Ford Focus dashboard repair, should I use lead or leadfree solder

[Zepnat]

Today we have traced a intermittent electrical fault on a 2005 Ford focus Tdci to CANBUS communication failure on the dashboard/instrument panel. Things like throttle and start signals go through the panel so the car is undriveable when the fault is present. Warming and slightly flexing the PCB behind the speedo makes the fault come and go with a simultaneous rise in the canbus H voltage.
Dry joints seem likely and tomorrow we are going to reflow as many solder joints as possible but should we use leaded or lead free solder?
Presume it has lead free originally. Both are available but I do prefer leaded! Flux is my favourite homemade rosin.
Thanks in advance.

[Phaedrus]

If you're not going to resell it, there's no real reason not to use leaded solder. It's better in pretty much every way.

[scientist]

The issue could easily be a damaged trace on the board. Check for that before you reflow. 

[Zepnat]

Just done some googling and it seems leaded solder on original LF joints is ok but not as strong. Not what you really want to hear on something like a car instrument panel!
Ok yes we should be able to trace the two wires along the board and check for breaks.
The pcb appears to be single sided but cannot see for certain until its stripped down tomorrow.
Its not my car hence the concern about getting a degree of reliability in the repair.
Thanks for the interest will post how things went tomorrow evening.

[Chris_PL]

I doubt if it is lead-free originally, automotive applications are RoHS-excempt (or they were?). But we're talking 200x.

[rexxar]

Just done some googling and it seems leaded solder on original LF joints is ok but not as strong. Not what you really want to hear on something like a car instrument panel!

You don't want to just add some leaded solder onto a lead-free joint, you need to suck all the solder out of the joint, then replace it with leaded solder. Unless the board is going to be exposed, and your customer is going to be eating off of it regularly, there's no reason not to use leaded solder.

[BurningTantalum]

This got me thinking, as I have just exited the workshop where I replaced components on a LF board with 60/40, and also resoldered a few joints with leaded solder to ease the solder sucking to remove some components.
I have always assumed that from an electronic/electrical point of view this was OK (other than the issue with lead in the environment). Have I missed something?
Regards,  BT

[Neilm]

Mixing the solder types could give some interesting results - you would in effect have contaminated solder joints. Depending on what was originally present I could easily see these joints becoming brittle and failing. (Chemistry was never my strong suit)

[scientist]

The product of mixing silver and lead solder is not pretty. You get this sticky, crunchy, half-melted texture that rips traces and flows like shredded cardboard. Please use solder braid to get rid of whatever solder is there already. 

[eKretz]

I concur with the others. Remove as much of the lead-free solder as possible before using leaded. I have seen the effects of combining them and it isn't pretty. You can almost see it when you first mix them, as a nasty oxide forms almost immediately on the surface (looks like a dull non-reflective skin). Mix them to make removal easier by melting a good blob of leaded solder onto the tip and bringing it into contact with the lead-free and agitate the joint a bit. They'll mix together and make it easier to remove the lot with a solder sucker or braid. If you are working on something you really don't want to fail again it might be advisable to repeat the process 2 or 3 times consecutively to make sure you've got the lead-free solder almost completely removed from the joint. In other words mix the blob of leaded solder in, remove it with braid/sucker, then re-tin the pad with leaded, remove it, etc.

[Macbeth]

Use leaded by all means, but don't rely on it to cover the hairline PCB cracks. You need proper bodge wires or bridges (made of cut off through hole component legs - always worth saving!).

[woodchips]

Umm, be careful. My first thing to do would be to get one of those lead solder test sticks, which will tell you what type of solder was originally used.

If leaded, then fine, go ahead.

If unleaded then the fun starts. Using leaded solder on an unleaded joint will, as scientist and eKretz say, be a mess. What seems to be the problem is that the joint ends up as a 90/10 leaded solder joint, and that amount of lead doesn't work. This is why cleaning all the old solder off is important, so that the joint is actually 60/40. Apparently you can get the same problem by simply using a leaded solder iron with lead free solder, disaster.

An alternative might be to use a small hot air pencil to reflow the joints, use a dab of flux on them as well.

[Andreas]

Hello,

it is very unlikely that in 2005 any car manufacturer used unleaded solder.

Most probably the problem is at the connector.
Either outside the pcb or the connector solder joints.

with best regards

Andreas

[Zepnat]

Thanks for all the advice, when the console was stripped down we could see strait away that the connector socket was dry jointed on every pin, phew! quite a relief to see such an obvious fault. Yep pretty sure they used lead solder, didn't get any weird effects when redoing the pins just nice shiny connections. Dashboards have sure changed since I was a kid.