[Moon] ON0EME beacon

[Eddy Jespers]

Hello Charlie,

I wonder if we need to chose the correct solder, to solder the devices to the PCB. I have been using Kester SN63 PB37  solder first to solder the device and the output capacitors. Melting temperature for this solder is 183° C
Last time I made some changements to the PA I used Cynel SN99,3CUO,7 solder, so lead free and a higher melt temperature ( 227°C )  These solder joints do not look as nice as the Kester joints. 

For the new repair I used solder paste Chipquik TS391AX ( SN63/PB37 ) to connect the damaged drain pad of the device to a new copper conductor, and soldered the whole with Kester SN63 PB37. 

In fact, this new solder SN99,3CU0,7 failed at one of the drains of the MRF13750H .

Are you aware of any recommendations of solder type being used for these devices ?  I now soldered the device again with Kester SN63 PB37 and solder paste SN63/PB37 but I am wondering what to use for the output circuit.

Vy 73’s Eddy ON7UN


> On 24 Aug 2019, at 19:42, charlie at sucklingfamily.free-online.co.uk wrote:
> 
> Hi Eddy
> 
> It looks like your recent failure is different to what happened to the
> DL0SHF 50W GaN PA.  In that case the pcb failed midway between the device
> and the output connector.  The device itself was fully functional.  The
> more recent failure again seems to be a pcb failure.  I do not know yet if
> the device is still functional.  There is no obvious reason why the pcb
> substrate failed.  In operation the calculated temperature rise of the
> microstrip carrying the output power is only 20-30C above chassis temp
> which should be totally benign according to Rogers.  There is a Rogers
> application note that discusses thermal cycling as a cause of cracking due
> to work hardening of copper.  The stresses result from differential
> thermal expansion of copper to the substrate.  Over time the constant
> stretching and compression forces can lead to failure. I could not prove
> this with the DL0SHF PA as all that was left was a charred mess on the
> pcb.  The DL0SHF PA had accumulated just under 500 operational hours, with
> some 30,000 temperature cycles.  This kind of operation is atypical of how
> PAs tend to be used in normal applications, so we are definitely in a
> learning mode here!
> 
> I have had some discussions with an LDMOS PA expert who  tells a tale of
> amplifier failures in a digital broadcasting application where the PA ran
> during the day and was switched off at night (to save energy) and failures
> of the soldered joint between drain and pcb occurred after only a few
> months. The solution was to run the PA 24/7, after which no more failures
> occurred.  The theory was that the solder joint was being mechanically
> stressed due to mechanical expansion/compression.  The stress on the
> solder joint is worse for devices (such as LDMOS or discrete GaAs or GaN
> transistors) since the joint has to carry both RF and DC currents.  The
> GaN device in the DL0SHF PA has separate DC and RF connections, so the
> solder joints are less stressed, and do not seem to have failed.
> 
> Looking at the temperature excursions in more detail, both LDMOS and GaN
> are operated Class AB so when RF goes on the devices will heat up probably
> in a few seconds and then cool down when RF goes off. The pcb temperature
> will rise further due to losses (and probably faster than the device due
> to the higher thermal resistance).  Additionally there will be larger
> temperature changes when the beacon goes on at the start of the moon  pass
> and off again at the end of the pass.
> 
> Another possible theory for failure could relate to environmental
> conditions. If for example a chilled water cooling system is used to cool
> the PA, when the beacon goes off at the end of a pass the amplifier
> chassis would end up below ambient temperature and if the dew point of the
> surrounding air is above that temperature,  water could condense on the
> amplifier, possibly causing corrosion or some other kind of damage when
> operation is resumed. I have seen such water condensation in the lab when
> using a chilled water cooling system so it certainly can happen.  In a
> professional lab, chilled water coolers have a case around the amplifier
> which is flushed with a dry gas to prevent such condensation.
> 
> I think what we are learning is that beacon operation with PAs that are
> built 'normally' is a whole different ballgame compared to typical amateur
> operation! With multiple potential factors at play, and root causes not
> yet identified I think users will have to accept that from time to time
> failures are unfortunately  inevitable.
> 
> 73
> 
> Charlie
> 
> 
> 
> 
>