A primer on the steps for building advanced PCBs.

To get more functionality out of boards but within the same or reduced board sizes or areas, OEMs are increasing the density of the product by means of high density interconnect (HDI) boards, which are PCBs with multiple layers vertically connected with blind or buried vias.

HDI PCBs use high-performance thin materials, and have fine copper lines and microvias. While various methods are available, some patented, some not, we use what’s known as Every Layer Interconnect (ELIC) technology, which produces very thin flexible PCBs with high functional density per unit area. Advanced HDI PCBs make use of multiple layers of copper-filled stacked in-pad microvias that enable interconnections with even greater complexity.

Soldering is the only process where the outcome can be impacted in real time.

One of AIM’s field engineers came back from a cross-country trip this week with stories of a profiling issue that was giving the client difficulties. Ultimately, the issue was design-related with a large ΔT that could not be overcome with the equipment used in production. It took a full day of attempts to make that final assessment.

Many engineers and technicians I work with rank reflow profiling alongside getting their teeth cleaned or an early morning workout. You know you need to do it, and the benefits are significant, but they aren’t immediate, and it is an unpleasant chore. Let’s take a minute to go over best practices for reflow profiling. Ideally, a “golden board” will have been supplied as part of the work kit by your customer or your design team. This board (FIGURE 1) will be a sacrificial, fully populated assembly with thermocouples attached (ideally five to seven) with high-temperature solder in strategic locations across the assembly. This board can be processed through the reflow oven to collect detailed information to ensure proper solder reflow temperatures are achieved within the temperature constraints of other components on the assembly.

An overview of the multilayer PCB fabrication process.

The actual process of PCB fabrication can begin on receipt of the necessary documentation from the designer. These data include the choice of materials for the substrate and cladding, the number of layers and stackup, the mechanical layout, and the routing. The documentation must provide individual details for each layer of the PCB.

Preparing the central panel. The fabrication process starts with obtaining the copper-clad substrate. For a multilayer board, copper will be on both sides of the substrate, which forms the innermost or central layer. Usually, such copper-clad substrates are supplied in sizes of standard dimensions, with the panel sized to match the specific mechanical layout. Otherwise, the fabricator will resize the panel the necessary dimensions by means of a shearing process. Depending on the size and total number of discrete PCBs to be made, the panel may be dimensioned to contain multiple PCBs: An 18" x 24" panel might, for instance, contain four 4" x 4" PCBs. The copper cladding is usually provided with a thin layer of protective coating to protect the surface from oxidation. This protective layer must be removed by immersing the panel in a weak acid bath.

In part I, the author reviews design steps and material choices.

Printed circuit boards are produced in different forms, e.g., rigid, flexible, rigid-flex, and high-density interconnect (HDI). The primary differences are in the materials used to fabricate them; these materials, by their properties, give PCBs their ability to flex or to remain rigid.

Regardless of the materials used, the primary steps in the PCB manufacturing process are generally the same. But before PCBs reach the manufacturing stage, the PCB designer must make some choices depending on the application:

• Selecting the type of PCB required by the application;
• Deciding the number of layers (one, two, four or more);
• Deciding the mechanical layout, the stackup, and the routing of tracks on different layers;
• Producing relevant documents and files for the manufacturing process.