Ian,
The melting temperature of most Pb-free solder alloys is a good 20-30
degrees C higher than 63/37.
On a design where many of the vias are connected to a large internal
heatsink (copper plane) such as the power and ground layers, much more
heat is required to overcome the heatsinking and get the lead-free
solder to melt.
For hot-gas reflow of BGAs, as an example, you will need to increase the
MRT of the profile to facilitate full reflow.
For removal of chip caps and resistors using a soldering iron or
hand-held hot gas (Hakko), you will need to increase the tip temperature
or hot air temperature. For example, I know that many of the components
require a 700 or 750 F. tip if soldered to the PWB with lead-free
solder, whereas the same part removal was easily done before with a 600
or 650 deg. F tip using leaded solder.
When removing a BGA that was originally soldered to the PWB with SAC 305
solder balls, it is much more difficult to do a site prep of the PWB
pads after removing the BGA, as the solder cools very quickly. You have
to be much more careful if using braid to prep the pads. If the board is
not required to be RoHS compliant, then what I have done is flood the
pads with standard 63/37 solder first, which makes it much easier to
wick off.
And your dwell time must be strictly controlled. Once the innerlayer
copper is heated up, its ability to wick away heat gets better and
better, making it more difficult to get the junction up to liquidus
temperature. So if you are having trouble, move on to another site and
come back to the first later after that area of the board has cooled
down. Don't "sit" on a single location, you will simply damage the pad
or the board.
The trick is to try to "thermally shock" the junction and quickly remove
the part, before everything gets too hot.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Cheryl L. Johnson
Sent: Wednesday, September 12, 2007 1:24 PM
To: [log in to unmask]
Subject: Re: [TN] Thermal issue's with unleaded solder paste.
What specific lead-free solder and pcb finish are you using?
--
Cheryl Johnson
(303) 485-8829 home
(303) 809-5815 cell
-------------- Original message ----------------------
From: "Thomas, Ian (Ian) %" <[log in to unmask]>
> Hi, Need some useful advice regarding a current design.
> I have a 12 layer FR4-370HR PCB, using stacked uvia's in pad which
> connect to planes on layers 2 & 3 which are flooded with no thermal
> relief.
>
> We have seen no issue's with leaded assembly, but on transferring over
> to un-lead, we are seeing solder issue's with the BGA's (0.5mm pitch)
> on assembly and also rework issuse's with 90% of the other components
> (including
> 0402's) in getting them off the board.
>
> Our first thoughts are to thermally relieve all the micro via's, but
> there are issue's because this may leave you with only one spoke on a
> pin in some areas.
>
> Has any seen these issue's before?
>
> Thank's in advance.
>
> Ian
> LSI Technology (Mobility)
>
>
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