Tolerance Analysis


Jim Shaw, EVP of Engineering at Crystal Group, discusses the importance of completing a tolerance analysis.

Tolerance Analysis

Hi, Jim Shaw here from Crystal Group.  Wanted to spend a just moment talking about when is it really important or essentially critical to do a tolerance analysis. Our general rule here is anytime you have an electrical interface with a mechanical interface that’s a must, a must do in terms of tolerance analysis. Also anytime when there are sliding interfaces. We’ll see that in drive carrier sleds, in removable DVDs. We’ll see that in essentially mounting and racks so we’ll do tolerance analysis on the mechanical and electrical parts when it’s necessary to make sure that the machining tolerances will always produce a product, including the assembly tolerances that meets the customer’s needs.

Probably one of the more critical aspects is anytime you have an electrical assembly that meets with a mechanical assembly. You really have to do a tolerance analysis. This is an example of a standard slot load DVD that we’ve created a circuit board and we always start from essentially from the wipe distance associated with the connectors. So you start with a connector, well what kind of connector? A lot of connectors you have to pick are super high speed connectors especially in backplanes for storage arrays. There’s a minimum amount of nickel under plate that you should be shooting for and there’s a minimum amount of gold over plate that you should be shooting for when you pick those connectors. And that’s essentially for wear.

You want to make sure that surfaces stand up. Then you center around that wipe, you create a datum for either one side or the other of the interface and then you take all of your tolerances from that datum. I always make a circle. I start with say the board interface, then the mounting surface and then back to its attachment, and then in this case the DVD sled and then back to the connector again. So what you do is you make a full circle. You start with that connector wipe dimension and then you make sure that all of your tolerances don’t exceed that acceptable wipe.

We always make sure there’s more than adequate wipe dimension on the connector so that in case there is any dynamic deflection and vibration you’ve always got contact for those electrical signals. The higher the frequency of the signals the more important it is to never lose contact. High frequency data rates are incredibly important for having constant and redundant contact so that you never lose the signal or blue screen the system. Essentially that’s the way that we work our tolerance analysis around the wipe dimension and then form that full circle to come back around. Have a great day.