[time-nuts] HP 5370B low frequency modulation

[Didier Juges]

When you solder a connector pin that is gold plated, the concentration of
gold in the immediate vicinity of the interface is maximum, since you go
from pure gold to pure tin/lead and you have all the gradient in-between
because the gold diffuses in the tin, so you will always have an area where
the compound will have the right concentration to cause problem. I have seen
many solder joints contaminated this way and experienced the problem first
hand, it's not just a theoretical problem. Yet, it's quite preventable using
either of these simple methods. As soon as you remove the excess gold, the
concentration drops below the level where it can cause problems and you will
be OK.

I am not aware of a method that does not entail removing the excess gold?

Didier KO4BB

> -----Original Message-----
> From: Magnus Danielson [mailto:magnus at rubidium.dyndns.org] 
> Sent: Thursday, August 30, 2007 6:41 PM
> To: time-nuts at febo.com; didier at cox.net
> Subject: Re: [time-nuts] HP 5370B low frequency modulation
> 
> From: "Didier Juges" <didier at cox.net>
> Subject: Re: [time-nuts] HP 5370B low frequency modulation
> Date: Thu, 30 Aug 2007 18:10:56 -0500
> Message-ID: <061301c7eb5b$063275b0$6501a8c0 at didierhp>
> 
> > The effect of gold contamination is to weaken the joint (the 
> > intermetallic compound of gold and tin is hard and brittle).
> > Over time, and particularly in high stress environments 
> (temperature 
> > cycling, vibrations), the solder joint eventually breaks 
> down at the 
> > interface and leaves you with an intermittent connection.
> > 
> > The way we deal with it in the military business (where temperature 
> > cycling and vibration is a way of life) is that we first remove the 
> > gold plating from the solder pin side of connector pins by 
> tinning the 
> > pin a couple of times and each time sucking the solder with fresh 
> > braid or other tool.
> > That dissolves the gold in solder that is then discarded, 
> leaving the 
> > solder end of the pin free of gold. At that point, the pin can be 
> > soldered normally and the solder joint will show good 
> characteristics.
> > 
> > Alternately, we use a solder pot and dip the end of the 
> part that is 
> > to be cleaned in it. The problem with that is that the 
> concentration 
> > of gold in the pot increases over time until the content of the pot 
> > has to be discarded and replaced, so if you try to use the 
> pot for too 
> > long, you may not get rid of the gold completely.
> > 
> > Our manufacturing engineering people have developped rules for how 
> > many times you can use the pot before replacement, and that 
> seems to 
> > work as long as it is controlled.
> 
> Even these measures can be argued as overdoing it, even if I 
> understand why they run popular. The key point is that it is 
> only when the gold concentration in the solder joint is too 
> high that you start to get these problems. For very low 
> levels of concentration there is no problem. The issues is 
> really about how much gold concentration do you end up with 
> and do you have sufficient control over amount of gold, amont 
> of solder and amount of dissolvement time for gold to 
> distribute in the tin. These are process control variables.
> 
> Cheers,
> Magnus