According to an IPC estimate, more than 50 percent of all electronic products are manufactured worldwide with lead free solder. Lead free soldering is thus long since state-of the- art, and practical work is now being concentrated on the optimization of the manufacturing process from a quality standpoint. The biggest problem in this respect is the narrow process window in reflow soldering. The process window is restricted at the bottom end as a result of the melting point of the selected lead free solder alloy. The favorite alloy combination for refl ow soldering is still tin-silver-copper (Sn-Ag-Cu). Within a range of 3.0 to 4.7 percent silver and 0.5 to 3.0 percent copper, this alloy demonstrates a very narrow melting temperature range of 216 to 217°C. Precise proportioning of the constituents of this alloy does not play a signifi cant role with regard to reliable solder joints, as indicated by an IPC study concerning 96.5Sn3.0Ag0.5Cu,95.5Sn3.8Ag0.7Cu and 95.5Sn4.0Ag0.5Cu solders. 
The fundamental upper temperature limit for the process window is dictated by the processing limitations of the utilized materials and components. In particular moisture-sensitive components and electrolytic capacitors are critical. The IPC JEDEC J-STD-020D standard classifies moisture-sensitive non-hermetic SMDs according to, amongst other factors, package dimensions, and restricts the maximum reflow temperature for package thicknesses of ≥ 2.5mm and package volumes of ≥ 350 square mm to 245°C. Warm-up and cool-down gradients (+3 K/s, -6 K/s), as well as limiting dwell times above certain temperature levels, represent additional restrictions. Figure 1 shows theworking window for moisture sensitive components. 
It must be noted that the temperatures specified here are package temperatures, not solder joint temperatures. These temperature profi les only make reference to the qualification of moisture sensitive components, and not to profiles for soldering electronic components in accordance with the specified quality requirements. However, soldering profiles must remain within the limitations stipulated here. In addition to moisture sensitive components, stress limits for other components must be observed as well (non-IC electronic components). For example, large aluminum capacitors (diameter > 10mm) are limited to a package temperature of 230°C in the IPC JEDEC J-STD-075 standard. Against this backdrop, the right refl ow technology must be selected by the party responsible for the process used for the electronic PCB to be soldered. Convection
Convection systems have controlled the largest share of the market since the 1990s. The term convection is understood here as heat transfer by means of flowing liquids or gases. When liquids and gases are heated up, their density is reduced and flow, circulation and convection are caused by the resulting lift. The heat energy does not flow itself, but rather the medium, which in turn conducts theenergy. Convection is forced from outside by means of fans or blowers in convection reflow ovens, which are usuallyoperated with an air or a nitrogen atmosphere. As opposed to vapor phase systems, convection systems are usually equipped with several process zones which can be adjusted independent of one another, thus allowing for variable reflow profiles. The temperatures of the process zones and the flow rate of the utilized gas, as well as the speed at which the PCBs to be soldered are transported through the oven, can all be varied. Figure 2 demonstrates the great flexibility of convection systems, with which linear as well as saddle-shaped profiles can be very easily created. 
Convective heat transfer results from contact of the gas molecules (air or nitrogen) with the colder surfaces of the PCBs. Transferred heat Q is a function of time t, contacting surface area A, temperature difference ΔT and heat transfer coefficient α, which essentially represents the characteristics of the system. Time is determined by conveyor speed. As speed increases the amount of transferred heat is reduced, thus reducing maximum temperature reached at the PCB. Simultaneously, dwell time and thus time above liquidus are reduced, which in turn affects a growing ΔTM (temperature difference between large and small thermal masses on the PCB). Thus conveyor speed is the greatest infl uencing parameter in convection soldering. Due to the fact that heat transfer depends upon the PCB’s dwell time in the oven as well as the temperature difference between the oven and the PCB, comparably higher gas temperatures are required in short ovens than is the case with long ovens, if identical cycle times are tobe achieved. The fact that the oven’s gas temperature is always higher than the maximum temperature achieved on the PCB is sometimes cited as a disadvantage for convection systems, because theoretically this could result in overheating. However, these concerns are unfounded in actual practice because modern oven systems are equipped with control and monitoring facilities which prevent process temperature and conveyor speed drifting and outliers (figure 3). Modern convection oven monitoring systems, such as CCS (capability control system), are also able to calculate machine capability coefficients for each heat zone online during production. 
Convection systems are frequently operated with a nitrogen atmosphere, which significantly improves the wetting results of the soldering partners, as is shown in figure 4. Nitrogen is an inert gas which displaces atmospheric oxygen in the reflow system and prevents oxidation ofthe parts to be joined. 
