Recent issue with DRO

I suffered a failure of my X Pos 3 DRO readout head and the cause may be of interest.

On a recent power up the DRO appeared to power up as normal then immediately shut down. I checked the mains supply to the head which was OK so I removed the head to explore the fault in my electronics workshop. I checked the 1 Amp fuse in the mains socket of the head and this measured O/C but visibly looked OK. I replaced the fuse and the same failure sequence occurred .

So time to have a look inside. Although my machine workshop is dry and the DRO head had never suffered any water ingress there was some corrosion on the non-component side of the PCB and especially around one of the decoupling capacitors. I thought that the electrolytic decoupling capacitor had leaked but on removal it was OK.

What I believe has happened is that when the PCB was made a "no clean" flux was used in the flow soldering process. As the name implies the PCB does not go through a post solder cleaning process and flux residues remain on the PCB.

This doesn't create an issue provided the PCB is only operated in a dry and warm environment. However my workshop although mainly dry is cold and unheated during the winter so may experience occasional humidity "episodes". Another factor may be that the flux residue may be hygroscopic and when damp, having an acid component, becomes conductive. If the DRO is powered up when this condition is present then a degree of current will flow and I've seen a metallic film, possibly tin, plated between PCB copper tracks particularly where through hole components are installed.

I have seen this before so a quick clean with acetone and a hard scrub where deposits appeared more stubborn and normal operation of the DRO head returned.

CS

PS if you ever drop your mobile phone into water immediately removing the battery may prevent damage because of this issue.

Good luck with that with a modern phone!