Reversed SMT oscillator - Help - Page 1
Forum section: electronics / repair

author: Jester
author_rank: Frequent Contributor
timestamp: May 10, 2018, 05:04:35 pm
content: I'm looking for advice on how best to repair a PCB (4 of them), the CM installed the oscillator backwards and we can't afford to ship them back due to time constraints.This oscillator is ultra miniature 2.0 x 1.6mm, this one:https://www.digikey.com/product-detail/en/abracon-llc/ASA-30.000MHZ-L-T/535-11715-2-ND/2809899Tools at hand:Hot air gun, this one:A couple of Metcal irons with small tips (however the pads of the oscillator are 90% under the oscillator)A tube of MG flux pasteA tube of MG leaded no clean solder paste (I never used this in the past)I can solder 0402, but this is obviously more difficult (not something I can get a solder tip on). There are capacitors about 0.5mm away, so either I remove them so they don't get blown away, or I could make a small cut-out in a piece of aluminum and place the cut-out over the oscillator and try and heat with hot air. Getting it back on and properly soldered is the real challenge.

author: dcarr
author_rank: Regular Contributor
timestamp: May 10, 2018, 06:36:49 pm
content: This seems pretty straightforward with hot air. I'd try one with the aluminum foil mask to see if it helps, otherwise just be prepared to replace the caps. You're going to need tweezers and a way to dispense small blobs of solder paste onto the pads (toothpick worst case). Shouldn't need extra flux. Might be smart to use solder wick to remove the original solder from the pads after you pull the parts. Microscope will make this job much easier, but you can probably do it without as the oscillators will self-center if they're close.Is the rest of the solder lead-free? I wouldn't mix leaded and lead-free if you can avoid it.

author: Jester
author_rank: Frequent Contributor
timestamp: May 10, 2018, 07:19:08 pm
content: Yes the board was done lead free.I'm going to give it a whirl.Suggested temperature for the heat gun?Thanks

author: SaabFAN
author_rank: Frequent Contributor
timestamp: May 10, 2018, 07:33:43 pm
content: Use a small nozzle on the hot air and turn the airflow all the way down to avoid blowing away the tiny caps.Capton-Tape or adhesive aluminium foil around the oscillator could help too.If possible (don't know if it's allowed for your product), go with leaded solder to resolder the oscillator. Makes it much easier in my experience.As for your question about the temperature: 330°C is a good temperature. I'd suggest preheating the board to with full airflow from the bottom first. If there's a ground-via near the oscillator, try heating that up with a soldering iron. Once the board is heated up to about 150°C, apply heat to the oscillator directly and it should come off basically by itself. Just don't forget to turn down the airflow, otherwise everything will be blown away

author: wraper
author_rank: Unknown
timestamp: May 10, 2018, 08:10:56 pm
content: Just apply some tacky flux and use got air. No need to cover anything. Preheating would be helpful but not required. If you are inexperienced with hot air, first try on some scrap boards.

author: wraper
author_rank: Unknown
timestamp: May 10, 2018, 08:31:59 pm
content: Quote from: dcarr on May 10, 2018, 06:36:49 pmFoil likely will do more harm than good. If it moves in the process and likely it will, it may knock off some components. And you won't be able to cover the caps, anyway. They are too close. The best is not using solder paste at all. There is already right amount of solder on the pads. Apply flux, remove the component, apply more flux and solder it with proper orientation. Using solder wick may be dangerous, may damage pads. Also it's not that easy to apply a right amount of solder paste without stencil.

author: Howardlong
author_rank: Super Contributor
timestamp: May 10, 2018, 08:43:21 pm
content: I agree, no need for a foil or Kapton surround. The most likely reason that other parts will come adrift is operator error, i.e. shaky tweezer action after one too many coffees!If you are on a multi layer board and/or large ground planes, you need to be patient and steady with your blowing action, it might take a little while to get up to temperature. Very slightly nudge the device to be removed every few seconds with your tweezers to see if it’s free, and then pull it off.

author: Jester
author_rank: Frequent Contributor
timestamp: June 10, 2018, 01:23:29 pm
content: I managed to correct the first board without a microscope, to do this my 3 layer combination of magnifying goggles and reading glasses had to be about 1" away from the oscillator to be in focus.So I ordered an AMScope SM-3T 7X-45X Trinocular Stereo Zoom Microscope. Oh what a difference, made the next three fairly trivial to correct. I'm still waiting for a Barlow lens, 0.5x on order, I think a 0.75 might be better suited but I will wait to try the 0.5.Update, I received the 0.5 Barlow lens, and it is near perfect, focus range is now about 8" from the working surface. For focus I needed to be 1" above the existing working surface, so I made a small base that I can move in when needed, just waiting for a static mat for the top.I mounted the boom to the metal rack that supports my Tek oscilloscope and this allows me to slide the AmScope up and out of the way in seconds while not consuming any work space for the base. Overall I'm quite happy with the AmScope and the setup.