Fluke 910R GPS controlled frequency standard. SMD IC replacement..

[See_Mos]

I have been tasked with repairing the serial interface on the Fluke 910R GPS controlled frequency standard. The interface is not the usual 232 interface chips. Instead there is a DS14C88 line driver and a DS14C89 for receive. The receive chip is faulty and it is the one nearest the rear panel.

I am looking for some opinions or help regarding replacing the IC. I have a hot air station so removing the device will not be a problem. I have done SMD before but never used the hot air station to refit a device.

The space is too tight to get in with the hot rod. I can make more space by removing the mains input filter and one or two of the BNC connectors however I feel that I can do a better job if I remove the back panel completely but there is a problem.

The pin and the earth tag for the BNC go through these ferrite rings. The rings are powder coated and the plastic has melted when the earth tag was soldered. Trying to de-solder the earth tags is only going to make things worse so I was thinking of breaking the ferrites and replacing them with new ones but the problem is to identify them and where to get some replacements.

The cores are on the 5MHz and 10MHz outputs.

[See_Mos]

In the end I left the everything in place, removed the old chip using the hot air station then got out my old RS 600-357 which has a longer barrel than my usual Weller WSP80. With a long 1mm tip and some tweezers I managed to solder the new chip into place.

[Nickthedentist]

I'm genuinely impressed, well done.

[ortek_service]

Yes, refitting with hot-air can be a bit tricky.

As really need to clean solder off the pads, and apply fresh solder paste.
Then position the new device into the paste and heat this with a rather-low air-flow setting, to not blow the paste away.
And keep moving the hot-air pencil around over all the pins, so that they are all molten at the same time when the IC should then also 'self-align' due to solder surface-tension so that it is exactly central to the pads.

In the absence of having solder-paste, an alternative is to leave some solder on all the pads, and apply a bit of liquid flux / fresh solder. Then rest the new device in position on top of solder and hold / push down with tweezers whilst heating all of the pins / solder with hot-air on a low air-flow. The device should then be gently pushed-down so that it drops flat on the board, with solder on pads then covering the tops of flat parts of all the IC's legs.

This 'sweating-down' method is also very useful for devices with centre heatsinking pads etc., that you can't get an iron to.


Alternatively, with devices like this, a 45deg angled tip might help get into awkward to reach places. And then, as usually best, just tack two diagonally-opposite corners in place, whilst getting the SMD exactly centrally aligned. Then go round and solder all the remaining pads, resoldering the temporarily-tacked corners, as required.

[See_Mos]

I have never used paste but I bought some last week as an option for the repair. I will have to dig out some old boards and have a play.

I suspect there are many different qualities of paste and a while ago I saw something about intelligent paste so something else to look into and it might help with the Panasonic machines that I am working on.

[Dekatron]

I saw a really neat way of removing SMD IC's the other day which I've never soon or heard of before (I don't do much SMD work) that I had to try and it works very nice.

Take a tinned copper wire which you bend like the letter U with the thickness of the copper wire and spacing just enough so that you can slide it under the pins of the IC, then bend the U-shaped end upwards so you can grab it with a pair of pliers, thin pliers so you don't cool the wire too much. Then you can put enough solder to loosen all pins and just lift the end of the wire pulling the IC up with the wire. After this it's just a matter of cleaning the pads.

The video went on to show how to remove other components with many pins in the same fashion with an extra copper wire either used for pulling the component away or in some cases the copper wire was thicker to allow for heat transfer on many pins to remove multi-pin contacts.

[ortek_service]

Originally Posted by See_Mos

I have never used paste but I bought some last week as an option for the repair. I will have to dig out some old boards and have a play.

I suspect there are many different qualities of paste and a while ago I saw something about intelligent paste so something else to look into and it might help with the Panasonic machines that I am working on.

Yes, there are many different grades of paste, with finer particles needed for the latest ultra-miniature SMD's. As well as slightly-different mixtures of Lead-free solder, with some optimised for shock-resistance for portable devices that might get dropped (as some lead-free solder can fracture in the 1m drop-tests, as much-more brittle than leaded-solder that will now be more-difficult to get in paste-form)

The main problem with paste (besides maybe limited shelf-life, having to keep in the Fridge) is getting the right and consistent amount on each pad when just using a standard syringe and fine nozzle. So there are system that apply short burst of compressed-air, to a syringe-attachment.
Or you can use a thin stencil for the footprint, and scoop paste across this with a suitable flat spatula so just the right amount goes onto the pads.

[ortek_service]

Originally Posted by Dekatron

I saw a really neat way of removing SMD IC's the other day which I've never soon or heard of before (I don't do much SMD work) that I had to try and it works very nice.

Take a tinned copper wire which you bend like the letter U with the thickness of the copper wire and spacing just enough so that you can slide it under the pins of the IC, then bend the U-shaped end upwards so you can grab it with a pair of pliers, thin pliers so you don't cool the wire too much. Then you can put enough solder to loosen all pins and just lift the end of the wire pulling the IC up with the wire. After this it's just a matter of cleaning the pads.

The video went on to show how to remove other components with many pins in the same fashion with an extra copper wire either used for pulling the component away or in some cases the copper wire was thicker to allow for heat transfer on many pins to remove multi-pin contacts.

Yes, it seems some Panasonic Service Manuals used to recommend doing that.
But it's only good for SMD's with legs on them, whereas many modern ones are now in QFN lead-less etc packages. Or even BGA.
- Sometimes with centre-pads underneath them, where only hot-air or (focussed etc) InfraRed heating will allow you to (de)solder these.

With SMD's that aren't too-large / Only have a few legs on two-sides, then you can also often remove these using two soldering to heat all the pins on each side at once (after applying fresh-solder / flux to these, to aid being able to keep all molten at the same time).

[Vintage Engr]

Years ago, I went on a Panasonic Broadcast training course, where the same method was suggested. It does seem to work fairly well.

David.