Mr. Franck, Procedure 1 (Figure 1) is for bow (only) and is determined by measuring the maximum deflection with all 4 corners in the same plane (touching a surface plate). Allowable bow is a percentage of the length of the side exhibiting the maximum deflection. Procedure 2 (Figure 2) is for twist (only). The board has twist when you can't get all four corners to touch the flat surface plate. Allowable twist is a percentage of the diagonal measurement. The two procedures are used for measuring two different characteristics, bow or twist. You are not given a choice of the two procedures for measuring bow. >From IPC-A-600D, pg 51, Table 10-3 "Bow and Twist Tolerances for Glass Base Material" For Multilayers, All Thickness categories: Class I = 2%, Class 2 = 1.5%, and Class 3 = 1%. On Wed, 31 Jan 1996 08:21:22 -0500 George Franck X2648 N408 wrote: > From: George Franck X2648 N408 <[log in to unmask]> > Date: Wed, 31 Jan 1996 08:21:22 -0500 > Subject: Bow and Twist > To: [log in to unmask] > > IPC-TM-650 2.4.22 is the test procedure for bow and twist. It is found in > various documents, mine is in the back of my trusty IPC-RB-276. In section > 5.4 Procedure 4, Production Testing using a "go/no-go" pin gague, there are > two methods and measurements. The first, in para. 5.4.1, (fig 6) show the > bowed board place cup side down and the measurement is the gap under the > cup. In 5.4.2, (fig 7) the board is turned over and the cup is up. The > measurement is at the edge or corner of the board. > > My question is this. The cup side up measurement is about twice the cup > side down measurement. Must both measurements meet the percent deviation > requirement? Is there a different way of calculating the maximum > percentage deviation for the two? I am confused. > > You can see what I am talking about by taking out a business card, and > curling it a little. Now put it cup side down, look at the gap. Turn it > over and hold down two corners (across the edge which does not have the > curl in it), the gap is about twice as much. > > Thanks and don't let your boss catch you playing with your cards at your desk! > > > ==================================================================== > George Franck > PWB Product Assurance Phone (703) 560-5000 x2648 > E-Systems M/S N408 Fax (703) 280-4613 > 7700 Arlington Blvd E-Mail: [log in to unmask] > Falls Church Va 22046 E-Mail: [log in to unmask] > ==================================================================== > > FRED J. PAUL FLUKE CORPORATION Sr. Process Engineer POB 9090 M/S 55 PCB Operations EVERETT, WA 98206 [log in to unmask] 206 347-6100 voice: 206 356-5734 (w Voice Mail) fax: 206 356-6070 [Ideas and expressions presented herein are my own, and in no way are they implied to necessarily represent those of the FLUKE CORPORATION]