No Waste: Beyond PCBs in Reflow Profiling

Article is from SMT Magazine

In many situations, EMS providers cannot waste a PCB for thermal profiling. Some ovens are equipped with profiling tools to generate an accurate reflow recipe without thermal profiling. This saves time, labor, money, and materials, but there are limitations.

By Brian O’Leary, KIC

There is a right way and a wrong way to set up a reflow oven to manufacture a new PCB assembly. This article suggests using the wrong method, but for the right reason. If an electronics manufacturer is prevented from following the correct method for setting up the reflow oven for a new production run, does a fallback position exist where they can still expect good results? For example, contract manufacturers find themselves in a not-so-uncommon situation where the manufacturer receives 100 boards and is expected to give a 100 assembled boards back. Sacrificing a single PCB to the profiling process is not an option. In another example, a manufacturer has PCBs that run in the several thousands of dollars. A suitable scrap board is not available for profiling, due to the cost incurred for the lost PCB.

Advantages and Disadvantages of Traditional Reflow Oven Set-up

The traditional method for setting up a reflow oven to manufacture a new PCB assembly is to attach thermocouples (TCs) to the PCB and run a series of profiles. Multiple profiles are usually required for the technician to adjust the oven recipe until an in-spec or deep in-spec profile is found. The introduction of lead-free assemblies has made this task more difficult and time-consuming. However, automatic prediction software and process optimization software have significantly cut down on the number of profile iterations required to determine the oven recipe that provides an in-spec process.

The benefit of this conventional reflow profiling method is clear: It achieves a deep in-spec and therefore stable process that is fundamental to good end-product quality. It also provides documentation to the client that proper process development work was performed.

These procedures, however, tend to sacrifice one or more PCBs. One reason for this concerns the TC attachment method. There are several TC attachment solutions, some more destructive to the PCB assembly than others. The use of high-temperature solder wire is a reliable method, but tends to damage the PCB assembly. Aluminum tape is also a reliable and repeatable method with the added benefit that the tape can be removed after the profile without damaging components.

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A second cause of PCB damage is the fact that each subsequent thermal cycle through the reflow oven raises the risk of latent or real defects as solder joints are re-reflowed, components are exposed to multiple reflow cycles, and the properties of the substrate changing. The PCB gets lighter, discolored, and more brittle with multiple profiles. Therefore, even with non-destructive TC attachment methods, such as aluminum tape, the PCB may need to be discarded when several profiles are run.

A final risk is that the technician selects, often guessing, a wrong initial oven recipe prior to the first profile. The initial recipe could damage the PCB. This could happen when the peak temperature is too high, the slope too steep, the soak prematurely dries out the volatiles in the paste, etc.

Profiling the Reflow Oven, Not the PCB

Modern reflow ovens are a far cry from their legacy siblings. Each oven model produced in volume tends to have very tight and similar thermal characteristics to each other. Equally important, these properties do not change over time as rapidly as in the past due to better flux management, improved oven control systems, more precise mechanics, etc. This enables new thermal process tools that “learn” the behavior of each oven model. To capture the thermal properties of a specific oven model, numerous profiles are run on a variety of PCB assemblies under differing process windows. This database will cover all but the most unusual applications encountered in SMT production. Once this work has been done, it is a simple matter of copying the information onto all the similar oven models. At that point, the operator could simply enter the basic information of the application, such as the length, width, and weight of the PCB assembly as well as the appropriate process window, and the oven will find its suitable recipe (zone temperatures and conveyor speed). This recipe will yield an in-spec profile in the vast majority of the cases without the need to run a profile or attach TCs. Experience with such technologies also suggests that when the recipe generated by the new thermal process tools does not yield an in-spec profile, it is usually very close.

Some U.S. oven manufacturers have completed this work. These reflow oven makers ship ovens with a fully functional database that essentially allows their customers to set up for new production runs without the need for profiling and sacrificing PCBs.

