All, There obviously is some confusion surrounding the calculation of land size for a given pad stack. Hopefully I will be able to clarify some of these issues. The calculation must start with a calculation for hole size. The hole is sized based on whether there is a component lead in the hole or whether the hole is used as a via. The tolerance selected for the hole size is based on these two criteria only. There have been several good suggestions on this forum for selecting a hole size. Paragraphs 9.2.2 and.9.2.2.3, of IPC-2222, discuss the criteria behind selecting a hole sizes for component leads or for vias. Once a hole size and tolerance has been selected we may proceed to calculate a minimum land size. For outer layers, one must use the maximum finished hole size. This includes the positive side of the tolerance. Everyone is correct when they speak about the fabricator drilling a larger size hole to accommodate the plating. The designer must be aware of this so that when requiring extremely small annular rings that the over drilled hole is not larger than the imaged land. The safest route is to use the maximum drilled size. Para. 9.3 of IPC-2222 will help you to determine drill sizes for holes as they relate to board thickness. Again, fabricators usually drill 0.003 - 0.004 in over the nominal hole size. For Inner layers we should only use the drilled hole size plus any etch back if required. The reason for this difference, is in the method of measuring annular ring between outer and inner layers. Outer layer annular rings are measured from the inside of the plated barrel to the outside of the land. Inner layers are measured from the outside of the plated barrel (the part that is adjacent to the laminate material) to the outside of the inner layer land. These numbers are then added to twice the annular ring requirement. Remember that annular ring is measured from only one side of a land, and we need to account for both sides of the land. See Figures 9-2 and 9-3 of IPC-2221. Last but not least we must add a standard fabrication allowance to the equations. This allowance takes into account variation in the fabrication process. The allowance includes items such as positional accuracy of the drill, registration between layers, image accuracy on each layer, etch factors, etc. This accounts for approximately 0.040in of individual tolerances. The tables in the IPC standards are based on industry survey and a statistical approach called sum of the squares. We know that all the tolerances will not go against us at the same time. By summing the squares we can statistically derive a safe number for process variation. The fabrication allowance will vary from one fabricator to another. Those fabricators that can hold their processes tightly will have a lower standard fabrication allowance. The tables in the IPC standards reflect the opinion of a majority of fabricators as to what is achievable. The table lists three levels of producibility, a clear reflection of cost. Column "A" is fairly common and reflects the ability of the majority of fabricators in our industry. Level "B" reduces that majority while level "C" reflects the highest cost and lowest yield of the group. It is intended for the designer to apply level "A" as his standard, selecting level "B" when he really must and avoiding level "C" as much as possible. As stated by one of our colleagues, the largest cause for deviation requests is in the ability to meet a minimum annular requirement. This is primarily because insufficient or no fabrication allowance has been added to the land size calculation. We also suggest that the designer not work to minimums but add a safety margin to all calculations. 0.001in or 0.002in annular rings for hand soldering or rework is just not practical, and not what the standard is recommending. The new table on standard fabrication allowances has recently come under fire since the different board or panel sizes have been dropped. The design committee will be meeting next week at IPC Expo and will evaluate any comments that have been received. Regards Gary Ferrari Executive Director IPC Designers Council (860) 350-9300 Fax (860) 350-9309 [log in to unmask] >>> "Uptain, George" <[log in to unmask]> 04/23/98 02:21PM >>> Greetings; Everytime I revisit hole size to pad size calculations I find a different road to follow. I just received my new copy of the IPC-2220 series that replaces IPC-D-275. While reviewing it, 2222 9.1.2, I am confused by the calculation for land size. It would seem that the maximum hole size did not account for locational tolerance, only size. We have always provided .022" over nominal hole size but the calculation provides only *.014" over max. My guess is that might be .017" over nominal. Does the fabrication allowance from 2221 Table 9-1 include the positional tolerance? and/or size tol? The 1st question is: How do you calculate maximum hole size? My usual method is (nominal) + (size tol) + (1/2 DTP) Usually that's nominal + .007" The 2nd question is: Do you have a "normal" value to add to nominal hole size for pad size? Ours is .022 or .026 depending on complexity. Our usual vendor recommends .021" minimum in their design guide. *The same calculation in IPC-D-275 shows .020" over max hole size. 5.3.2.3 Thanks; George W Uptain, Jr. Austin Product Center of Schlumberger Oilfield Services (512)331-3183 ################################################################ DesignerCouncil E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE DesignerCouncil <your full name> To unsubscribe: SIGNOFF DesignerCouncil ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. 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