How to evaluate Hole Registration Accuracy in PCB drill

2019-12-03 14:59Writer: qyadminReading:

 What is PCB Hole

      Printed circuit boards have many different features. all which affect the physical appearance and function of the board. Some of these features about more often. such as how many layers of PCB you have or the type of material supporting your board. but there are other design elements that may get less consideration. including, for example, holes in the board.
      In particular, some printed circuit boards need screw holes. to attach them in your applications. and how you drill these screw holes is significant. The two main methods of drilling screw holes in PCBs are counterbore and countersink. Different types of screws call for different types of screw holes. so the kind of screws you will be using dictate whether you are drilling counterbore holes. countersink holes or both for your PCBs.

PCB drill hole composition

    PCB drill hole is one of the important part for multilayer PCB. usually drilling cost account for 30% to 40% of the total cost in PCB fabrication.
     for the function, the holes can divided into two categories. one used for electrical connection between each layers. The other used to fix or position the components.
      about technology in PCB manufacturing. the holes are generally divided into three categories. which are blind vias, buried vias. and through holes. Blind via located at the top layer and bottom layer surface of the printed circuit board. which used for the connection between surface layer and inner layer. the depth of the hole is usually not exceed a certain ratio.
      Buried hole refers to connection hole. that located in inner layer of the printed circuit board. buried hole does not extend to the surface of the circuit board. Through holes drilled through the whole printed circuit board. which can used for internal electrical interconnection. or used as a positioning hole to assemble components. Because the through holes are easier to achieve. in the pcb manufacturing process with lower cost. so most of printed circuit boards choose to use it rather than the other two holes (blind via and buried via).
       about pcb design, the hole is consist of two parts. one is the drill hole in the center area, the other is the pad around the drill hole. The size of these two parts determine the hole size. in the design of a printed circuit board with high speed and high density. the designers always hope that the holes the smaller the better. so that it can leave more space for circuit layout.

        But the hole size decrease will bring the increase of the pcb fabrication cost. moreover, the hole size is impossible to decrease. because it's limited by the process of drilling and plating. The smaller hole size, the longer drill time required, and the easier off center position. When the depth of hole is more than 6 times of hole diamete.  the copper plating process won't guarantee that the hole wall could plate copper . For example, a standard 6 layer PCB with thickness (depth of the hole) about 50 mil. so the PCB manufacturer can provide the minimum hole diameter to 8 mil.

What is de diffference between Counterbored Hole and  Countersink Hole ?


Counterbored Hole

The first type of hole you might have to drill into your printed circuit board is a counterbored hole. This is a standard screw hole. It is a cylindrical hole that you drill into the surface of the board to fit a typical socket cap screw. with a somewhat rounded head. But, a standard socket cap screw is not always the best choice for all printed circuit boards. which is where countersink holes come in.

Countersink Hole

As opposed to a counterbore hole, a countersink hole has a more conical shape. Drilling this cone-shaped hole requires a little more calculation. and precision than drilling a counterbore hole. but the result is that you can screw a flat head screw into the hole. and leave it completely flush with the surface of the board.  thereby keeping the board’s surface smoother and not interfering with the flow of the board’s design.
When do you drill a countersink hole vs. a counterbore hole? A counterbored hole is easier to drill because it is a vertical hole. But, a countersink hole has to have a chamber angle for the flathead — usually an 82-degree.  90-degree or 100-degree angle.
That said, your counterbore hole will only be suitable for socket cap screws. .that feature a hex hole that you would screw in with an Allen wrench. The countersink hole is more flexible. with room for flat head screws that either have a hex hole top or a traditional flat head screwdriver notch. But which type of screw do you need for your printed circuit board?
If saving space and flexibility is your priority.  you will usually choose countersink holes. With the screws flush with the board. you will have an easier time fitting your PCB into devices where space limited. like watches or smartphones.
But, if secure mounting of your board is vital. and more important than saving space or having a smooth surface design. you will prefer counterbore holes, as the socket screws will allow you to create a more secure fit.

How to do  accuracy evaluation ?

     For the hole registration accuracy evaluation. the displacement of the drilled hole from the target  measured. All holes  measured using a PWB hole tester (AOI machine). All the holes  measured for each drill bit used. The exit side of the bottom panel  measured for the hole registration accuracy. when stack drilling  carried out.The centroid method, shown below,  used. 
       The measurement data  analyzed by "Largest displacement (Max)", "Average value + 3s, (Ave+3s)". and "Cpk value", etc.In "Average value + 3s", "s" means standard deviation value. Centroid MethodTo counterbalance the amount of displacement. caused by external factors of the drilling machine's registration accuracy.  the center of the data group of the displaced drilled holes is set to origin (0,0) as shown in the figure below. The evaluation  carried out using the centroid method. L: The center displacement of the group data due to external factors.
How to evaluate Holle Registration Accuracy in PCB drill
  Raw Data Hole Distribution Chart

