Backdrill refers to the removal of stubs present in copper-plated vias. A stub is the unused portion of a via that does not connect to any layer or serve any function. To better understand back drilling, let's consider a simple example: suppose we have a ten-layer board and a high-speed signal only needs to be transmitted from the first layer to the seventh layer. However, we typically place a via that passes through all the layers of the board, including the eighth to the tenth layer. The eighth and tenth layers are not connected by any trace and form a pillar, which can cause signal integrity issues and needs to be drilled out from the backside (secondary drilling), hence the term back drilling.

Unused vias or stubs in multilayer PCBs can cause impedance mismatches in the signal path.

For high-frequency PCBs (such as those requiring impedance control), techniques such as back drilling, blind vias, and buried vias can be helpful so that they can form part of the solution to address such issues.

General Characteristics

   1. Typically used for boards with four layers or more

   2. No specified board thickness; 0.8mm is acceptable

   3. Usually, rigid boards are drilled for back drilling

   4. Minimum diameter for back-drilled through holes is 0.15mm, and for blind vias is 0.25mm

   5. Back-drilled holes are 0.1mm larger than through holes

   6. Tolerance for back drilling depth is +/-0.1mm

The Advantages of Backdrill

   1. Improved signal integrity

   2. Better impedance control

   3. Reduced signal crosstalk between vias

   4. Reduction in local board thickness

   5. Reduced reliance on blind and buried vias, which can simplify PCB fabrication.

The Manufacturing Process of Back Drilling

   1. Use the positioning holes on the PCB for positioning and perform the first drilling.

   2. Seal the positioning holes with dry film, then electroplate the PCB after the first drilling.

   3. Create the outer layer pattern on the PCB after electroplating.

   4. Seal the positioning holes with dry film, and then perform pattern plating on the PCB with the formed outer layer pattern.

   5. Use the positioning hole used for the first drilling to locate the back drilling, and use a drill to perform back drilling on the plated holes that need to be back drilled.

   6. After back drilling, wash the back drilled holes to remove any remaining debris.

The Function of Back Drilling

The purpose of back drilling is to drill off the unused portions of plated through holes that do not serve any connection or transmission functions, in order to prevent issues such as reflection, scattering, and delay in high-speed signal transmission.

The Application Fields of Back-drilled Boards

The application fields of back-drilled boards mainly include communication equipment, large-scale servers, medical electronics, military, aerospace, and other related industries.

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