The 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 various components, such as copper layer, prepreg (resin/glass fiber) and inner layers. These are key components and can be supplied in many thicknesses and material variations.

This is what an inlay looks like:

An inner layer is typically pre-laminated by the material supplier, who will supply this to the printed circuit board 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: 0.1mm, 0.2mm, 0.3mm, 0.51mm, 0.76mm, 1.0mm, 1.2mm and 1.55mm.

This is what a Prepreg sheet looks like:

The thicknesses and their properties 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 are typically a combination of fiberglass weave and a resin. Prepregs vary in many thicknesses and weave types.Prepreg sheets are used in the lamination process to glue inner layers together or to glue individual copper sheets and an inner layer together.

The material supplier can also supply a laminated dielectric with only one copper sheet. This is known as a one-sided lamination and looks like this:

Multilayer lamination

If you need more than 2 layers of copper on your PCB, the PCB supplier will use all of the above sheet types as components to build a multi-layer PCB.

A 4-layer PCB is laminated by the PCB factory using 2 inner layers, which are separated by one or more prepregs. Adding more prepregs helps you meet the overall thickness. This is the most commonly used 4-layer PCB and is also the cheaper option as each inner layer will be etched and processed simultaneously before being laminated. This type is usually not relevant for HDI.

The alternative 4-layer PCB is actually HDI-friendly, but also more expensive, as the PCB factory needs more components and more time for processing. It can be laminated using individual copper sheets for the top and bottom layers, with an inner layer in the middle. The inner layer and the top and bottom copper plates are separated with prepreg. First, the inner layer is treated (etched), and then it is laminated. After lamination, the top and bottom copper plates will be etched. Building up an even greater number of layers will require the factory to apply additional inner layers or prepregs and copper sheets on examples over 4 layers.

If considering HDI, sequential lamination may be required. For sequential lamination, the board will undergo at least two lamination processes, but depending on the interconnection of vias between the layers, it may be necessary to undergo additional lamination processes. Sequential lamination is generally driven by computer chip packaging, mobile phone, medical products and satellite industries, where the customer use of small BGA packages is high and the thin and tight fanout is required.

Designing a printed circuit board, choosing the use of technology and design rules for optimized and cheapest manufacturing requires extensive know-how about the limitations of the technology used. Keep in mind that there is often more than one technology that meets your requirements, but understanding the best alternative usually takes years to master. If you need guidance, seek out people with expertise in the field and they will help you navigate through the jungle of PCB technology choices that will best meet your demand.