That's how I see it.
As Ian said though commercial grade alloy is not always at the theoretical
eutectic, but I suggest that the alloy we use in PCB assembly is
functionally or practically eutectic.
So returning to the meaning of the original question, [is 105 seconds a long
time for the solder to be molten?]. My answer would be: well 105 seconds is
certainly towards the long end of what we would regard as usual practice.
Doesn't mean it's wrong though.
Keep in mind that paste manufacturers' data sheet profiles are not
instructions carved in-stone. They are suggestions for a start point to
which the most people most of the time will have to make the smallest number
of changes. The correct profile for an assembly is the one which works, and
this actually has more to do with the assembly and reflow oven than the
solder paste. [In that respect you could say a "good" solder paste is one
which supports your required profile].
So far as formation of intermetallics, joint structure and so on is
concerned - in other words long time reliability in service - the only way
to definitively answer that question would be by sections and reliability
trials; and field feedback. Best manufacturing practices would always be to
keep process temperature excursions as short/small as possible, and that's
why that suggestion is included in the profile.
Regards
Mike Fenner
Bonding Services & Products
T: +44 [0] 1865 522 663 E: [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Karen Tellefsen
Sent: Thursday, April 05, 2012 4:40 PM
To: [log in to unmask]
Subject: Re: [TN] Max time above liquidus
Wouldn't liquidus and solidus be the same temperature at the eutectic
point? I thought the eutectic point was at the intersection of the
liquidus and solidus curve, and one of the advantages of eutectic solder is
that it didn't have a mushy range between the liquidus and solidus
temperatures.
Karen Tellefsen - Electrical Testing
[log in to unmask]
908-791-3069
Re: [TN] Max time above liquidus
Stadem, Richard D.
to:
TechNet
04/05/2012 11:09 AM
Sent by:
TechNet <[log in to unmask]>
Please respond to TechNet E-Mail Forum, "Stadem, Richard D."
Werner emailed several times to me directly, and to others on the forum
that the term "above liquidus" should not be used when dealing with
eutectic solder, as there was no such thing as being above the liquidus,
ie, above the molten state. The solder is liquidus above the solidus
temperature, solid below it. So, to describe the molten state of the
solder, the correct term would be "above solidus". Was he wrong?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
Sent: Thursday, April 05, 2012 9:51 AM
To: [log in to unmask]
Subject: Re: [TN] Max time above liquidus
Agreed. I believe Richard is referring to the seldom referenced Vapidus
temperature where vapor and liquid can co-exist.
Doug Pauls
From: "Fox, Ian" <[log in to unmask]>
To: <[log in to unmask]>
Date: 04/05/2012 09:26 AM
Subject: Re: [TN] Max time above liquidus
Sent by: TechNet <[log in to unmask]>
Richard, the liquidus temperature is the maximum temperature at which
solid crystals can co-exist with liquid. Above the liquidus the material
is completely liquid. The solidus temperature is in contrast the
temperature below which liquid can't exist. In terms of a tin/lead phase
diagram it is the upper boundary of the liquid+solid "mushy" region
Regards
Ian
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: 05 April 2012 14:32
To: [log in to unmask]
Subject: Re: [TN] Max time above liquidus
You mean time above solidus. Time above liquidus is vapor.
Yes, 105 degrees in liquidus does seem excessive, but it depends on the
size/mass of the CCA being soldered. It might be required for very large
CCAs.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Fabien Guizelin
Sent: Wednesday, April 04, 2012 6:04 PM
To: [log in to unmask]
Subject: [TN] Max time above liquidus
Hi,
Should I be concerned by excessive TAL ? (inter-metalic, etc)
Do you consider 105sec TAL with a Indium 5.1 paste excessive ? (Indium
recommends 45-60sec)
Thanks,
Fabien
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