Page 1 of 6 Roter Sand 5 • D-97877 Wertheim • Tel: +49/93 42/970-0 • Fax: …/9 70-8 00 • [email protected]• www.SMT-Wertheim.de Reflow Soldering – Solving A Solder Wicking Problem By Raimund Faber/Manfred Maehl, SMT North America, Inc., Glen Allen, VA Soldering is quite a complex science. Although the underlying scientific facts are the same for the various soldering processes each one has to overcome their unique issues in addition to dealing with the inherently common ones. Among the variety of potentially serious problems is solder wicking; in this case a challenge to be submitted by a demanding automotive customer and solved by the process engineers of a first-class reflow oven manufacturer. For those less familiar with this phenomena a few words to the theories and root causes of solder wicking which basically is defined as solder climbing up on a lead: • The pad reaches the liquidus temperature later than the lead, therefore the melted solder is tightened completely by the lead (wicking-up phenomena) • The solder alloy pulls to the hotter surface if sufficient wettability • Leads are melting faster than rivet solder joint – the delayed liquefaction of solder paste at the rivet solder joint pulls the leads away from pads • All non-wetting problems like plating too thin or contamination/oxidation can cause wicking-up In preparation of testing the printed circuit board consideration was given to following conceivable wetting parameters: • Temperature difference • Wettability of pad • Heat sink at conductor • Gap dimension between pad/lead • Melting time of solder at rivet The major conclusions to the above: The heat sink had to be heated up first, and increased convection heat had to be applied from the bottom! For the following test a well-proven 8-zone (5 pre-heat, 3 peak zones) forced convection reflow oven SMT Quattro Peak L Plus (N2) was selected suitable for processing very complex boards, with optimum process stability and unmatched reproducibility. This reflow oven model also features perfect heat transfer, excellent zone separation, and achieving unique cross and load profile performance. The manufacturer of this reflow oven is globally active SMT Maschinen und Vertriebs GmbH, Wertheim, founded in 1987 and a European leader in the manufacture of reflow ovens and other thermal process equipment. The Company has caught the attention of the industry for its environmental commitment. SMT offers the most energy efficient thermal equipment in the industry worldwide; as a result they have been accepted and are active as a partner by the governmental Energy Efficiency Initiative. The revolutionary ABS process gas cleaning system and the intelligent low consumption nitrogen system are further examples of not only conserving environmental resources but also significantly contribute to lowering cost and improving customer’s cost of ownership.
Reflow Soldering – Solving A Solder Wicking Problem
By Raimund Faber/Manfred Maehl, SMT North America, Inc., Glen Allen, VA Soldering is quite a complex science. Although the underlying scientific facts are the same for the various soldering processes each one has to overcome their unique issues in addition to dealing with the inherently common ones. Among the variety of potentially serious problems is solder wicking; in this case a challenge to be submitted by a demanding automotive customer and solved by the process engineers of a first-class reflow oven manufacturer. For those less familiar with this phenomena a few words to the theories and root causes of solder wicking which basically is defined as solder climbing up on a lead: • The pad reaches the liquidus temperature later than the lead, therefore the melted solder is
tightened completely by the lead (wicking-up phenomena) • The solder alloy pulls to the hotter surface if sufficient wettability • Leads are melting faster than rivet solder joint – the delayed liquefaction of solder paste at the rivet
solder joint pulls the leads away from pads • All non-wetting problems like plating too thin or contamination/oxidation can cause wicking-up In preparation of testing the printed circuit board consideration was given to following conceivable wetting parameters: • Temperature difference • Wettability of pad • Heat sink at conductor • Gap dimension between pad/lead • Melting time of solder at rivet The major conclusions to the above: The heat sink had to be heated up first, and increased convection heat had to be applied from the bottom! For the following test a well-proven 8-zone (5 pre-heat, 3 peak zones) forced convection reflow oven SMT Quattro Peak L Plus (N2) was selected suitable for processing very complex boards, with optimum process stability and unmatched reproducibility. This reflow oven model also features perfect heat transfer, excellent zone separation, and achieving unique cross and load profile performance. The manufacturer of this reflow oven is globally active SMT Maschinen und Vertriebs GmbH, Wertheim, founded in 1987 and a European leader in the manufacture of reflow ovens and other thermal process equipment. The Company has caught the attention of the industry for its environmental commitment. SMT offers the most energy efficient thermal equipment in the industry worldwide; as a result they have been accepted and are active as a partner by the governmental Energy Efficiency Initiative. The revolutionary ABS process gas cleaning system and the intelligent low consumption nitrogen system are further examples of not only conserving environmental resources but also significantly contribute to lowering cost and improving customer’s cost of ownership.
