Having gotten involved in printed circuit board for a long time, I’ve seen a whole lot of errors and similar drawing mistakes. During the declaration of PCB tolerances for a specific design, the PCB designer has to consider the board manufacturing itself.
Here, we will consider some problems usually seen in printed circuit board (PCB) designs, which prevent the right and proper dielectric thickness tolerances and inner layer clearances for plating, drilling, and routing processes.
It is not possible to have a standard size with a plated hole with a +/-0.001” finished dimensional tolerance width, dielectric thickness, and adhesive thickness. At times, I see drill charts having tolerances of +/-0.000” and or +/-0.001” for the finished holes. In addition, for standard plated through holes, the drilled hole size must have about 0.006” more compared to the finished size. This permits 0.0008” – 0.001” copper that needs plating within that hole.
In addition, the table whereby the printed circuit board sits will hold down the board with little movement. However, drill wander happens. Therefore, there is a need to ensure that these tolerances could be important for all these processes.
Anytime there is an order for a large quantity of circuit boards, manufacturing will end up stacking the panels of production on the top of each other and themselves. In addition, you may find three panels present on each other’s top and held down on a drill table.
Therefore, if there are about 4 drill heads that are set up using the necessary tools to drill your pre-plated holes, as well as three panels held underneath each of the drill heads, which would result in 12 production panels that have been drilled at once. You must conduct this process for better and efficient use of your production time.
One issue with the scenario above is, the drill bit has to drill via the backer board utilized in the cutting down on burrs and smear. Also, this drill passes through the three panels of production that is made of 0.062” FR4 as well as other material. Once the drill bit isn’t as sharp as this new deal, then its drill size is already off from the nominal.
When the hole sizes are at 0.250” and more, this drilling now becomes a more mechanical process. This leads you to what we call routing. This process of routing a printed circuit board (PCB) needs more of tolerance compared to drilling.
Furthermore, if the circuit board design requires 0.250” plated hole that is finished, a router will have to cut a 0.256” diameter open for the plated hole. This will have a preferred tolerance of +/-0.005. Once again, for this case, the completed 0.250” size has to have a tolerance of +/-0.005. Once again, asides the chemistry of having the variables, the hole size will add another variable. Importantly, the small holes will plate faster compared to the large holes.
These printed circuit boards (PCBs) are held onto the table with little movement of the board. Moreover, with bit wander, in addition to the router bit’s tolerances is needed, most especially the +/-0.005”. Furthermore, as mentioned earlier, when the volumes of mass production are needed, the panels will become more stacked compared to one high. When a shift in the panel stack happens, the end routed size will become affected.
Cutouts and slots either plated or not, needs the same or similar router tolerance placed on them. Also, as you use the router bits, they also wear down, and these cut openings become smaller. This holds true for production environments whereby the use of heavy routing is necessary.
When you plate the copper, the size of the hole shrinks to a finished size. The solder mask is also important here. Also, plating isn’t a complete science, same way a tolerance of +/- isn’t practical. In addition, it is preferable to use a tolerance of +/-0.003” which can reach 0.249” on the hole sizes.
Also, the process of copper plating usually requires time necessary for the plating tanks, the tank’s temperature, as well as the amps that have been sent to the printed circuit boards.
Furthermore, the standard time for plating will take about 45 minutes to one hour 30 minutes, to ensure that enough copper gets into those holes. Due to the variables coupled with the time taken in those plating tanks, then the tanks’ temperature coupled with the amps required, the plated holes’ finished size will required that +/-0.003 tolerance.
The panels of the circuit board production enter the department of copper plating when the images of the circuit become available and the holes are already pre-plated. The panels of the PCB pass through a process of cleaning. This again slightly etches the hole sizes. Note that before the panels in the tanks for plating, you must calculate several variables.
You must calculate the area requiring copper plating to know the necessary amperage to send via the chemistry without having to burn the boards. Also, you must rack the production panels specially to even the current flow out the best way possible.
Internal Cutouts and Non Plated Slots
Cutouts and slots is another factor affecting the process of plating meeting the right drilling tolerances. These slots and holes and plated cutouts behave in a different way within the process of plating. The chemistry has to flow via the plated holes and adhesive thickness. You can achieve this through panels moving within the tanks back and forth, as well as air agitation that moves the chemistry round, thereby ensuring that it is evenly mixed.
Though chemistry will find it very difficult flowing via small holes, the small holes plate faster. These large slots and holes, though the chemistry flows easily through, it plates slower.
Here comes the end of our article. There is a need for PCB tolerances to ensure the product is properly processed. Using the right base material thickness tolerances for the non-plated and plated features plays a very important and relevant role. If you have any questions concerning the topic, please reach out to us here. You can also contact us to order printed circuit boards.