|Ask the Experts|
November 2, 2017
Reflow oven testing and calibrating concern
We currently use a test board supplied by our oven manufacturer to run samples. Is there an industry standard test board we should use? Does this industry standard board have specific pad and test point sizes with specific distances from one pad or test point to another, and to the circuit board edge?
|Expert Panel Responses|
Although there does not appear to be an official "industry" standard, there is a standard tool used by several Reflow Oven OEMs, in-plant, for "fingerprinting" and "verification" of their ovens, prior to shipment to their customers. This same tool is used by Reflow Process Engineers in the field as a "standard" against which they measure oven uniformity, brand-vs.-brand in the oven selection process, and verification that their ovencontinues to produce the same thermal performance. Many engineers have told us that their final selection of a reflow oven was a result of use of this tool. Some might call that a 'standard' against which multiple alternatives may be measured. The tool is the ECD OvenRIDER, powered by the SuperM.O.L.E. Gold thermal profiler. The tool can be used to measure ambient temperature profiles, belt speed, and the effects of convection, and is available in various widths from 6.6" to 18". The pad/test points you mention are consistent in mass, independent of the board width, however spacing necessarily varies as the width of the chosen tool varies. You are wise to ponder the location and spacing of parts tobe measured thermally through the oven. On a real PCB, location of a component-of-interest relative to other, larger components, or the circuit board edge you mention, has significance. A small component, in open territory, would experience a different thermal profile than the same component, in the "shadow" of a much larger component. This behavior is part of what we refer to as the "characterization" orthermal behavior of a specific board. The board geometry and relative loading density of components are necessary information to the process engineer, but certainly not sufficient to ensure that components are properly soldered, and NOT over heated. In pursuit of "good looking" solder joints, it is entirely possible to over-temp critically sensitive components such that they pass a functional test in production, but leave that plant with significantly reduced life expectancy. These are additional discussion topics, addressed through a recent study of Thermocouple Attachment techniques, and in the recently published J-STD-075, which relates to "Sensitive Components". Both topics are available at www.ECD.com/blog, or by contacting email@example.com. Short-answer: ECD OvenRIDER can tell you things about the thermal environment that neither the oven diagnostics, nor a stand-alone thermal profiler can even detect. It is the common tool for "oven testing, verification, and feedback" post-maintenance, pre-production or pre-purchase.
Vice President; Marketing & Sales
Numerous people have attempted to develop a standard test board for reflow ovens without success. Real world surface mount boards vary widely and there are many different thermal requirements. Much of the variation is due to board and component weight, component density, and solder paste profile requirements. Oven manufacturers normally use custom designed test fixtures to simulate a board but their real purpose is to measure uniformity across the oven and confirm that the oven is working correctly. The test board might match a small percentage of boards actually being produced but is not close to many more and is not intended for calibration. I have personally seen companies place unrealistic performance specifications on reflow oven testing with boards that have little to do with actual production needs. For example, we once were required to show that an oven could reproduce an inspect ramp soak spike profile on two 12 X 18 inch aluminum sheets that were 0.040 and 0.080 inches thick without changing any recipe parameters. Needless to say the thin board was at the top of the specification and the thick board at the bottom. But the trial had little to do with the 8X8 inch product they were manufacturing. On the other hand another company asked us to help them develop a recipe for their package that would reflow lead free solder on exposed pads while not exceeding a specified lower temperature on the inside of a component. The only way to develop the recipe was to run the actual product with TCs placed at the critical locations. From a surface mount manufacturing point of view - single board oven performance testing has little benefit. The real answers are to use actual boards with TCs placed on the critical components. I would also add that the board needs to be run numerous times to obtain process stability and repeatability data (CpK etc.), but that is a separate discussion.
