Tech Tips

Roy AkberAn 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.

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Roy AkberIn 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.
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Roy AkberThink ahead, because the cost of a PCB is essentially designed into it.

The many different factors and variables of producing PCBs complicate the task of estimating the cost to manufacture them. Considerations for the cost factor depend primarily on the different production strategies manufacturers use, the varied production equipment employed, and the range of technologies available for creating the final product.

Regardless of the factors responsible for the cost buildup, it is critical to control costs in the early stages of the PCB design process. This is because the cost of a PCB is essentially designed into it, and it is impossible to reduce it later without redesign. Although additional process steps do add to the associated cost in terms of materials, consumables, process times, waste treatment, and energy, the process cost impacts the PCB price regardless of the manufacturer.

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Timothy O'NeillPicking the right tool for electrochemical contamination at the rework bench.

A half-dozen versions of the same scenario occurred in the past month, all having to do with materials and processes used in post-op/rework applications. This step of the production process often escapes the attention of engineers because there’s no cool machinery or any real engineering that takes place. Most hand-solder operators are highly proficient and have developed techniques that get the job done, which can lull a supervisor or production manager into a false sense of security. Electrochemical contamination doesn’t normally appear until it has become a dreaded field failure. In fact, if the issue is contamination/corrosion/leakage-related, the first place I look is the rework bench, and eight times out of 10 that’s where the trouble spots lie.

Manual soldering applications have different requirements than upstream processes, and it’s worth detailing these differences to understand the importance of materials selection and proper usage.

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When it comes to application, less is more.

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Tim O'NeillWith RoHS exemptions set to expire, can SAC 305 hang on?

It is not the strongest of the species that survive, nor the most intelligent, but the one most responsive to change.
– Leon Megginson

The final steps of RoHS will be phased in over the next 24 months. Once implemented, lead will be virtually eliminated from solder in the electronics assembly supply chain. With the last exemptions applying predominantly to high-reliability applications, materials in use are being scrutinized to determine if they can perform to the mission requirements of high-reliability PCBs.

Concurrent to this material concern is the unrelenting trend in microelectronics: more functionality and performance in smaller spaces. As circuitry becomes denser and more power is inserted into smaller spaces, an inevitable byproduct is heat. These two realities, high-reliability requirements and increased power density, are exposing deficiencies in the de facto lead-free solder alloy, SAC 305. SAC 305 has performed adequately to this point. The processing temperatures are acceptable. It has proved sufficiently durable and is largely compatible with other common materials, albeit at considerably higher cost than the SnPb it replaced. Typically, if an alloy other than SAC is in use, it’s for cost containment rather than solder joint reliability characteristics. But the needs of the industry are evolving based on the aforementioned changes in regulation and reliability requirements. As a result, SAC 305 may fall out of favor, as it has a variety of undesirable inherent characteristics.

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