Re: gEDA-user: RoHS compliant fab & assy drawing notes

[Marvin Dickens <marvindickens@xxxxxxxxxxxxx>]

joeft wrote:
> I have found similar info as what Steve has noted. Many vendors
> are letting their parts automatically "roll over" to lead free
> compliant versions.  This means that if you manufacture something
> now (during the transition) you will have a mix 
> of RoHS and non-RoHS compliant parts. 

In fact, a lot of parts have been manufactured for years
without lead. Thing is, you don't know which ones because
the manufacturers will not give a guarantee in writing. But,
they will tell you _verbally_ that they are lead free - A
lot of good that does.


> Some will solder better
> than others depending on your assembly shop's process.  And you
> might find this leads to extra documentation overhead should
> someone want to know what your compliance status is down the road. 
> Lead free solders will be mostly tin with a small amount of other
> metals (usually bot not always silver), and they will indeed require
> higher soldering temperatures. Board materials and plating will have to
> change if you were using HAL, as Steve points out.  Many shops will
> not have gold available as a plating option however and will use
> matte tin instead.  This brings into play the "tin whisker" problem
> which it seems not everyone has solved yet.

Research that has been done recently indicates all metals
(Including gold) used in PCB manufacture can grow whiskers.

See:

http://nepp.nasa.gov/whisker/other_whisker/

Even so, we have never been bitten by whiskers. We think
(But, how do we really know?) this is because the
manufacturing lines that have done our assembly are _very_
clean. all of the valid literature regarding whisker
reduction emphasizes cleanliness (Which makes sense when
you think about it).

> Gold does not always
> solder as well as you might like since it can actually diffuse out
> into your solder.  Unless your shop uses some really bizarre fire-retardant,
> the plating should be the only board material problem. One note on part 
> compatibility - watch out if your are using certain plastic packaged ICs 
> (QFP, QFN, some BGAs).  The higher solder temperature profiles can exacerbate
> problems with moisture.  Some parts manufacturers already require a "bake out"
> for certain parts.  When you go to the higher solder temps required for
> lead-free soldering, the "moisture sensitivity level" can increase by two
> levels (usually from MSL 2 to MSL 4).  This means your pre-loading handling,
> moisture exposure, and bake out may have to be changed.  Be sure to ask
> whoever places and solders your parts how they deal with this issue.

We had a bad experience due to moisture - As far as we are
concerned, baking is a must.

> I regards to what you need to put on the drawings - I suggest contacting
> your vendor(s) and ask them exactly what wording to use to get the process
> you want.  And then make sure they label the finished boards with date codes,
> UL file numbers etc. so there is some traceability or record of what they did.
> 
> Joe


Regards

Marvin