1. How does the diameter about come into play with the true position? Table 9-3 defines the diameter about true position, or tolerance around true position expressed as a circular value, compared to the square tolerance area with non-GD&T dimensioning. IPC-2615 does a pretty good job explaining how GD&T works (for the curious). Figure 6-8 on page 28 of IPC-2615 helps answer your question. 2. What tolerance of diameter about true position do you specify on your fab prints? Though Table 9-3 specifies 0.2mm for Level B boards, I typically specify 0.15mm, or 0.006, MMC. As the drill/hole size gets larger, the tolerance zone increases in size. For non-critical features, I specify 0.2mm or 0.25mm. 3. What is the typical capability of diameter about true position that PWB fabricators hold? I'll leave this to one of our fabricator friends. Roger M. Stoops, C.I.D., PCB Designer [log in to unmask] Trimble Engineering and Construction Division 5475 Kellenburger Rd. Dayton, OH 45424-1099 USA Ph: +01 937.233.8921 or +01 937.233.4574 ext 288 Fax: +01 937.233.7511 Scott Kauling <skauling@TRI- To: [log in to unmask] ONICS.COM> cc: Sent by: Subject: [TN] PWB Fab Note Question - Diameter True Position TechNet <[log in to unmask] RG> 12/07/01 03:01 PM Please respond to "TechNet E-Mail Forum." In IPC 2221 section 9.2.2 describes hole location tolerance, it is expressed as "diameter about true position". IPC T-50 defines "true position", not diameter about true position. My questions are: 1. How does the diameter about come into play with the true position? 2. What tolerance of diameter about true position do you specify on your fab prints 3. What is the typical capability of diameter about true position that PWB fabricators hold? Thanks in advance for your input. Have a good weekend! Scott Kauling --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------