These systems do have limitations. The first was alluded to above, namely that there will be a small percentage of the applications that will not be processed in-spec. The fail-safe method is to wait for the oven to stabilize on the suggested recipe and then run an old-fashioned profile to verify whether it is in-spec. If out of spec, it should, in the vast majority of the cases, be close enough to achieve an in-spec profile on the second try. One profiling pass through the reflow oven, with aluminum tape used for TC attach, should not damage the PCB assembly.

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Another limitation is that an oven will, given enough time, eventually change its thermal properties. Wear and tear, changes in exhaust conditions, preventive maintenance, and a host of other factors will have an accumulative effect on the behavior of the reflow oven. Therefore, the initial database will need to be updated. This can be achieved by running some real-life profiles from time to time, and feeding this fresh information back into the database.

The final limitation is the fact that a system that eliminates profiling, by definition, does not have a profile recorded for the specific assembly. This means that there is no documentation or evidence that the PCB was indeed processed in-spec. Some customers will accept this, while others will not and require reflow profile documentation.

Conclusion

The correct method for reflow oven set up with a new application is to profile the PCB and dial the processed deep in-spec using prediction software. If the electronics manufacturer either cannot or will not perform this task, there are now thermal process tools available that achieve a more than 80% effective solution. Oven-inherent programming produces an in-spec recipe in the vast majority of the situations with no need to profile or sacrifice a circuit board. This technology also saves set up time and associated labor.

Using a profiling technology without an actual PCB profile run is also far better than doing nothing. Many manufacturers in our industry currently do not profile at all, or they limit their profiling to a single application a few times a year. If you do not want to do traditional profiling at all, oven-generated recipes can be an intermediary, rather than blindly reflowing.

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BGA Profiling Webinar Recording

The following presentation was first held as a 30 min BGA Profiling webinar in July 2009, with over 120 participants.  Due to its popularity an abridged 8 min version was created.

To subscribe to my Podcast for iTunes (click here).

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Energy Consumption Reduction for Reflow | Better Thermal Management

Is your reflow oven throwing money away with a poor thermal management?

To subscribe to my Podcast for iTunes (click here).

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How to Maximize Reflow Ovens Throughput

The following video shows you how to in the fewest steps possible in the least amount of time increase your reflow ovens throughput while maintaining an in-spec process.   In this real life example, throughput was increased by 20% in 20 minutes time!

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Profiling BGA Webinar

Profiling BGA Webinar Supplemental (July 1, 2009):

Component Specific Specs

We discussed the need to define BGA specs separate from other components that have different reflow requirements.   BGAs typically require more heat to reflow properly but typically there are many other “smaller” components that also populate a PCB that will overheat if you develop your process solely around the BGA.   The following 2:40 min video reviews how you can bring both your BGAs and other temperature sensitive components into spec, striking a thermal balance that results in quality products.

Thermocouple Attachment

The following 1 min video shows one of the most reliable direct methods of TC attachment for BGA profiling.

…..but, who can always sacrifice a PCB in the process?   We talked about some indirect/non-destructive methods for profiling BGAs that are suggestive, but inconclusive.   In the fall I hope to have some results of a study that will help our industry come up with solutions that one can reasonably predict the temperature/profile of a BGA without destroying the PCB in the process or worse the BGA!

BGA Inspection

First there was SPI (solder paste inspection), then there was AOI, now there is RPI (Reflow Process Inspection)

rpi-smt-linerev-11

You can see a prior blog posting discussing RPI at:   http://profilingguru.com/reflow/what-is-reflow-process-inspection/

RPI works in the world of continuous reflow monitoring, where a profile is created for each and every production board.

In order to automate reflow profiling, a baseline/virtual profile is first established, where one runs a traditional profile with PCB, TC attachment and profiler while the on-board system of 30 thermocouples gathers the same profiling data and reconstructs and converts the traditional profile to a virtual representation. Once a virtual profile has been established, profiles can be collected for all production boards.  SPC charting, cPk, traceability and process control are all possible.

