I just wasted a couple of hours finding a self-created problem with a 27MHz crystal oscillator rework.  The SMT crystals were changed in a batch of 6 prototypes, but one of the units was failing with baffling results, intermittently seeming to almost work . . .  The problem was finally discovered as a PCB trace that was broken during rework (by me), but I noticed the reason for the break and a solution to prevent future occurrences of the problem:



The SMT crystal package (HC49/US) has leads that exit the metal case near the center, bending over the plastic base to extend past the ends of the package, allowing visual indication of proper soldering.  One of the tiny traces on my board came to the inside end of one of the PCB pads (see http://stevezeva.homestead.com/CrystalRouting.jpg for a picture of the PCB artwork.  Thanks, Steve)



Rework of this part is difficult to do with a single soldering iron, heating one end and prying up gently on the package until that end lifts slightly, repeating on the other end, back and forth . . .

Unfortunately, the inside corners of the pads are the last to melt, and have the most mechanical advantage to lever-up the end of the pad.  My pad lifted slightly, invisibly tearing the connecting trace.



The solution to this special case combination is to not make electrical contact to the inside end of the pads (under the part, and out of view), but to connect the PCB traces to the outside ends, eliminating this fringe problem.  I'm calling this approach DFR (Design For Rework), and am adding this to my PCB checklist.



Later,

Carl (attractor of fringe problems)



Carl Van Wormer, P.E., AE7GD
Senior Hardware Engineer
Cipher Engineering LLC
    21195 NW Evergreen Pkwy Ste 209
    Hillsboro, OR  97124-7167
    503-617-7447x303
    [log in to unmask]<mailto:[log in to unmask]>     http://cipherengineering.com

This message may contain confidential and/or proprietary information, and is intended for the person/entity to whom it was originally addressed. Any use by others is strictly prohibited.  If I sent this to you by mistake, please be nice and delete it, and then tell me of my mistake so I can send it to the right person.



______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________