Yes it's the total thermal input that is the worry. Quick and hot may be
better than low but slow - the area under the graph if you like.
So I would go along with Rex's suggestions as far as possible to avoid
having to change your BoM with alternate alloys or bonding methods.

If you go down that route you will need to specify all the base metals and
finishes, plus existing solder used, whether lead containing or lead free
solution required, plus service temperatures in conditions of use, plus duty
cycle etc in order to pick your way to a suitable alloy, [and depending on
what you find there may not be one]. 

This is because finishes, termination materials are made in the expectation
of a certain type of solder being used (i.e. Sn/Pb or high Sn). If you
change from those then you have to do diligent plod of all the details to
make sure you have compatibility and service life etc. with candidate
alloys. That's probably an off-line with supplier of your choice. In this
respect conductive epoxies are unlikely to be reliable on Sn containing
surfaces and may not have the current capacity for LED anyway.

Regards 

Mike Fenner 
Indium Corporation 


 
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Rex Waygood
Sent: Thursday, November 20, 2008 8:38 AM
To: [log in to unmask]
Subject: Re: [TN] Low temp solder alloy

It strikes me that whoever wrote the hand soldering spec either doesn't
understand hand soldering or is doing a CYA. 
In doing the rework I would ignore the 35W 'command' as that will just
get you into difficulties. 280C from a 90W iron is the same 280C as from
a 35W iron but my guess is time for soldering with a 90W iron will be a
lot less than a 35W iron and therefore potentially less damaging. If
Cree are making assumptions about thermal impedance of a 35W vs a larger
iron they are in dangerous territory (35W laser?). I would preheat the
board assembly prior to reworking with the iron to the maximum you think
you can get away with.
If you wanted to be really clever and enthusiastic you could calibrate
the 'Vbe' of the led vs temp and monitor that during a rework to
characterise your process. Making connection would require a bit of
inventiveness! :-)

More of interest, why are the leds failing?
Rex


Rex Waygood
Technical Manager
 
PartnerTech Poole Ltd 
Benson Road
Poole
Dorset BH17 0RY
United Kingdom
 
Tel: +44 (0)1202 674333
Fax: +44 (0)1202 678028
DDI: +44 (0)1202 338222
Mob: +44 (0)7887 997403
 
[log in to unmask]
www.PartnerTech.co.uk

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: 19 November 2008 16:50
To: [log in to unmask]
Subject: [TN] Low temp solder alloy

Dear Technos,

 

I have another one for you.

 

We do these boards having 100 to 200 TH high power LEDs (5mm in
diameter) and see a fairly high return since after a few weeks in the
field some LEDs will die or become intermittent. So we sat down and
started reading the app notes, see an example at
http://www.cree.com/products/pdf/LED_Lamp_SolderingHandling.pdf

 

Well, it turns out that these components cannot be touched up with
solder irons at more than 280C, which is 530F. This is rather low and I
wonder if we are going to be able to get the solder up the hole.
Anyways, I am heading towards the shop, in order to do some testing.

 

In the mean time, I would like to have your opinion on low temp alloys I
could use for touch-up:

o        What alloy should I look at?

o        Can I use a low temp alloy to touch up eutectic SnPb joints?
Any compatibility / reliability issues related to the eventual mixture?

o        Any particular low temp alloy that is more compatible with
SnPb?

o        Anything else I should think about?

 

Thanks again,

 

Ioan


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