So rather then the reflow process being a black box, where anything and everthing can go wrong…..

illustration_5….alternatively, do you not only know what is going on continuously, but your BGAs using the techniques above are being monitored on a continuous basis.

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Your Questions:

Q: Doesn’t the thermocouples utilized by the oven itself (assuming that they are calibrated and verified) provide the same basic information as the secondary set of TCs you are referring to?

ANSWER:  No, the oven thermocouples and the secondary KIC  TCs have completely different and separate functions.  The oven TCs are typically located close to the heaters since their job is to turn the heaters on and off as the temperature drifts from the set points.  The KIC 24/7 (or KIC Vision) TCs, located along the conveyor, help to automatically measure the profile that each PCB experiences as it is processed through the reflow oven or wave solder machine.  This function is called Virtual Profiling.

Virtual Profiling (VP) provides process traceability as it logs the profile for each PCB, along with information on how this profile fits the established process window.  The VP works in real time and offers instant alarm when the process (profile) drifts out of spec.   Because it provides basic SPC charting, it acts as an early warning system for trouble ahead.  Think of the KIC24/7 or KIC Vision as an automatic profiling system in real time.

Q:  I encountered wetting issue with CSP and BGA, how do I solve them?   /   Q: How about wetting issue?

Answer; In some cases, but of course not all cases, wetting issues are a result of incomplete flux activation in the solder paste and an overall low temp soak, where the components did not reach sufficient energy levels before entering the reflow, TAL stage of the process. Many of these issues are related to Pb – free solder pastes, mixed RoHS components or a number of other variables.

I suggest that the best answer is to research the publications available on the Web for the most relevant solution. The following is a link that closely resembles the issue, but again, you will need to research the most relevant to your situation.

http://www.emsnow.com/cnt/files/White%20Papers/Henkel_Leadfree_Designing_Reliability.pdf

Q:  How do you take measurements on each board without TCs?

Answer: KIC software algorithms compare what was observed at the time of the Baseline Profile to what is present within the oven during production. Using the 30 thermocouples in the oven, this data is communicated to the eTPU and the output is the PWI based on the specific process and the specification of that process.

Q:  How well does the DPMO relate to the actual defect where there could be placement defects interacting with reflow?

Answer: DPMO is a parameter of only the thermal reflow process. If issues exist in placement or screen printing, it will not be reflected in the DPMO, since KIC is only monitoring the thermal process. Given that all other aspects of the SMT line is functioning properly, DPMO will give an assessment of the thermal defects assuming that the proper solder paste and placement is present at the time the product enters the oven.

Q:  What about paste formulations?

Answer: KIC works with any solder paste manufactures to build the solder paste library that is present in the KIC software. This library is updated periodically and verified by the solder paste manufactures in most instances. The library however does not at any one time contain all information about all possible solder pastes. We try our best to be certain the information is present, but changes in formulation and engineering at the solder pate manufactures sometimes causes gaps that are beyond our control.

Q: How important is it to drill into the BGA ball and put the TC in it, vs. putting on the package, slip under the package, and on the bottom side of the board?

Answer: There are many variables in PCB design and component placement that directly and indirectly affect other components, in this case BGA. The best possible answer to this question is in the amount of data that is collected, how it is collected and how this information is applied to the specific PCB and BGA directly. Gathering as much information as possible, charting this info and drawing data driven values is the best possible formula for successful BGA reflow. Using all available data collection methods and positions aids in successfully reflowing this package.

As indicated during the webinar, we are currently commissioning a study to see if non destructive methods can be used in place of drill a hole.

Q: Does your software always choose an extended peak recipe?

Answer: No. Based on the type of recipe and profiles that are part of your normal production determines what path the KIC Navigator (auto-prediction) directs the profile. If your profiles are mainly RTP, the software looks at the values of the library data and suggests set points that will lead to a RTS profile. If your profiles are largely RSS, then the suggested set points will tend towards a RSS profile.

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How to add a Component Specific spec to a Profile

What do you do when you have temperature tolerant components on your PCB that require a different reflow profile?

