Tin/lead solders do NOT melt at 183C, they solidify at this temperature,
called the solidus temperature. For eutectic tin/lead solder, consisting of
61.9%Sn/38.1%Pb, the liquidus temperature is also 183C, for all others the
liquidus temperature is higher depending how far from eutectic the mixture
is. Other constituents, e.g. copper, also will raise the temperature above
which solder is liquid. But it is not temperature, that makes a solder joint,
but heat. It takes a certain amount of thermal energy for the solder to flow,
go into solution with the base metal (e.g., copper, nickel-requires even more
heat, Alloy 42-requires even more heat, Kovar-requires even more heat). The
thermal energy available depends on the temperature, the heat transfer
conditions, the time at temperature, for the most part. That is why one gets
good solder joints at lower temperatures for vapor phase reflow than for
forced convection reflow, and lower temperatures for forced convection reflow
than infra red reflow.
A good rule of thumb for forced convection reflow soldering to copper
surfaces is liquidus+20C (so your 213C may not be high enough for larger
components depending on the solder composition you use) and liquidus+35C for
soldering to Alloy 42. These higher than liquidus temperature assure good
wetted solder joints even for larger components, that heat slower because of
the larger thermal mass, without overheating and damaging the components.
If you are interested, I give a workshop "Practical Design and Prediction
Methods for Surface Mount Solder Joint Reliability" at NEPCON West'97 that
would help you.
Course Content:
The objectives of this course are to make the participants familiar with the
underlying technical issues; provide insights in the relative importance of
product quality resulting from workmanship standards and process controls,
and focused 'Design for Reliability;' give them an appreciation of the
reliability pitfalls in the design, testing, as well as environmental stress
screening (ESS) of electronic assemblies; and to present state-of-the-art,
but simple to apply, tools for the 'Design for Reliability.' The attendees
of the course will have an understanding of the solder behavior under load
and fatigue which is necessary for the successful design, manufacture, and
testing of electronic assemblies. Processing issues to achieve the consistent
solder joint quality necessary to assure reliability will also be discussed.
The information presented has been included in the industry documents
IPC-SM-785, 'Guidelines for Accelerated Reliability Testing of Surface Mount
Solder Attachments', IPC-D-279, 'Design Guidelines for Reliable Surface Mount
Technology Printed Board Assemblies', ANSI/IPC J-STD-012, 'Implementation of
Flip Chip and Chip Scale Technology', and ANSI/IPC J-STD-013, 'Implementation
of Ball Grid Array and Other High Density Technology'.
Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ 07945 USA
Phone & Fax: 201-543-2747
E-mail: [log in to unmask]
***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to: *
* [log in to unmask] with <subject: unsubscribe> and no text. *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] *
***************************************************************************
|
