Beta Test: Can Software Create a Self-profiling Reflow Oven?
If effective, results from this test could reduce production setup time, increase overall process accuracy and eliminate the need for daily product-profiling delays.
By Curtis Kee
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At contract manufacturer (CM) Benchmark Electronics' Beaverton, Ore., facility, a beta test was established for a new thermal-profile-planning and automatic-setup system for a solder reflow oven. Its goal was to cut reflow-process-setup time in half and create a self-adjusting profile by communicating directly with the unit's controller. The plant's production level was conducive to this activity. It runs two shifts daily of high-mix board configurations and solder paste types. Part of the goal was to test the software against a cross sample of printed circuit boards (PCB) using a working oven from the production line and a standard pass-through profiler. The software would get a thorough workout and any problems would surface during the process.
Software Concept
The software is based on the premise that if it contained a thorough database
of profile formulas derived from those established by solder paste manufacturers,
it could combine any formula with specific user-supplied assembly data
and known oven capabilities to develop the optimum reflow profile for
any board configuration. The new profile would be communicated automatically
to the oven controller, which then would adjust the unit to the required
parameters. Although modern reflow ovens have substantial capabilities
in automated control, the "true" self-profiling oven has been
elusive. The problem lies with communication between the oven and the
product profiler.
Getting its controller computer to adjust the reflow oven's key parameters to a new recipe is a normal occurrence. Also, a profiler that accurately collects product thermal data has been a reality for decades. The challenge has been to achieve a method that connects the two without requiring substantial human interaction and manual iterations. A "language" was needed that would be understood easily by both the oven controller and thermal profiler and be supplied through a bi-directional mailbox file shared by the controller and the new software. This combination would report oven information — zone numbers, lengths and temperatures, convection rates, and conveyor speed — and suggested set points to achieve the robust target profile.
Test Setup
Over a two-month period, many experiments were conducted in a working
environment rather than in the isolated conditions of a dedicated site.
The tests were run between or after working shifts with the need to reconfigure
the oven profile for each test a constant reality.
The equipment used for the tests included: 1) a pass-through profiling data logger*; 2) a forced-air convection oven with eight heating zones**; 3) a docking station attached to the oven at its computer platform (for data-logger recharging and downloading to the oven's computer); 4) various assembly sizes, e.g., small, medium and large boards with mixed components, including 0603s, mini-quad flat packs (QFP) (some greater than a 1 sq.2 ball grid array [BGA]) and various ceramic parts; 5) RMA and water-soluble solder pastes; and 6) new profile-planning and automatic-setup software*** installed in the oven-controller computer.
The Test Process
To accommodate the software package, the oven-controller computer required
a minor upgrade to add memory, which was provided by the oven manufacturer.
With the software installed, the first activity established the oven to
be at optimum performance levels. The initial setup consisted of characterizing
the unit, which was conducted by the software developer. All oven parameters
had to be qualified and documented, including knowing the maximum heat-output
capability of each zone and ensuring that all oven variables were in optimum
condition.
The oven-calibration process is a password-protected procedure that asks questions based on the program's built-in database for the installed oven brand and model. After accepting the zone-convection rates descriptor — on, medium, RPMs, percent or Hz (depending on built-in convection-rate control by brand) — that rate is set manually via the oven controls. Next, the software automatically sends zone-temperature setpoints between 100° and 180°C. The pre-instrumented calibration board, included with the system, is run while attached to the pass-through profiler. When the latter is downloaded, the software compares zone coefficients just generated with those in the database and "asks" for acceptance of the new values. If accepted, the program automatically exits from calibration mode with the software and oven ready to operate normally.
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To form a solid database, each test follows the same formula using different
pastes and board configurations (Figure 1). PCBs from 0.062 to 0.100"
thick are used. The pass-through profiler is attached to the test board
to be profiled via three thermocouple wires (T/C), which are attached
to the board at key locations: at sensitive, cold and hot components (Figure
2). For example, a cold component could be under a ceramic BGA while the
hot could be a device with less mass (e.g., an 0603 capacitor) on the
board surface. These then are plugged into the ready ports of the pass-through
profiler.
