Earl,
To get a good handle on the physics behind IMCs, I suggest you take a
look at Colin Lea's book - A Scientific Guide to Surface Mount
Technology, Electrochemical Publications, 1988. The book may be
beginning to age but the physics isn't. This explains the dynamics for
solders in both solid and liquid states, with both lareg and small
volumes of solder available to take part in the reactions.
It does not, however, say what is safe and what is unsafe ina any
particular cincumstances.
Regards,
Roger Bilham
In article <[log in to unmask]>, Earl Moon
<[log in to unmask]> writes
>Thanks folks for all your great input, but I'm still without a clear
>understanding of the IMC subject and may never be.
>
>It is not hard for me to get confused at my advanced age and congenital
>ignorance. I?m just not getting a consistent picture. Could someone paint it
>for me ? for us all. First, the following is what I think I know:
>
>1) I should have said non-surface scientists (me especially) look at a
>surface as what it is apparent as an objects area having no depth.
>
>2) Surface scientists clearly understand surface as definitely having depth
>down to the surface?s beginning (about 1,000 angstroms or so) and understand
>that atomic layers (not mono-layers ? very poorly put) comprise this ?depth
>to surface.?
>
>3) Soldering is defined as the joining of two metal surfaces, with a solder
>medium (eutectic most commonly composed of tin-lead but changing to
>something lead free soon) requiring both diffusion and intermetallic
>formation (not Manko?s original definition) at temperatures below 800 F.
>
>4) I know all processes are capable of being mis-managed including HASL. I
>just find it more difficult to manage effectively across so many suppliers.
>I'll not go farther with this. Each of us has to make a decision concerning
>a process and its effects.
>
>5) I recognize that rework cycles, be they two or three, have more impact on
>circuit integrity as pad bond strength, etc. I still need to know how much
>is too much concerning intermetallic growth and its correlation to solder
>joint reliability.
>
>What follows is what?s confusing to me:
>
>1) I THINK Werner says thick IMC formations are essential and good for
>reliable solder joints. Is this right?
>
>2) Aside from board/pad damage, can there be too many soldering operations
>performed
>
>3) Is there a maximum or minimum IMC thickness to assure reliable solder joints
>
>4) Does Phil agree about IMC thickness
>
>5) Does Peter Duncan agree about IMC thickness
>
>6) Do Manko and Rahn agree
>
>7) Is this what Wassink says and knows and is it converse to all the above
>except Werner
>
>The main reason I?m asking is to gain objective evidence concerning eutectic
>lead soldering techniques having been around since before the time of
>Christ. Then, I would like to apply some of this understanding to what
>concerns us all as lead free soldering using who knows what. So, this means,
>I?m really no longer interested in stirring the pot, nor am I interested in
>becoming an expert. I just want a handle on this stuff as there still seems
>to be too much opinion.
>
>I just want to learn more about what I once thought I knew a little
>something about.
>
>Earl Moon
>
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--
Roger Bilham
Roger Bilham Consultancy
Tel: +44 (0)20 8467 8819
Fax: +44 (0)8700 548 613
Mobile +44 (0)7 941 122 446
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