Convection runs at faster cycle times, while condensation is superior for heavier PCBs.

Within a range of 3.0 to 4.7% silver and 0.5 to 3.0% copper, the SnAgCu alloy demonstrates a narrow melting temperature range of 216° to 217°C.¹ Precise proportioning of the constituents of this alloy does not play a significant role with regard to reliable solder joints, as indicated by a recently published IPC study concerning 96.5Sn3.0Ag0.5Cu, 95.5Sn3.8Ag0.7Cu and 95.5Sn4.0Ag0.5Cu solders.²

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 are critical. IPC/JEDEC J-STD-020C classifies moisture-sensitive non-hermetic components (MSDs) according to package dimensions, among other factors, and restricts the maximum reflow temperature for package thicknesses of >=2.5 mm and package volumes of >=350 square mm to 245°C.³ 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 the working window for the maximum temperature of 245°C.

Against this backdrop, the right reflow technology must be selected for the PCB to be soldered. This article reviews the differences between convection and condensation (vapor phase) soldering.

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 a flowing liquid or gas. When liquids and gases are heated, 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 in turn conducts the energy. Convection is forced from outside by fans or blowers in convection reflow ovens, which are usually operated in an air or a nitrogen atmosphere. As opposed to vapor phase, convection systems are usually equipped with several process zones which can be adjusted independent of one another, allowing for variable reflow profiling. The temperatures of the process zones, the flow rate of the utilized gas and the speed at which the PCBs to be soldered are transported through the oven can all be varied. Figure 2 demonstrates the flexibility offered by convection systems with the help of three characteristic temperature curves, measured on the same PCB using different parameter settings. Differing dwell times (preheating and time above liquidus) have been achieved with nearly identical temperatures in preheating and in the peak zone.

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 (DT) and heat transfer coefficient (h), which essentially represents the characteristics of the system [Eq. 1]:

Achievable cycle time per PCB or panel is determined to a great extent by the selected conveyor speed [Eq. 2]:

The relationship between cycle time and oven length is discussed at length by Bell.⁴ Since, as Eq.1 shows, 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 with long ovens, if identical cycle times are to be achieved.

The fact that the oven's gas temperature is always higher than the maximum temperature achieved on the PCB is cited as a disadvantage for convection systems, because theoretically this could result in overheating. However, these concerns are unfounded in actual practice because modern ovens have control and monitoring facilities that prevent drifting and spikes in process temperature and conveyor speed. Figure 3 depicts the actual temperature of a convection oven heat zone, which fluctuates within a range of <1 K. Monitoring systems are also able to calculate machine capability coefficients for each heat zone online during production.

Condensation. Condensation soldering, also known as vapor phase, is the older of the two reflow processes. It was patented in 1975 by R.C. Pfahl and H.H. Ammann.⁵ Condensation soldering makes use of the latent heat (also known as specific vapor enthalpy or phase change enthalpy dH) released by a special medium when it changes from the vaporous to the liquid state to heat the PCB to be soldered. Temperature remains constant during the medium's change of state (phase transition), which ensures 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. Herwig et al.⁶ established heat transfer coefficients of up to 300 W/m²K for condensation soldering, whereas values from 20 to 50 W/m²K are typical for convection soldering (in air or nitrogen).

The PCB's individual DT. The greater, more uniform heat transfer associated with condensation soldering results in smaller temperature differences between small and large thermal masses (components) located on the PCB (designated individual DT). The difference becomes clear when we compare Figures 4 and 5. The same PCB achieves DT = 10 K during convection soldering and DT = 3 K during condensation soldering.

In convection soldering systems, the oven can be laid out with the soldering zone subdivided into several peak zones, each of which can be controlled separately (Figure 6). The individual DT value was halved for a number of PCBs using a triple peak zone included in a novel system (Figure 7).

The heat gradient. 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 characterized by nearly linear, very steep increases. Steep temperature increases may lead to damage (e.g., popcorning) during reflow. In addition to MSDs, electrolytic capacitors are also critical components. Tests conducted by BC Components and Rehm have shown that the capacitors' loss of capacitance after condensation soldering is, in some cases, greater than after convection soldering.⁷

With the help of specific vaporization enthalpy (which specifies how much energy is available within one kilogram of a given material), available condensation energy Q_cond can be calculated for the entire volume of vapor [Eq.3]:

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. Figure 8 depicts a set of curves for a single test PCB, which was measured using different process parameters. The temperature gradient corresponds to the volume of injected medium.

When soldering heavy PCBs, condensation soldering is superior to convection soldering due to the large heat transfer coefficient. Large masses require proportionately more heat 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 results more or less in a single-zone reflow system, in which the PCB is at a standstill for the duration of the processing time span. The PCB's overall temperature profile must be subordinated to this reality. Relative cycle time is thus always equal to process time (reflow profile time + handling time) divided by the number of PCBs or panels which are located inside the process chamber. A cycle time of 1 min. can be expected with fourfold loading of the process chamber and a PCB with a length of 300 mm. On the other hand, a convection system achieves a cycle time of less than 30 sec. with the same PCBs. With regard to cycle time, convection ovens are superior to condensation systems.

Summary

The following table compares the benefits of convection and condensation reflow:

A reflow technology that 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 ensures an inert process atmosphere in convection ovens, and the use of several peak zones reduces 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 heavier PCBs. The condensation temperature of the medium limits the maximum attainable temperature on the PCB. High-level, uniform thermal conduction results in the smallest individual temperature differences. However, limit gradients can very easily be exceeded if the parameters are not correctly configured.

References

1. Jennie S. Hwang, Environment-Friendly Electronics: Lead-Free-Technology, Electro-chemical 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," July 2005.

3. IPC/JEDEC J-STD-020C, Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices, July 2004, Table 4-2.

4. Hans Bell, "Reflowlöten," Leuze Verlag 2005, p. 116.

5. Robert C. Pfahl and Hans H. Ammann, Method for soldering, fusing or brazing, Western Electric Company, Bell Laboratories, U.S. Patent 3,866,307, 1975.

6. Hans Bell, Harry Berek, Heinz Herwig, Andreas Moschallski, Mathias Nowottnick, "Inline-Kondensationslöten," VTE, vol. 14, 2002, Notebook 2, p. 66.

7. Franz Wieser, "SMD Alu Elkos für bleifreie Prozesse," 8th EE-Kolleg, March 2005.

 

Dr. Hans Bell is head of development and technology at Rehm Anlagenbau GmbH (rehm-anlagenbau.de); h.bell@rehm-anlagenbau.de.