PCB Depth Slot Technology

What is PCB Depth control slot technology?

PCB Depth control slot technology is the stepstair or groove shaped by partial routing in printed circuit boards(PCBs) manufacturing. It has great advantages in PCB assembly as the special shape of depth routing.

  1. Using the depth slot of the PCB edge makes the card slot favor for PCB fixed and installation.
  2. It can avoid cold soldering as the siphonic effect uses an NPTH depth slot.
  3. Use the depth slot in the middle of the PCB applied in special module applications, and degrade the height of the PCB assembly, achieving refinement and miniaturization.
  4. Use special size NPTH depth slot as the resonant cavity of microwave signal, degrade signal loss, and other special functions.

Methods Of Manufacturing Depth Slot

There are two main methods of manufacturing depth slots.

1. Depth Control Routing:

The study for Capability

Mechanical milling:

For the depth slot PCB of mechanical milling, the precision can be +/-4mil is very high as an equipment factor.

The depth slot is always 0.2-0.4mm at some layers. So, it is impossible to mill to a particular layer due to depth tolerance and the limits of precision. At present, main use two main ways to control the depth of milling in stepstair milling process.

  1. Optical rule control Z axis height: Use optical rules to control the Z-axis height for depth routing.
  1. Use a milling tool and a copper layer to form a closed loop. This will measure and control the height of the principal axis to finish the depth-controlled routing process. The copper layer can be a surface layer or a particular layer. This method can achieve a precision of +/-15um. So, the minimum copper thickness for the stepstair is 30um. It risks copper remaining at the bottom layer after the depth milling process.

For this kind of stepstair milling PCB, all use FR4 materials and CORE+CORE to lamination. The prepreg thickness is 20mil, and the chip thickness is 20mil. There are different sizes of stepstair, minimum 20x20mm, maximum 200x200mm, the depth milling is 1.0mm.

Conclusion and application prospect

The research on the manufacturing process of stepped plates found three main control points and methods.

  1. A depth-milling machine makes stepped groove circuit boards. It mainly controls the Z-axis depth. To ensure depth accuracy, compensation adjustment of the milling pad and first board is necessary. The depth accuracy can reach 3mil.
  2. The gasket size and thickness are key to controlling the flow of glue in the stepped groove of the gasket press plate. The right lamination typesetting method can also control warpage and glue flow. This method allows all ordinary FR-4P sheet pressing plates to be used.
  3. The size and thickness of the gasket are the key factors that affect the control of the flow of glue. At the same time, the selection of the gasket material is the key factor for the production of different types of stepped plates. At present, different sizes can be used. Gasket material (silicone sheet and PTFE gasket), this method can be pressed by one-time pressing, and all ordinary ER-4P sheet pressing can be used by batch method.
  4. The ladder board is a special type of circuit board. Its use in device installation, module design, and special materials drives this. Its application range is constantly expanding and developing. The ladder board is now a favorite design for electronic designers. Its production technology and methods are a key focus for circuit board makers to improve their tech and regain their edge.

References: Joseph Fjelstad, Kevin Grundy and Gary Yasumura (Next Generation Copper Based PCB Interconnection Technologies)

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