The following screen shot is from an eptac webinar presentation titled:
Interpreting IPC-A-600 Requirements for Annular Rings and Laminate Cracks
Source: webinar_eptac_07_21_10.pdf
PCB fab notes for commercial board designs typically call out IPC6102 Class 2 requirements.
However it is not uncommon to see exceptions in fab notes which call for the annular rings of the vias to meet a minimum finished width of 2mils.
To meet these minimum annular ring requirements the fabricators may need to increase the pad diameter.
Example 20/10:
If you have used 20/10mil VIAs and stated in the fab notes that minimum 2mil annular rings are required, the fabricator will need to increase the pad size to 23mils to meet this requirement.
The variables include finished hole size requirements shown in the drill table. The actual drill size used to drill holes is +4mils to 5mils larger than the finished hole size to allow for plating in the barrel. Plus true position tolerances (+/- 3mil) for the drilled holes, plus layer to layer registration tolerances (+/-3mils).
Do NOT make Assumptions:
You should not assume that the 13mil difference in the above example will apply to any size via. As the hole size increases the fabricator will require proportionally larger pads.
Let say you have assumed the 13mil difference above would be good enough for a 43/30mil via or plated hole. To meet the 2mil annular ring requirement your fabricator will ask for a pad size that is ~16mils larger than the finished hole size. So in this example you would need a 46mil pad, to meet the 2 mil annular ring requirement.
To avoid surprises, talk to your PCB Fabricator.
No comments:
Post a Comment