Re: Modamps

Neil Smith G4DBN

re: stereo microscopes and teensy devices:  

I do like the look of those devices, thanks for the heads-up Andy.  Not sure what their longevity of supply is though. Mouser show them as obsolete/discontinued, and even the VMMK-3803-BLKG is marked as obsolete, but RS and Mouser have stock of those at about a fiver each.  2dB NF, 18dB gain at 10GHz, same package.  I can’t see anyone doing an alternative, so maybe time to find a cheap source and stock up on them like I did for the old Modamp devices in 1983 or so.  I am down to my last two Avanteks...

I use a scope like this:  and it has been superb for working on even the teensy DFN packages with 0.5mm spacing.  The working distance is about 100mm, which is a bit too close if you are working on boards which are seam-soldered inside a box. The very long reach arm is a joy to use, leaves plenty of free bench space.  I had to drill some small holes in the conical lens shroud to fit my ultra-cheap LED ringlight, but luckily it is radio-silent.

I have a 0.5mm needle tip on a temp-controlled iron that works for most devices, but I have done a few boards using a 100x100x12mm aluminium slab on top of a single mains cooking hotplate.  I use a fine hypodermic to apply medium-temp solder paste to the pads, then pick-and-place the devices under the microscope with tweezers. 

I have drilled a hole into the ally block and fitted a thermocouple so I can ramp up the heat to just under soldering temp, then use a hot-air reflow blower to do the final melt.  I then move the block on to a cooling plate to ramp the temperature down.  Haven’t killed any devices yet.  If/when I mess up, I use one of those 15 quid soldering tweezer things to remove the offending device. Main use of the hotplate is for soldering LDMOS devices to copper heat spreaders, but for those I use 138C melting point solder paste rather than the higher-temp stuff. 

Neil G4DBN

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