Condensation
Condensation soldering, also known as vapor phase soldering, is the older of the two reflow processes discussed here. It was patented in 1975 by RC Pfahl and HH Ammann. Condensation soldering makes use of the latent heat (alsoknown as specifc vapor enthalpy or phase change enthalpy dH) released by a special medium when it changes from the vaporous to the liquid state in order to heat up the PCB to be soldered. Temperature remains constant during the medium’s change of state (phase transition), which assures that the maximum temperature of the PCB cannot exceed the boiling point – the condensation temperature of the medium. This form of maximum temperature limiting represents a significant advantage in favor of condensation soldering. A large amount of heat is released during the medium’s change of state, which results in a rapid rise in temperature at the PCB. Bellet al. established heat transfer coefficients of up to 300 W/ m²K for condensation soldering, whereas values within a range of 20 to 50 W/m²K are typical for convection soldering (in air ornitrogen). Greater, more uniform heat transfer associated with condensation soldering results in smaller temperature differences between small and large thermal masses located on the PCB (designated individual dT). The difference becomes quite clear when we compare the soldering processes (figure 5). The same PCB achieves a ΔT value of 10K with convection soldering and 3K withcondensation soldering using Galden LS230. 
In the case of condensation, the flow of heat into the PCB depends primarily on the mass flow rate of the medium which is condensing onto the surface of the PCB. If enough vapor is present, the mass flow rate remains constant, resulting in a reflow profile for condensation processes which is characterized by very steep increases. Steep temperature increases may lead to damage during reflow soldering of electronic PCBs, for example popcorning and delamination. In addition to moisture-sensitive SMDs, electrolytic capacitors are critical components as well. Tests conducted by BC Components and Rehm have indicated that the capacitors’ loss of capacitance after condensation soldering is, in some cases, greater than after convection soldering. Latent heat energy available to the PCB can be influenced by decreasing or increasing the amount of vapor available within the process chamber, i.e. the heat gradient can be controlled. This process concept—injection—makesit possible to create different reflow profiles within the process. If the vapor is evacuatedfrom the process chamber after an initial warm-up phase,no further condensation can take place and the temperatureprofile resembles a saddle. 
When soldering very heavy PCBs, condensation soldering is superior to convection soldering due to the very large heat transfer coefficient. Figure 6 demonstrates that the convection system has not succeeded in heating up a mass of 0.5 grams to the expected final temperature with 240°C temperature zones; this was no problem with the condensation system. Large masses require proportionately more heat in order to reach the soldering temperature. The larger the mass of the PCB, the flatter the heat gradient becomes, assuming that the volume of available vapor remains unchanged. Normally, only one process medium with a single condensation temperature is used in condensation systems. This more or less results in a single zone reflow soldering system, in which the PCB is at a standstill for the duration of the processing time span (figure 7). The PCB’s overall temperature profile must be subordinated to this reality. Relative cycle time is thus always equal to process ime (reflow profile time plus handling time) divided bythe number of PCBs or panels which are located inside the process chamber. As opposed to this, a convection systemachieves a cycle time of less than 30 seconds with the samePCBs. With regard to cycle time and flexibility, convectionsoldering systems are superior to condensation solderingsystems. 
Summary
Table 1 summarizes the characteristics of both convection and condensation soldering systems. A reflow technology which fulfills the requirements of the PCB to be soldered should be selected. Convection ovens are suitable for flexible manufacturing with requirements for minimal cycle times. Nitrogen assures an inert process atmosphere in convection ovens, and the use of several peak zones makes it possible to reduce the PCB’s individual temperature difference. In comparison, reflow profiling flexibility is restricted by condensation systems, but they are superior to convection systems for soldering especially heavy PCBs. The condensation temperature of the utilized medium limits the maximum attainable temperature for the PCB. High level, uniform thermal conduction results in the smallest individual temperature differences. However, limit gradients can very easily be exceeded if the parametersare not correctly configured. About the author
Dr Hans Bell is Manager Research and Development at Rehm Thermal Systems.He can be reached at h.bell@rehm-group.com. References
[1] Jennie S. Hwang, Environment-Friendly Electronics: Lead-Free-Technology, Electrochemical Publications, 2001, p. 232
[2] IPC Round Robin Testing and Analysis of Lead Free Solder Pastes with Alloys of Tin, Silver and Copper — Final Report 2005
[3] IPC/JEDEC J-STD-020D, Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices, July 2007
[4] Hans Bell, Reflowlöten, Leuze Verlag 2005, pp. 116 ff
[5] Robert C. Pfahl, Hans H. Ammann, Method for soldering, fusing or brazing, Western Electric Company, Bell Laboratories, US Patent 3,866,307, 1975
[6] Hans Bell, Harry Berek, Heinz Herwig, Andreas Moschallski, Mathias Nowottnick, Inline-Kondensationslöten, VTE 14(2002) booklet 2, p. 66
[7] Franz Wieser, SMD Alu Elkos für bleifreie Prozesse, 8th EE-Kolleg, Sant Jordi, March 2005
[8] IPC/JEDEC J-STD-075, Classification of Non-IC Electronic Components for Assembly Processes, August 2008
[9] Dr.-Ing. H. Wohlrabe, Dr. H. Bell, Rehm, Dipl.-Ing. J. Trodler, Analyse vonMaterial- und Prozesseinflüssen auf die Reflowqualität – part 1, PLUS 5, 2008 | 
