Dear all,
The aspect of soldering peak temperatures has been discussed in this forum before on several occasions. Let me try to add some data from experiments. (By the way, I'll use degrees Celcius. I always have to re-calculate Farenheit to Celcius to get a
feeling of the order of magnitude. (Maybe a good suggestion to the American collegues to do the same ?!))
In the BRITE/EURAM IDEALS project wave and reflow soldering have been exercised, mainly with the SnAgCu eutectic alloy. Thorough process window studies have been performed. We at PHILIPS CFT did the wave soldering part (together with Alpha Metals), other
partners did the reflow part. We ourselves continued that work from the point onward were IDEALS stopped, both for wave and reflow and with the SnAgCu eutectic alloy.
Considerations for wave soldering temperatures:
Yes, those who state that 250C for SnPb40 (Tm = 183C) allows for some play with most types of printed boards, are right. That provides enough super-heat in most cases, in particular with single-sided boards and simple double-sided boards. However, for
thick multilayer boards, even with SnPb40, the contact times with the solder waves have to be increased and design rules which prescribe thermal barriers in the connected copper of the inner layers and of the top layer have to be obeyed. Otherwise you
will not get the heat in and the solder will not flow up completely into plated holes.
We have done a systematic process parameter study with SnAgCu eutectic (Tm = 217C) on single-sided boards and on double-sided boards with plated holes, board thickness 1.6mm. We found that the absolute minimum required bath temperature was 265C. In that
case we had to increase the contact times considerably already. Details can be found in our review in EGG2000+ - Berlin Germany and IPCWorks 2000 - Miami (counting)Florida as presented before to the forum.
Of course, the thermal barrier design rules have to be obeyed even more strictly and, for multilayer boards, times and temperatures may have to be raised even further in order to get the solder into the plated holes correctly.
Considerations for reflow soldering peak temperatures:
Yes, 235 - 240 C is OK as the minimum required reflow peak temperature for the SnAgCu eutectic alloy. However, the coldest spot on a board should reach that temperature. With the variety of components, small and large, the smallest components and bare
board areas will reach temperatures around 260C, depending on the chosen throughput rate of the reflow oven. (This temperature difference can be smaller in principle, but only at the expense of production rate. Usually applied production rates arrive at
this order of magnitude.) It's as simple as that.
Of course, there is some tolerance on temperature measurements and some variation on process parameters. It is a matter of discussion which safety margins should be added to the values mentioned above. I only mention this here without entering that
discussion.
Depending on how a component's thermal mass fits in the total range of components to be soldered, one can estimate the worst case peak temperature that it will undergo in a soldering process. Test conditions for resistance to soldering heat can be
determined, based on this principle. Component damage can be popcorn/delamination of moulded semiconductor devices (MSL = Moisture Sensitivity Level) leaching of metallisations from chip components, thermal shock cracking of glass and ceramic components,
film capacitor plastic degradation; over-heating of plastic bodies of mechanical components such as SM switches and potentiometers, boiling of electrolytes in elcos, unwanted reflowing of soldered joints in components, etceteras.
With best regards,
Erik
PS:
Regarding the environmental impact of nickel and noble metals, I based my statement on literature data (i.e. a recent paper in EGG2000+) and on our work performed in IDEALS, which I have issued to the forum already a long time ago. As a responsible human
being, I may be emotional regarding environmental issues and a sustainable world but, for lead-free soldering, no one will trap me on emotion without technical background.I leave that to politicians. I still have to read through the recent multitude of
lengthy contributions on this subject. After that, I will react in greater detail.
E.
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Erik E. de Kluizenaar
PHILIPS CFT - Electronic Packaging & Joining (EP&J)
Building SAQ-p, p/o box 218, 5600 MD Eindhoven - The Netherlands
Tel/Fax: (+31 40 27) 36679/36815; E:mail [log in to unmask]
PHILIPS homepage: http://www.philips.com; PHILIPS CFT homepage: http://www.cft.philips.com
Internal PHILIPS only: http://pww.cft.philips.com/cfteurope/electronics/elpajo/index.htm
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