With many electronics manufacturers still running leaded and lead-free assembly jobs, reflow ovens are tasked to run both solder-type profiles. Implementing both processes simultaneously with a single reflow soldering system can be approached with leaded and lead-free reflow profiles set up next to each other within a thermal system with two conveyor lanes.
Bans on the use of hazardous substances, set forth by RoHS, have become daily routine in manufacturing facilities around the world. Meanwhile, a few temporary exceptions to the ban on lead are forcing many electronics manufacturers to work with tin/lead as well as lead-free solders. The separate SMD production lines this demands, or the continuous changing of process parameters to match the respective solder at a single production line, can increase manufacturing costs. Implementing both processes simultaneously with a single reflow soldering system is an inexpensive alternative.
From a technical standpoint, implementing both processes simultaneously with a single reflow soldering system can be approached in two ways. Manufacturers can set up two thermal systems next to each other within one basic system; i.e. in addition to the conveyor lanes, there are two separately controllable heating systems for reflow profiling. Stated briefly, this involves the use of two ovens in one. This adds significant cost to the reflow equipment.
The other alternative is that different reflow profiles can be set up next to each other within a thermal system for two conveyor lanes. This is accomplished by taking advantage of the thermodynamic characteristics of the heat transfer system. In this case, investment costs are only increased by the amount required for a two-lane conveyor system.
Heat transfer between a reflow system and the PCB to be soldered is generally described by means of: q = α × t × A × ΔT, where α = heat transfer coefficient W/m2K; t = time in seconds; A = surface area in square meters; and ΔT = temperature difference (K).
The amount of time specified in the equation is determined by the speed at which the PCBs move through the system. Therefore, differing conveyor speeds with constant temperature differences (oven zone temperature minus PCB temperature) must result in different amounts of heat flow within the PCB. In other words, changing the conveyor speed inevitably alters the temperature profile for the PCB. As shown in Figure 1, as conveyor speed is reduced (i.e. longer dwell time in the oven), all reflow times (preheating time, time above liquidus [TAL], and cooling time) increase, maximum temperatures rise, and the difference in temperature between small and large thermal masses on the PCB is increased.
It becomes apparent that conveyor speed has an immensely complex influence on the reflow profile, eliciting the question: “Can tin/lead (SnPb) and tin/silver/copper (SAC) soldering processes be implemented in the same temperature zones simply by using two different conveyor speeds?”
To answer this question adequately, we must first define the working windows for both processes. IC standards JEDEC J-STD-020D.1 and J-STD-075 are helpful references, defining maximum reflow temperatures (maximum process temperature TP < classification temperature TC) for the respective component types and classes. The lower limits of the process window can be derived from general experience with necessary overheating above the liquidus temperature of the respective solder for the production of reliable solder joints.
Definition of the process window must always be based on the “weakest link,” namely the component with least amount of thermal stability during the soldering process. If two different processes are to be set up next to each other in the same reflow system, and if thermally sensitive components are included on the PCB, great flexibility is required for parameters configuration.
The technical layout of a dual-lane reflow soldering system allows for highly flexible process set ups, opening a wide process window for soldering. One system in this configuration* is equipped with two conveyor lanes that can be operated asynchronously at different speeds. With the multi-track variant, two lanes can also be operated asymmetrically with different lane widths (Figure 2).
Heat zones that are thermally well-isolated from each other, and which are arranged in a grid pattern above and underneath the conveyor lanes over the entire length of the process chamber, will allow for greatly varying reflow profiles. A single homogenous heat zone, with a single fan, spanning the width of the process chamber and both conveyor lanes at each respective location, means that there are no thermal barriers. Thermal barriers can be gas deflector plates or divided nozzle sheets between the conveyor lanes that would individually influence the flow of gas for the left and right-hand sides. The gas temperature and speed at which it flows is kept identical for both lanes. If the two conveyor lanes are simultaneously loaded with lead-containing and lead-free PCBs, respectively optimized reflow profiles can only be achieved by means of different conveyor speeds through this homogeneous reflow tunnel.
SnPb and Lead-free Reflow Profiles
Minimal temperature differences of less than 2 K within the transverse profile for the respective lanes contribute to obtaining the largest possible process window. It has been substantiated that SnPb and lead-free PCBs can be processed next to each other at the same time in a homogeneous, dual-lane reflow system. During the course of the experiments that prove this (which made use of various boards), two identical test PCBs were run through the reflow system at different speeds in the right and left lanes.
SnPb reflow profiles for large and small thermal masses on the PCBs are always shown on the left-hand side of the graphs in Figures 1 and 3, and the lead-free profiles (SAC) are shown on the right-hand side. The horizontal lines delineate the working windows:
SnPb process: 183°; 220°; 235°C
SAC process: 217°; 245°; 260°C
The dashed temperature lines (220° and 245°C) indentify maximum temperature for the more thermally sensitive components. The reflow profile is always represented with the thinner line width for the measuring campaigns.
As conveyor speed is reduced, maximum temperature and time above liquidus are increased, and the temperature difference (dT) between large and small thermal masses is decreased. Within the stipulated tolerance range of ±15% for a conveyor speed of 1200 mm/minute for the SnPb process and 700 mm/minute for the lead-free process, the resulting reflow parameters remain within the specified limits. Similarly good results have also been obtained for the saddle profiles. This demonstrates that an adequately large working window for parameters configuration is ensured.
SnPb and lead-free PCBs can be soldered simultaneously without investing significantly in duplicate reflow ovens, oven changeover, or segmented reflow soldering systems. The different reflow profiles required are achieved solely through the use of different conveyor speeds for two parallel lanes. The temperature settings for the heating and cooling modules are identical, because the modules span both conveyor lanes in a homogenous fashion. There are not any additional technical fixtures, such as separate nozzle sheets or gas deflector plates, that would influence heat transfer at the two sides of the system. This increases ease of use for the operator, and keeps investment costs down.
Electronics manufacturers benefit from the following advantages. Tin/lead and lead-free processes can be run parallel to each other with a single reflow soldering system. There’s no need to change system settings for the respective process, thus providing significant time savings for the manufacturing process. Nor is there any need to invest in a second reflow system, or a special system with separate heating units for the parallel lanes. A standard system can be used for both processes — one heating system, two conveyor lanes. Homogenous heat and cooling zones provide operating convenience, while allowing flexibility for reflow profiles.
*The Dual Lane version of the Rehm VX Reflow Soldering System.
Hans Bell, Ph.D., Rehm Thermal Systems GmbH, www.rehm-group.com.