone, medical products and satellite industry, where the customer use small BGA packages is high and for which the thin and tight fanout is required.
Designing a PCB, selecting the use of technology and design rules for optimized and cheapest manufacturing, require extensive know-how about the limitations of the used technology. Keep in mind that there is often more than one technology that will meet your demand, but understanding the best alternative usually takes years to master. Should you require guidance, seek for people with expertise in the field and they will help you navigate through the jungle of PCB technology selections, that will serve your demand best.