SVX II Silicon Strip Detector Upgrade Project
Readout Electronics
J3 Backplane Assembly and Repair Procedures
--PRELIMINARY--
October 21, 1997
John T. Anderson
John Chramowicz
This document provides information on the assembly and repair of J3 backplanes used in 9U VME subracks. It contains specific information regarding the assembly and maintenance of press-fit hard metric connectors used in the SVX VRB/FIB custom backplane. For specifics regarding the printed circuit board itself the reader is referred to Document # ESE-SVX-960130, VRB/FIB Custom J3 Backplane, by K. Woodbury and J. Anderson.
Assembly of a custom backplane using press-fit connectors requires tooling and fixtures which are capable of providing sufficient force to insure that the pins of the connectors are firmly inserted into the backplane. This is normally accomplished by using an arbor press to provide the pressure, a press plate to distribute the force evenly throughout the connector being mounted, sufficiently rigid mounting of the PC board to insure no flexing, and a press anvil which receives the force required without compression. This arrangement is shown in Figure 1.
Figure 1
The hard metric connectors used in the VRB/FIB custom J3 backplane require approximately 30 pounds of pressure per pin when inserted. The 125-pin connector commonly used thus requires approximately 3750 pounds of pressure to insert - thus necessitating the arbor press. To distribute the load a set of spacer boards are used which have the same drill pattern as the backplane. The long Transition Module side pins of the connector pass through the spacer boards and are protected from damage by them during the press operation. If insufficient spacers are available to protect the entire length of the pin, a support anvil as shown may be used to raise the assembly high enough off the base plate of the arbor press to insure that the pins are not bent at the end of the press’ travel.
The press plate used for the hard metric connectors must fit inside the body of the connector to distribute the press force to the plastic body. The pins of the hard metric connector are weak and cannot withstand any significant pressure. AMP sells a press plate for use with the type ‘b’ connectors used in VIPA J5/J6 backplanes. Their part number is 354698-1. The plate requires inserts that fit the various hard metric connector types; the most common for VIPA use is 354696-2. Other backplanes also use number 354696-5.
The spacer boards are designed to provide extra support for the main printed circuit board during the press operation while also protecting the fragile pins of the connector. A spacer board may also be required in order to provide the correct mounting of the backplane into a VIPA subrack. The main backplane in a VIPA subrack is quite thick in comparison with user backplanes. If alignment of the Transition Module side shrouds is important to the final application (e.g. a Transition Module which only connects to the user backplane) then a spacer board may be necessary to properly set the shroud dimension while still allowing the aspect ratio of the plated holes in the user backplane to remain within tolerance limits.
A support anvil is required if the thickness of the spacer boards permits the pins of the connector to stick through and the throat or clearance slot of the press base is too narrow to admit the pin extension. In this situation an anvil which lifts the assembly high enough off the press base is required so that the pins are not bent during insertion.
The support anvil is also useful in those situations where a partially stuffed backplane later has more connectors added to it. The vast number of pins in connectors previously installed prohibits the re-threading of multiple spacer boards onto the existent board.
Connector shrouds are installed after the all connectors have been pressed into the backplane. Care should be exercised to insure that the shroud is fully seated and is on straight. Different manufacturers provide shrouds with different locking mechanisms. Some locking mechanisms can be released to allow field replacement of broken shrouds, whereas others cannot be released once installed.
After installation in the subrack normal wear and tear may result in bent or broken pins. The hard metric connector provides for the field removal and re-insertion of individual pins. ERNI corporation sells two tools (remove: p/n 220-150; insert: p/n 220-151) for this job. To remove a pin, the removal tool is applied at the Transition Module side and the pin backs out to the point where it can be removed with a needlenose pliers at the Module side of the backplane. The insertion tool fits across a row or column of pins from the Module side and presses against the shoulder of the pins. A light rap with a hammer re-seats the pins.
The connector body has cross-shaped holes at the base which allows the new pin to be rotated 90 degrees from the orientation of the old pin. It is recommended that the new pin be installed in the rotated position to provide maximum press-fit contact with the sides of the hole.