I should not have used the words I did in my first response, interchanging the words "melt" with "dissolve". This led to a lot of confusion. I apologize to those who were confused. Following is an explanation of what I was trying to say. I hope it is clearer.
First, soldering is the science of joining metals using a low-melting-point alloy (tin-lead solder is only one example) with a higher-melting-point base metal such as copper, gold, silver, or nickel. When soldering, heat is applied to the base metal to be soldered merely to heat it up, but not to melt it. When you solder, the molten solder wets out over the base metal. If heated properly, a small amount of the base metal dissolves into the molten solder, much like honey dissolves when added to hot tea. It does not actually achieve a molten state like the solder does, but the small amount that dissolves provides a good bond between the base metal and the molten solder.
This area of dissolved metals is the intermetallic formation (IMF) between the metals. Some metals such as copper, gold, and silver dissolve quite rapidly into the IMF, others take a much longer time to dissolve. Each metal has a certain rate of dissolution. This rate of dissolution is determined by the applied temperature and time during which the solder is in contact with it to achieve a corresponding IMF. Nickel's rate of dissolution is approximately 8 times longer than that of copper, and 10 times longer than noble metals such as silver or gold. In other words, nickel dissolves much more slowly into the IMF than other metals, and as such takes a longer time and hotter temperature to form an IMF that can be reliably depended on.
My question below should have been: How does one achieve a good solder joint without the proper amount of dissolution?
Because of this, in the example below where an assumption is made that if the solder melted over the nickel and "wetted", that a good IMF is formed; this may or may not have happened. Just because wetting is achieved does not mean that an IMF was formed that will provide reliable solder joints. Solder wetting can be achieved with a wooden toothpick. It does not mean that there is a good mechanical bond between the solder and the wood underneath.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Tremmel
Sent: Friday, October 28, 2005 4:48 PM
To: [log in to unmask]
Subject: Re: [TN] LEAD FREE PART IN SNPB PROCESS
Hello,
When someone equated soldering to that of salt dissoving in water, it threw me for a loop. Now I am wondering if someone willing to explain how an inter-metallic bond is formed when one of the metals (Ni, in the example
below) is not in a liquid state?
Thank you in advance,
David
>From: "Barmuta, Mike" <[log in to unmask]>
>Reply-To: TechNet E-Mail Forum <[log in to unmask]>, "Barmuta,
>Mike" <[log in to unmask]>
>To: [log in to unmask]
>Subject: Re: [TN] LEAD FREE PART IN SNPB PROCESS
>Date: Fri, 28 Oct 2005 12:57:03 -0700
>
>Richard: Affinity of the solder and base metal to form an intermetallic
>reaction.
>I hope you're not running your reflow at 2500F!
>
> Regards
>
> Mike Barmuta
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]]On Behalf Of Stadem, Richard
>Sent: Friday, October 28, 2005 11:46 AM
>To: [log in to unmask]
>Subject: Re: [TN] LEAD FREE PART IN SNPB PROCESS
>
>
>Mike, how do you form a solder joint with the nickel if the nickel does
>not melt?
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Barmuta, Mike
>Sent: Friday, October 28, 2005 10:43 AM
>To: [log in to unmask]
>Subject: Re: [TN] LEAD FREE PART IN SNPB PROCESS
>
>Hi Susan: No, there is no need to increase the reflow oven to Pb free
>temperatures.
>
>First the flux in his Sn/Pb solderpaste is not desinged to go to those
>temps, so the results will probbaly be worse.
>
>Second it is not nessesry to get the SAC coating on the part up to it's
>melting point to create a good solderjoint. Think of soldering to Ni.
>You don't have to reach the melting point of the Ni.
>
>Since you have not provided any information on the reflow times and
>temps or type of flux all I can do is give you a general direction. I
>would start with an increase in the dwell time above liquidous, 70-80
>secs, and a peak of 220-225 C.
>
>
>Regards
>
>Michael Barmuta
>
>Staff Engineer
>
>Fluke Corp.
>
>Everett WA
>
>425-446-6076
>
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Susan Mansilla
>Sent: Friday, October 28, 2005 8:08 AM
>To: [log in to unmask]
>Subject: [TN] LEAD FREE PART IN SNPB PROCESS
>
>
>Happy Friday to All
>I have a customer who is now being supplied with parts that the
>terminations are SnAgCu (95.5/4/0.5) instead of the previous SnPb.
>
>His process is not lead free and the parts are not always wetting and
>forming an adequate joint.
>
>What are his choices? Can he/should he take his SnPb line to lead free
>temps in an attempt to solder these parts - reflow process?
>
>Thanks for your advice
>
>Susan Mansilla
>Robisan Lab
>
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