Manager, Process Technology
I am not aware of any industry standard test board or recommended pad location test points. There is a JEDEC publication, JEP140, a guideline which provides a procedure to accurately and consistently measure the temperature of semiconductor packages during exposure to thermal excursions. The specific test board supplied by your oven manufacturer would be used to ensure proper functioning and measurement of the various controlled devices within your reflow oven. Depending on the design or layout of this specific board, does it verify and or calibrate your TC's, heater's, conveyor speed, etc. As far as testing or running samples to show the process capabilities of your oven within a given set of parameters, profile. What better way to validate... run actual product and measure (profiler). Your end result must be consistent thermal repeatability. Different boards, board size, components, location of components, thermal mass or a high thermal mass differential, board loading or space between boards when conveyed are all considered oven and or product related variables that could affect non-uniform heating. Again the manufactures test board should be used to confirm correct operation of the oven and profiling your product should be conducted to validate your ovens process capability.
VP of Engineering
APS Novastar LLC
Test boards can be created to illustrate specific characteristics of a reflow system, be it heating / cooling capacity, thermal repeatability, thermal uniformity across a conveyor system or designed to emulate a particular type of product. It's very difficult for one test vehicle to do it all well. A test board supplied by an oven manufacturer or independent supplier will likely address one or two of the aforementioned. For example, a test vehicle designed to compare several ovens across multiple lines can be vastly different from a test vehicle designed to measure cross belt uniformity. Similarly, a test vehicle designed to gauge percent infrared, may not be well suited for CpK measurement. In addition to consulting your reflow oven manufacturer, consider contacting some of the profiling companies like ECD, KIC and Datapaq to learn their unique perspectives and approaches. Depending on the scope of your project, a few of our industry consultants have significant thermal expertise to create custom solutions. The first step and typically the most difficult is to determine what you are really trying to measure and why.
Regional Sales Manager
First of all, nothing can take the place of running profiles of your actual PCB's. It's very important to know that the assemblies you are passing through your oven are meeting the specifications of the solder paste and/or components. As far as a test board is concerned, there are various tools on the market that will monitor the performance of your reflow ovens over time. These fixtures will ensure that your oven is stable and performing the same today as it was yesterday. There is really no industry standard test board available. To suggest otherwise would be dangerous whereas this would assume that all assemblies are identical and this is not the case. If you set the oven up to the test board, it would invariably be different than your own assemblies. This is not a risk worth taking. If you would like to discuss test boards or any other profiling situations off line, feel free to contact me at firstname.lastname@example.org.
National Sales Manager
IPC has several test board configurations for reflow furnace studies. If you on the IPC website, you can review the board configurations they offer under the "Technical Resources" section of their website.
Global Segment Manager
Air Products & Chemicals, Inc.
There are various industry accepted test panels and measurement systems that provide a "baseline" for comparative analysis purposes. As an industry we build PCBs that range from the sublime to the ridiculous in terms of mass, materials, complexity and volumes produced. No one standard would work and consequentially it is a case of finding the best tools for your application but then using those tools consistently.
The short answer to your question is no. There is no industry standard test board. Test boards, also sometimes called "golden boards," are an imperfect measure. Often, they are used for calibration purposes, but keep in mind every time you run the same PCB through an oven, some mass of the board is lost. For this reason, a true GOLD standard that is identical to your production board is difficult to achieve, unless you can somehow recreate the exact same conditions each and every time you profile your standard test board. Since PCBs lose mass, some manufacturers will create calibration tools out of plates of stainless steal and use metal slugs to simulate components. Of course, a hunk of metal is no closer to a production board than a golden board, but at least it gives you a relative measure that is repeatable. So what is the best answer if there is no perfect tool? There is no better representation of what is going on with your process than running an actual profile of a production board. The good news is that there are tools available that do not necessarily mean running a profile equals destruction of a sell-able product nor does it mean that you need to waste the next few hours profiling. Both oven manufacturers and profiling companies have developed on-board databases that allow you to develop in-spec profiles before you even profile (see http://profilingguru.com/reflow/how-to-establish-the-%E2%80%9Cfirst%E2%80%9D-profile-for-a-new-product/ ) so when you run a verification profile you can at least do so knowing that the PCB being used can still be sold! Another method of ensuring your process is continuously in spec and can serve as an early warning if things are going astray is the use of systems designed to monitor your oven. For example, KIC's 24-7 and Vision will create virtual representations of your PCB all based off of a true "golden board," since the PCBs used to set up the system to create these virtual profiles are run through your process as actual profiles. As an added bonus, these same boards do not suffer from the repeated use problem described above with golden boards.