The above chart shows the different machine settings for lead as well as lead free temperature profiles – note the remarkable and considerable temperature difference of up to 30° C (zone 5) for the “Special Lead Free” profile between top and bottom heaters. Chart II: Time to Liquidus:
Chart II. explains in detail the melting process of the solder paste. For conventional lead solder paste the desired time to liquidus is usually achieved earlier. The reflow oven used in this test using lead free solder paste achieved similar results due to its advanced and mature control concept.
Sample of temperature profile for the test with Special Lead Free solder paste for sensors 1 and 2 per above Chart II. Chart IV.: Reflow Oven – Zone Definition of Test Machine:
Above Arrangement Chart defines 5 pre-heat zones, 3 peak zones and the cooling section for the reflow oven used for this test. The heated process chamber length includes the active convection zones consisting of pre-heat and peak zones. These are perfectly proportioned at a ratio of 3:1.
Explanation of high efficient heat transfer due to vertical air guiding: Well Known Reflow-Systems: Well known reflow systems are mostly using a laminar airstream. Laminar airstream creates no air circulation on the surface and thus is generating ⇒ isolated laminar air layer ⇒ weak heat transfer
In addition laminar airstream causes inhomogen temperature settings on the PCB and heat shadows behind bigger components and thus a bigger delta T on the PCB. SMT - Power Nozzles: SMT reflow systems with Power Nozzles are using a vertical airstream. Vertical airstream creates homogen air circulation profile on the surface and thus is generating ⇒ balanced airstream ⇒ excellent heat transfer
In addition the vertical air guiding reaches every single spot on the PCB and the heat will be spread out homogeneously. The short distances guarantees high efficiency and a lowest possible delta T
Efficient and optimum heat transfer is not only a guarantee for best soldering results, it also ensures a positive energy balance. SMT, Wertheim has equipped all its machines with Power Nozzle technology. The enhanced geometry allows an even more perfect process gas conveyance. Power Nozzle is a special nozzle system that operates on the basis of a high fan performance while simultaneously maintaining low flow speeds. Absolute temperature stability in the soldering chamber is the result and this in turn ensures consistent warming of the modules. Agitating large air masses while at the same time maintaining low flow speeds results in hot air flow directly over the module where it is most effective. By introducing Power Nozzle technology, SMT has not only taken another step forward in the direction of quality enhancement but has also quite consciously and consistently advanced its existing energy saving policy. Summary: The selected Reflow Oven model was an excellent choice and fully capable of adjusting the required profile such a way that even the heat sink soldering joint, sensor 1, was melting 1.5 sec. before the component lead /pad sensor. The soldering results were achieved due to perfect heat transfer as well as bottom side heaters in all zones and independently controlled fan speed settings. Temperatures on the bottom side were set approx. 20 degrees higher than on the top side. The fan speed at the bottom side was set at maximum, and to minimum at the top side. These settings were chosen to get the biggest possible difference in heat transfer between top and bottom. Contact: SMT North America, Inc., 5905 New Harvard Place Glen Allen VA 23059 Phone: 804 360-0144 E-Mail: [email protected] Web: www.smtna.com
