The drilling machine is a preprogrammed Computer Controlled (CNC) machine. The drill takes place based on the XY coordinates fed into the CNC system. The spindles rotate at a high RPM and ensure an accurate drill hole in the PCB. When the spindle rotates with a rapid speed, heat generated. due to the friction between the hole wall and the spindle. This melts the resin content on the hole walls and results in a smear of resin. Once the required holes drilled the exit and entry panels discarded. This is a small gist of what goes happens on the shop floor.

 

 

     As the packing density of the through holes in the PCB assembly line increases. the demand for smaller holes increases . Two main technologies that used to produce accurate. and repeatable holes of microns in diameter are mechanical drilling and laser drilling. The through hole diameters can range from 50-300 microns. and run about 1-3 mm deep using these PCB drilling techniques.

 

      The drill press consists of a high-speed spindle, which rotates at speeds of about 300k RPM. These speeds are essential to produce the required accuracy. for micron-level drilling on the PCB.

 

      To maintain accuracy at high speeds, the spindle makes use of an air bearing and a direct drill bit assembly. which held by a precision collet chuck. Additionally, the vibration of the tip of the bit controlled within a 10-micron range. To maintain the exact position of the hole on the PCB. the drill mounted on a servo table, which controls the motion of the table on the X and Y-axes. A tunnel actuator used to control the motion of the PCB in the Z-axis.

 

       As the pitch of the holes in the PCB assembly line is  decreasing. and the demand for higher throughput is increasing. the electronics to control the servo may lag behind at some point. The use of laser drilling to create through holes. for manufacturing PCBs helps reduce. or drop this lag and is the next generation’s need.

 

    Laser drills used in the PCB fabrication process consist of a complex set of optics. which used to control the pinpoint accuracy of the laser, required to make a fine hole.

 

      The size (diameter) of the hole to drilled in the PCB controlled using the aperture of the setup. and the depth of the hole controlled by the time of exposure. Also, the beam split into various energy bands to further provide control. and precision. A moving focal lens used to. concentrate the energy of the laser beam exactly at the drilling location. A Galveno-sensor used to move and position the PCB with high speed and accuracy. Galveno sensors capable of switching at speeds of 2400 KHz are currently. used in the industry.

 

Additionally, a novel method known as the direct exposure technique. can also used to drill holes in the circuit board. This technique based on the concepts of image processing. where the system increases the accuracy. and speed by creating an image of the PCB and converting that image into a position map. The position map is then used to align the PCB under the laser for the drilling process. 

 

Advanced research in image processing algorithms and precise optics will. further improve the throughput and yield of PCB fabrication. and high speed drilling used in the process.

 

Classification of Holes

The drilled holes classified into Plated Holes (PTHs) and Non-Plated Holes (NPTHs).

The plated holes (PTHs) are the signal carrying conductive vias. that establish interconnection between the different layers in the PCB.

 

The non-plated holes (NPTHs) are non-conductive. These used to hold the components in position during the PCB assembly process. The component mounting holes are NPTHs. There is no tolerance level for these holes since the components won’t fit in if the hole size is too small or large.

 

A rule is a rule even for a hole

Non-Plated Through Hole (NPTH)

Finished hole size (mimimum)= 0.006″

Edge to edge clearance (from any other surface element) (minimum)= 0.005″

Plated Through Hole (PTH)

Finished hole size (Smallest) = 0.006″

Annular ring size (Minimum)= 0.004”

Edge to edge clearance (from any other surface element)(minimum) = 0.009″

Drilling Disasters

After repeated usage, the drilling tools tend to wear and break. This leads to the following problems:

 

The accuracy of the hole location compromised:

When the drill bit fails to hit the preferred spot and shifts away in the same axis. The shifts in the drilled hole will give rise to tangency or breakout in annular rings.

 

Roughness inside the drilled hole:

Roughness leads to non-uniform plating of copper. This results in blow holes and barrel cracks. It can also result in lower insulation resistance. by penetration copper plating solution to the hole wall.

 

Resin smear:

The resin in the board melts due to the heat generated during the drilling. This resin sticks to the hole walls and called as resin smear. This again results in poor copper plating and leads to conductivity failure. between the via and the interior layers of the circuit. The resin smear removed by a chemical solution.

 

Presence of entry and exit burrs:

Burr is the unwanted part of copper sticking out of the hole after the drilling process. They seen both on the top surface of highest stacked of printed circuit board. and on the bottom surface of lowest-stacked of the printed circuit board.

 

Nailhead:

Exposed copper of inner layers on through holes. formed the shape of nail heading during drilling. Such a huge burden to hole brings non-uniform surface of. through holes and may cause conductivity failure of plating.

 

Delamination:

The partial separation of the PCB layers.

 

All these irregularities ruin the integrity of a PCB. These problems have been a nightmare for the PCB manufacturers. For these reasons our in-house PCB engineers define PCB as, “Problems Come Back!”

 

To drop these flaws. scholars have researched on drilling process and PCB design structure. and have come up with the following solutions:

 

The Remedies

Desmear process

It is a chemical process where the melted resin that deposited on the hole walls removed. This process eliminates unwanted resin and enhances electrical conductivity through the vias.

 

Deburr process

It is a motorized process that. eradicates the elevated ends (crowns) of the metal (copper) called burrs. Any debris that’s left out 
 

• Excellent flatness and thickness tolerance
• Homogeneous high density core provides for good cleaning. and cooling of the drill bit
• offers a smooth surface. providing good face to face contact with the PCB giving good burr control. through a wide range of drill diameters
• Can be ‘flipped’ and used on both sides, maximizing use and material yield