What is PCB back drilling?
Back drilling, a specialized form of depth control drilling, is utilized in the
production of multi-layer boards, like the 14-layer boards shown in the attached
image. In connecting the 6th layer to the 10th layer, the conventional approach
involves drilling through holes (using a single drill) and then applying PTH copper.
However, this method results in a direct connection from the first layer to the 14th
floor. In reality, the desired connection is from the 6th layer to the 10th floor,
treating the 10th to 14th floors and the first layer to the 6th layers as a column with
no interconnecting lines.
This column, known as a ‘STUB’ in the industry, introduces potential issues with
signal integrity in communication signals due to its impact on the signal path. To
mitigate this, the excess column is drilled out from the reverse side, a process
referred to as secondary drilling or back drilling. Despite the name, back drilling is
not entirely pristine due to the subsequent electrolysis of a small amount of copper
and the inherent sharpness of the drill tip.
To address this, PCB manufacturers intentionally leave a small point, and the length
of the residual STUB is termed the ‘B value.’ Typically falling within the range of 50-
150 micrometers, the B value ensures the necessary connection while minimizing
signal integrity concerns.
Why PCB need back driling( the advantage for back drilling)?
1. Reduce interference noise.
2. Enhance signal integrity.
3. Thin out a section of the PCB thickness.
4. Decrease reliance on buried blind vias and simplify PCB production
challenges.
How to make PCB back driling?
1. Supply the PCB, which is equipped with a positioning hole. Utilize this
positioning hole for one-drill alignment and perform one-drill drilling.
2. Conduct electroplating on the PCB after drilling, and apply dry film sealing
treatment to the positioning hole before electroplating.
3. Create the outer layer pattern on the plated PCB.
4. Perform graphic plating on the PCB after forming the outer layer pattern,
and apply dry film sealing treatment to the positioning hole before pattern
plating.
5. Use the positioning hole from the drill for back drilling alignment. Employ
the drill cutter to carry out back drilling on the electroplating hole requiring
this process.
6. After back drilling, cleanse the back-drilled hole to eliminate any remaining
drill cuttings.
What are the key features of Back Drilling Technology?
1. Most backboards are rigid PCB.
2. They typically have 8 to 50 layers.
3. Thickness: 2.5mm or more.
4. Large aspect ratio.
5. Generous board size.
6. Minimum first drill hole diameter is usually >= 0.3mm.
7. Fewer outer layer traces, often designed in a staggered pattern.
8. Back drilling holes are typically 0.2mm larger than the holes to be drilled
off.
9. Back drilling depth tolerance: +/- 0.05mm.
Where is the back drilling primarily used?
Back drilling is primarily used in industries such as communication equipment,
large-scale servers, medical electronics, military, aerospace, and more. Given the
sensitive nature of military and aerospace sectors, domestically produced back
drilling is typically supplied by research institutes, development centers associated
with military or aerospace systems, or PCB manufacturers with a strong military or
aerospace background. In China, the demand for back drilling is primarily driven by
the communication industry, particularly in the steadily growing field of
manufacturing communication equipment.