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Via Tenting, Plugging, and Filling

There are many reasons a printed circuit board designer might want to have a via tented, plugged or filled. First, let’s start with defining these terms since they can be frequently misused and misunderstood.To accurately drill your countersunk holes, we will need the following information.

Via Tenting

This is the easiest and least costly process—actually there is no added cost for this process. Simply remove the mask clearances from the vias you wish to have tented. Tenting a via simply means to cover the annular ring and via hole with solder mask. No special steps are taken to ensure the hole opening remains closed. Tenting a via will sometimes result in the hole remaining covered but it isn’t guaranteed. Smaller diameter vias (12mil diameter or less) have the best chance of remaining closed. The main purpose for tenting shouldn’t be to close the opening of a hole but rather, cover the annular ring to prevent exposure to the elements and reduce accidental shorting or contact with the circuit.

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Copper Thickness FAQ

The most common unit of measure for the copper thickness on a printed circuit board is ounces (oz). But how thick is that? It’s the resulting thickness when 1 oz of copper is pressed flat and spread evenly over a one square foot area. This equals 1.37 mils (1.37 thousandths of an inch). Of course not everyone thinks in mils so please refer to the chart below to convert into your favorite unit of measure.

1 oz Copper Thickness Conversion *To determine the thickness of 4 oz, simply multiply 1 oz thickness x 4.  1.37 mil x 4 = 5.48 mils = 4 oz
1 oz
1.37 mils (thousandths of an inch)
0.00137 inch
0.0347 mm
34.79 µm  (micron/micro meter)

 

Most PCB’s are constructed with 1 oz copper thickness.  At PCB Universe, if we are not given specific specs, we will assume 1 oz when quoting and building your design.  If you have determined that your design requires more current than 1 oz can carry, you’ll need to consider increasing the copper weight or increasing the width of your traces.  Of course the thicker the copper the higher the cost, but there are certainly times where this is necessary.  The cost increase is not only due to the raw material costs but processing thicker copper weights takes more time and is a little trickier to do.  Keep the following chart in mind when designing your board.  The more space you can allow between copper features the better.  Etching the spaces (air gap) between traces is more difficult than etching a trace of the same width as your space.

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Countersink vs. Counterbore

Countersink

A countersink is a cone shaped hole cut into the laminate.  It is typically used to allow the tapered head of a screw to sit flush with the top of the laminate.  By comparison, a counterbore makes a flat-bottomed hole and its sides are drilled straight down.  This is usually used to fit a hex-headed cap or screw.

To accurately drill your countersunk holes, we will need the following information.

  1. Angle of the drill
  2. Major Diameter (finished diameter of the hole at the surface)
  3. Depth the countersink is to be drilled
  4. Which side of the board is the sink on?  Top or Bottom?
  5. The finished diameter of the shaft of the hole
  6. Is the sink and shaft to be plated or non-plated?

Countersink Anatomy.png

Counterbore

A counterbored hole is typically used when a fastener such as a bolt or cap head screw is required to sit flush with or below the level of a surface.

To fabricate your counterbored holes, we would need to know the following information.

  1. Major Diameter (finished diameter of the hole at the surface)
  2. Depth the countersink is to be drilled
  3. Which side of the board is the sink on?  Top or Bottom?
  4. The finished diameter of the shaft of the hole
  5. Is the bore and shaft to be plated or non-plated?

Because the sides of the hole are always parallel, there is no need to specify an angle.

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Controlled Dielectric or Controlled Impedance?

Controlled Dielectric ExampleConfusion abounds regarding the topics of controlled dielectric and controlled impedance in the printed circuit board industry. We’ll try to clarify the subject.

What is the difference between Controlled Dielectric and Controlled Impedance regarding PCB fabrication?
The purpose of controlling dielectric or impedance is essentially the same, to achieve a target impedance on one or more signal lines on a printed circuit board.  For example, your design has a USB signal pair that must have an impedance of 90 ohms (±10%) to function properly. To make sure this will happen, there are many factors that need to be considered and calculated such as trace width, spacing between copper features on the same layer, distance between copper features on other layers, the Dk (dielectric constant) of the laminate used to manufacture the board, as well as a few other factors.  more…

Array Design

Read our latest Array Design Tips article at http://www.pcbuniverse.com/articles.php

The primary reason for having your boards delivered in an array is to make automated assembly faster and less expensive. Running an array of boards through a pick-and-place machine is far more efficient than sending them through one at a time. Arrays are also desirable because they allow the addition of tooling rails, tooling holes, and fiducials, all of which help your assembler.     more…

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