Global Account Manager
I agree with the responders that there is no real industry standard, and I also agree with the general sentiment that the "canned" products provided by manufacturers of profiling equipment can be useful. I do feel, however, that the canned products fall short of mimicing the thermal behavior of a PWB, and therefore are less useful than they can potentially be. They are also less than well-thought-out as far as layout of sensors, and as a result don't provide as much spatial (across and along-conveyor information) as they could. I initially designed a tool for reflow system benchmarking and monitoring in 1996, and have updated it and continue to use it today. The tool interfaces with a standard thermal profiler, and provides information on differences in profile due to:
From my perspective using 1 board for everything is a bad approach. From the oven performance (daily/weekly check) - temperature, speed, etc. a test board is acceptable. However, when running live product everything changes from one assembly to the next
Engineering and Operations Management
Some of the advice given in this column makes sense, somedoesn't. It is true that scrap production boards are good for establishing aprofile for just that specific board (but only one time for the loss-of-massreasons mentioned). A PROCESS CONTROL TOOL is required for the separate anddifferent task of daily or weekly oven validation. Therefor it is a good ideato use a scrap PWB to create the profile and try to match the suggested profileon the solder paste manufacturer's Technical Data Sheet for best results. Butuse a good process control fixture that utilizes thermocouples that are clampedin place (not taped, nor epoxied). That way when the TCs wear out or changeover time, you can just strip out the old TC and clamp in a new TC and it willnot change the characterization of the process control tool more than +- 1 deg.C (the standard tolerance of most thermocouples). If high-temp adhesives ortapes are used, the problem is that when you replace the TC the amount of tapecan insulate the new TC differently, and the TC weld MUST be making goodmechanical contact and that is difficult to do with tape that peels or fallsoff in the oven. Likewise, replacing any of the TCs bonded in place will alsochange the operating characteristics of the process control tool depending onhow well the location and amount of epoxy are matched to the original. So the best thingto do is to make your own process control fixtures using a flat piece of CAS116 Durostone that utilizes TC clamps across both the top and the bottom of thefixture. The process control fixture does not need to match any of theproduction pwbs you build, nor does the process control profile need to matchany of your production profiles. If the process control profile shows exactlythe same pattern each time it is run with the same process control program,then you can assume that all of the heaters, fans, and conveyors are operatingexactly the same, and therefore your production programs (by implication) arealso running exactly the same as the day you first created them using scrapPWBs. Also, the most overlooked factor in creating both theproduction profiles and using the process control fixture is the simple factthat neither takes into consideration the oven loading caused by preceeding orsuccessive CCAs being placed into the oven at the same time. A single CCA goingthrough the oven will show a completely different thermal profile if it is allby itself than if it goes through the reflow tunnel right behind or in front ofother CCAs. So to correctly profile a given production board, that board shouldbe run both by itself and also behind and in front of other boards, which donot necessarily need to be monitored because they only serve the purpose ofunderstanding their effect on the profile (thermocoupled) CCA. By doing thisyou can understand the minimum spacing required for given production CCAs. Forsome reflow ovens, their recovery capability is such that they cannot handlemore than one CCA all the way through the tunnel before they can recoversufficiently to reflow the next. But for other ovens, they can handle a muchlarger thermal load, and thus can almost run each board one right behind theother with little recovery time. The factors here are both the mass of theproduction CCAs being run, the spacing between, and the oven's recoverycapability. If you ignore these factors you will certainly run into problems nomatter how good your initial profile is, and no matter how repeatable yourprocess control check is. So good luck with that profiling!
After using your reflow oven for a certain time, when you get a bad profile do not always blame it on the recipe. Some times the reflow oven itself may be the cause and may need to be checked.Fred Baker, DENSO, USA
IPC-7801 Reflow Oven Process Control shows examples of substrates and thermocouple locations suitable for verifying the oven is stable and repeatable.Scott Homan, IEC Electronics
There is now a standard, IPC-7801 "Reflow Oven Process Control Standard" which could help address C.J's question.
Senior Project Engineer
Electronic Controls Design Inc
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