ROBUST REFLOW PROFILE DESIGN
By Bob Rooks
There is little literature on the engineering design of robust solder reflow profiles. This paper goes step by step through the process of engineering a reflow profile. The component requirements, solder paste requirements and reflow oven limitations are combined to produce a logical engineering approach to reflow oven profiling.
The time temperature requirements of the components and solder paste are equated to the oven construction. The component and material profiles are in temperature (degs. C) and time (sec.). The reflow oven is defined in zones and distances. The times of the profile parts are equated to the reflow oven zone lengths. The resulting sets of equations are solved to produce a robust fit of process requirements and oven dimensions.
The approach results in a logical repeatable robust design process. Once the process has been completed engineering decisions about solder paste selection and oven capacity / selection can logically be made.
Robustness is the ability of a process to have a stable out-put with a highly variable in-put. Today’s solder reflow ovens are superior to the IR reflow ovens of ten years ago: the PWA temperature is closer to the oven zone temperature. This makes oven profiling simpler and more intuitive. For the design of a robust profile, a change in profile regime (ramp, soak and reflow) must begin at the start of an oven zone. From the PWA’s perspective, the beginning of a zone is the only location the process can be changed (move in new direction). If the profiles are not aligned to the beginning of the oven zones, changes in plant temperature will shift the profile in the oven, resulting in an unstable process.
The fundamental activity of Engineering is model (mathematical, mechanical, computer, etc.) development and exercise to explain and predict the response of a physical, chemical, thermal or electrical system to changes in the input function. The model of interest for this paper is the reflow process for SMT soldering. Reflow profiles involves three of the four phenomena. Soldering is a process that is carried out at the edge destruction. Care is given not to thermally stress or shock the PWB and it’s components, but raise the temperature of the solder paste in a way which slowly evolves the volatile, activates the fluxing system and insures good metallurgical bonding.
The first step is to define the desired thermal profile. The profile requirements are determined by looking at the material requirements of the components and the solder paste. Component manufactures publish process guidelines for solder reflow requirements. Examples of the requirements are:
Max solder time and temperature
Time above 183 degs. C for 30 – 60 seconds
For this example the solder paste process guidelines are the most conservative of the above requirements. The resulting ideal profile is (figure 2):
For a robust process the ideal reflow profile must fit the reflow oven (figure 3)! Unfortunately this is not often the case. The closer the fit the more robust the process.
The ideal profile can be partitioned using ratios and the ratios can be applied to partitioning the reflow oven.
If the oven was partitioned exactly right the optimum conveyor speed would be 32.6 inches / min. (rate = distance / time). Overlaying the ideal oven on the reflow oven highlights the mismatch (figure 4).
Complying with the definition of robustness: The beginning of the ideal soak regime can be moved to the beginning of oven zone three or the beginning of oven zone four. The beginning of the ideal ramp 2 regime can be moved to the beginning of oven zone seven or the beginning of oven zone eight. There are four combinations (figure 5): A - low, A - high, and B - low, B - high.
A and B can be evaluated for its fit by dividing the misfit of each regime by the ideal zone length and summing the absolute value of each regime. The combination of A and B with the lowest value is the best fit (sum of squares can also be use).
A low and B low (figure 6) fit evaluation:
Absolute sum of fits = [ -.25 ] + [ .061 ] + [ .29 ] = .60
A low and B high fit (figure 7) evaluation:
Absolute sum of fit = [ -.25 ] + [ .43 ] + [ -.31 ] = .99
A high and B low fit (figure 8) evaluation:
Absolute sum of fits = [ .13 ] + [ -.26 ] + [ .29 ] = .68
A high and B high fit (figure 9) evaluation:
Absolute sum of fits = [ .13 ] + [ .06 ] + [ -.31 ] = .50
The sum of the fits is shown in (figure 10). A high and B high is the best fit. The problem with A high and B high is that ramp 2 fit is a negative -.31. This makes the ramp to reflow difficult. The three regimes are not equal in importance. The reflow is the most significant: if the solder paste does not get hot enough to reflow it does not matter that ramp 1 and soak are perfect. If solderbility is assured of being perfect the soak can be shorten. If the solder paste has good out gassing properties ramp 1 can be steeper. A -.1 or less for the ramp 2 regime disqualifies that A and B combination. The closer to the exit end of the oven the more significant the regime is.
A low and B low is the next lowest sum. Ramp 2 is positive and the soak is very close to ideal. Ramp 1 is a negative .25 more than the negative .31 of A high and B high for ramp 2. The conveyor speed for the best-fit combination is calculated using the oven segment (divisions) and the ideal times.
Ramp 1 conveyor speed = distance / time = (25.8 inches / 63.3 secs) X (60 secs. / 1 min.) = 24.4 inches / min.
Soak conveyor speed = (43.3 inches / 75 secs.) X (60 sces. / 1 min.) = 34.6 inches / min.
Ramp2 conveyor speed = (27.9 inches / 40.0 secs.) X (60 secs. / 1 min.) = 41.9 inches / min.
Ramp 1 conveyor speed is the slowest and yields the best profile to oven fit at 24.4 inches / min.
Robust profile development now moves to a first guess of the oven zone temperature settings with the conveyor speed set at 24.4 or as close to 24.4 as your oven can be set. A profile is run and the predictive function of your profiling software is then used to determine the oven zone temperature for each zone of the reflow oven. Run a second profile to check that your oven zone predictions were correct.
The importance of this model is that it yields a robust process. It provides a consistent method that lends itself to automation. The model can be used to select a solder paste and determine reflow oven requirement for capacity and capability.