Footprint holes are predefined within each footprint and layer characteristics are predefined in the layout file. Consequentially, hole -sizes may vary among footprints or not match a vendor???s available drill +sizes may vary among footprints or not match a vendor's available drill availability unless manually adjusted. Likewise, predefined layout layer -characteristics may mismatch a vendor???s specific requirements. For +characteristics may mismatch a vendor's specific requirements. For example one type of Through-Hole resistor footprint may have 40mil holes while another may have 43.5 mil holes.
Some PCB fabricators have a finite set of drill sizes available. In this -case loading a vendor file will automatically adjust the layout???s hole -sizes to agree with the vendor???s drill availability. +case loading a vendor file will automatically adjust the layout's hole +sizes to agree with the vendor's drill availability.
Many current PCB fabricators offer any drill size within a tolerance with -just a minimun and maximum. In this case the layout???s holes do not need +just a minimun and maximum. In this case the layout's holes do not need to match specific drill sizes but it may be desirable to normalize the hole sizes. While the user may want all holes consistant for asthetic reasons there may be a fabriction cost cost savings if hole sizes are @@ -43,7 +43,7 @@ sizes listed in the vendor file and layer characteristics will be used in any subsequent Design Rule Checks (DRC). Some critical parts where you may not want to change the drill size can be identified to be left alone -- ???skipped???. +- "skipped".