How to evaluate Holle Registration Accuracy in PCB drill

  Centroid Data Hole Distribution Chart


pcb hole tolerance

     How much larger should the PCB hole be than the part? A rule of thumb is that you should make a PCB hole 0.007 inches larger than the part lead diameter.
     For example, if you have a resistor with a 0.022 inch lead diameter, the hole diameter should be at least 0.029 inch.
      Wait a minute! If you have a 0.022 inch part, why not a 0.022 inch hole? Well, manufacturing isn’t perfect. On the datasheet for the part. you will find a tolerance specified with plus/minus some amount to. compensate for variations in machining. material, plating, temperature, wear, aging, and so forth.
      For example. the datasheet for a specific manufacturer of 1/4-watt resistors says the lead diameter is 0.022 ±0.003.
      subtract the tolerance from the standard value. to find the minimum, and add the tolerance to the standard value to find the maximum. Thus, the actual part can vary from 0.019 to 0.025.
      In practice, you will find most of the parts are close to 0.022. you’ll find a part that is outside of the specified range. which would considered a defective part because it didn’t meet specifications.
      Okay, if the resistor can only be as large as 0.025 inch, why make the hole 0.029? Ahh -- because the hole has tolerances, too!
       As each hole drilled, the drill bit wears down a little bit. Or, the drill may wiggle or vibrate in a hole, causing a oversized hole. After that, the hole plated. The plating may be thicker or thinner for each batch or position on the board. You also have to account for thermal expansion or shrinkage of the PCB substrate. as it is being processed.
    Many PCB manufacturers specify a hole tolerance of ±0.004. So, 0.022 resistor lead + 0.003 resistor lead tolerance + 0.004 PCB hole tolerance = 0.029 inches.
     The 0.007 inch rule of thumb is adequate for hobbyists and prototyping most of the time. Of course, it is better to check out the datasheets. and online specifications for the part and PCB manufacturer you will be using. Often, the specified tolerance will vary based on hole size and type of plating. In fact, professional designers also take into account the type of solder. and the soldering process.

Drill hole application in PCB

     Why use through-hole mounting technology in printed circuit boards (PCBs). when it seems surface mount technology (SMT) is a better choice? Like most design decisions, there are tradeoffs involved using each type of component.
      Through-hole components come in one of two types of packages: radial and axial. Axial through-hole components have electrical leads. that run along the axis of symmetry of the component. Think about a basic resistor. the electrical leads run along the cylindrical axis of  the resistor. Many diodes, inductors, and capacitors mounted in this way.
      Meanwhile, radial components have electrical leads. that protrude in parallel from the same surface of the component. Many large electrolytic packaged in this way, allowing them to mounted to a board. while taking up a smaller amount of space. Illustration showing the component side of the PCB design
      Through-hole components came about at a time. when designers were more concerned with making electronic systems stable. and were less concerned about aesthetics and signal integrity. There was less of a focus on reducing space taken up by components. and signal integrity problems were not a concern. Later, as power consumption, signal integrity. and board space requirements began to take center stage. designers needed to use components that provide. the same electrical functionality in a smaller package. This is where surface-mount components come in.
      If you take a look at any modern PCB design, you’ll likely see boards that dominated by surface mount components. Newer designs still use through-hole components. but these components tend to used more often in power electronics. and other devices that generate a lot of heat. Surface mount technology is the most used component package technology today. These types of components do not use pins for electrical leads. Instead, the leads appear as small pads of metal on the same side of the component. The primary purpose of these pads is to allow soldering onto the surface of a PCB during assembly.
      The use of pads in surface mount technology. compared to through hole technology provides certain advantages. which will discussed below. In addition, the smaller pad size and component size. causes these components to have less prominent parasitics. This allows them to operated at higher speeds/frequencies. before you start to notice signal integrity problems.
      Surface mount components tend to be smaller than an equivalent through-hole component. BUT, this does not mean the cost of a surface mount component is always cheaper. because less raw materials used in manufacturing these components. Surface mount components themselves might cost a similar price. as an equivalent through-hole component. BUT, once automated assembly costs per component considered. the total cost per surface mount component tends to be cheaper. than a through-hole component with the same component values, power/voltage ratings. and tolerances.
      This difference arises because placing through-hole components requires drilling holes in your PCB. which incurs tooling costs. In contrast, drilling is not required with surface mount components. which accounts for the cost difference. All this begs the question: if surface mount components are smaller, faster. and cheaper, then why use through-hole technology at all? The answer depends on the use case for your PCB design. Yes, through-hole PCB technology is old, big, and expensive. but there are some advantages.

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