Initiating Automatic Profile Selection
Planning Stage: The paste to be used is selected from a pull-down
menu. This generates the target profile that the oven must match to the
PCB being processed (peak temperature, preheat ramp, soak time, time above
liquidus [TAL] and conveyor speed). Conveyor speed may be limited to the
realities of the production line while the target also may be adjusted
here for desired exit temperature, etc. (Figure 3). For example the paste
spec may have a peak temperature at 230°C although the engineer might
prefer a peak at 220°C, in which case the zone exit temperature box manually
is dragged down 10°.
Setup Stage: The "cold component" and "board thickness" assembly properties then are selected. Based on these variables (plus the solder paste and oven characterization), the software displays suggested setpoints (Figure 4). When "send setpoints" is hit, the software "talks" to the oven controller, which adjusts the oven heaters and conveyor speed to the new values, and "waits" for the oven to stabilize at the new settings.
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Verification Stage: A green light (tower on oven) indicates the oven is ready and the profiler and board are sent on the conveyor. Afterwards, the profiler is plugged into the oven-mounted download/charging station to transfer the thermal data directly to the oven computer. The process engineer then compares the live profile to the target (software-generated) profile and selects "auto predict," which automatically adjusts individual zone temperatures to match the target to the current PCB profile (Figure 5). This creates an optimum set of points for sending to the oven. The process is complete after the oven stabilizes to the new parameters, i.e., the optimum product profile had been achieved with a single pass. However, to further validate for the test report, each board is sent through the oven for a second profile pass.
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Further Testing
Repeating these basic steps over a two-month period, different board configurations
were processed and the steps required to attain ideal profiles were logged.
It was found that only highly unusual board configurations required additional
interfacing to provide ideal profile parameters. For example, to stretch
software capabilities, a particularly challenging board of approximately
16 x 14" with large BGA components was tested. Owing to its thickness
and potential for warping, the PCB was held in a fixture during reflow.
In response, the software defined a belt speed 27 ipm (which complied
with the paste manufacturer's specifications), but because of the unusual
configuration, this proved to be too fast. Mainly because of its size,
the speed was reduced manually to 19 ipm to give the board a longer soak
time. The software permitted this adjustment and created a new set of
product profile data.
For the basic array of high-density surface mount devices and mixed-technology boards tested, the software provided a workable profile at the first run without serious problems or errors. This was validated by a second test run performed for this purpose. For unique board configurations, the software accommodated single (manual) adjustments followed by automatic changes of all other parameters to create a new robust profile.
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Summary and Conclusion
Traditional Product Profiling: This method sets the first profiling
run using a thermally similar board and profile. Depending on board complexity,
oven parameter adjustments are made after the data from the first pass
are downloaded. Then the product is passed through the oven again.
This procedure is repeated until all minor adjustments are made and a
satisfactory profile is achieved, a process that easily can run to 20
to 30 minutes for each pass, plus data download time and oven-parameter
adjustments. Prime examples include an assembly with both ceramic BGAs
and highly sensitive components that cannot withstand high temperatures
(over 217°C), or when running lead-free solder with an extremely tight
process window. The self-profiling oven took much of the time, risk and
speculation out of process setup in those situations.
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Tested Software: After the initial "target profile"
is established (based on oven characterization) and the paste menu and
information concerning the assembly is entered, software-generated setpoints
are sent to the oven as the operating recipe. After the oven stabilizes,
an initial profiler pass is made and the data are downloaded, prompting
the software to auto-predict which heating zones must be adjusted to match
the target. The new setpoints then are sent automatically to the oven
as the verified operating recipe.
With these results, it can be stated that new software will reduce costs
associated with reflow processing. These costs are comprised of time and
labor. For example, if 20 new profiles are required within a work period,
even using a low average of three passes per standard profiling process
and two passes with the new software, this would calculate to a saving
of more than an hour per oven setup or almost two 10-hour working shifts
saved over 20 oven setups. Labor also is reduced because of the software's
ease of use. Accordingly, daily profiling tasks have been turned over
to technicians, freeing up process engineering resources at several CMs
who have conducted the test and adopted the software.
- M.O.L.E. Gold (ECD
Milwaukie, OR).
** XPM 820N (Vitronics-Soltec, Stratham, NH).
*** AutoM.O.L.E. Xpert (ECD, Milwaukie, OR).
M.O.L.E. is a registered trademark of Electronic Controls Design, Inc (ECD)
CURTIS KEE may be contacted at Benchmark Electronics Inc., 3725 SW Hocken Ave., Beaverton, OR 97005; (503) 626-2279; E-mail: curtis.kee@bench.com
Surface Mount Technology October, 2001