To subscribe to my Podcast for iTunes (click here).

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What is Reflow Process Inspection?

Reflow Process Inspection is catching on as the next advancement in-line inspection systems for SMT Reflow.   I have pulled together the various aspects of RPI to better explain how it works and what are its benefits.

How does RPI fit into the inspection processes in the SMT factory?

Unlike SPI and AOI that are defect inspection systems specifically designed for viewing solder deposition and component assembly respectively, RPI complements these systems by inspecting the performance of the thermal process IN-LINE.   RPI inspects the thermal process for any joint, including those that are not visible to the AOI system such as BGA components.

What is being measured?

RPI charts the thermal Process Yield and DPMO

What are the RPI benefits?

RPI provides information on the “health” of the thermal process over time.   The Yield and DPMO charts provide instant understanding of detrimental changes in the process.  The following format is easy to read and understand and often used by management as well as engineers.

dpmoyield1

For an Overview:

mbrpi

Awards for RPI:

2009 Innovation Award

KIC’s RPI Wins a 2009 NPI Award

Innovative Technology Center Award at Apex 2009

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Is there a standard calibration tool for the reflow process?

A question was posted on Circuitnet (May 18, 2009) asking if there is a standard test board that can be used for profiling/calibration of a reflow oven.   Answers were provided by profiling companies, oven and rework station manufacturers.

The consensus from all groups was:

  • There is no standard test board.
  • There is no substitute to creating an actual profile of your product.

Here is a summary of panelist replies, including from yours truly (for a full transcript go to http://www.circuitnet.com/articles/article_59131.shtml).

Brian O’Leary – KIC  (full version)

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 Reflow 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 sellable product nor does it mean that you need to waste the next few hours profiling.

Both oven manufacturers and profiling companies have developed onboard databases that allow you to develop in-spec profiles before you even profile (see this link) 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.

Fred Dimock – BTU

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….

….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….

Richard Burke – Datapaq

First of all, nothing can take the place of running profiles of your actual PCB’s…..

…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.

Al Cabral – VJ Technologies

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.

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Minimize Energy Consumption through profiling software

The days of cheap energy are gone and in this economic environment, we are all looking to consume less and cut costs!   You can start looking for energy savings in your solder paste specification and the component layout of your PCBs.  Using simpler designed products with fewer components is an excellent way to begin. This, of course, is not always an option, especially for contract manufacturers.

What if your profiling software can reduce your oven’s energy consumption, while maintaining a process that’s in spec? I can do this by defining the minimum allowable conveyor speed for the reflow oven.  Many engineers will set the conveyor to be a little faster than the bottleneck on their SMT line. Also, the engineer will define how much of their process window can be used or how close to the limits of their specification are they willing to run their process.

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In a few seconds, your profiling software searches billions of combinations.  It will settle on a profile that uses the lowest oven set points.

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Minimize Product Changeover through Profiling Software

Since it is not always possible to run all products under a single profile, most factories require product change over time in order to reset the oven to a new set of temperatures, per product class or board. Depending on the variations of PCBs, changeover time can add up to hours, days or weeks of lost production time.

So, what can you do to minimize this dead period? Determine, in advance, the best grouping of products that have shared profiles and then schedule product runs over available equipment that minimizes changeover. It is worth every minute of company effort and time to review daily production in order to run products that require small set point changes.

It is entirely possible, in an organized methodical way to consolidate groups of PCBs under the umbrella of one or more Thermal profile, I like to think of putting them into small, medium and large buckets.  Testing can be done to prove to yourself and/or your customers that the profile is both producing a product within specification and, quantifiably, how deep within spec.  Yes, it is possible to have your cake and eat it too!

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Profiling software can do some of the heavy lifting for you.  You can reload past profiles to see whether or not they can be produced at given oven set points.  Every time you ask the profiling software to run a prediction analysis, it searches billions of possible combinations.  If you want the deepest in spec process, it will pick the one closest to the center of all variable specs.  If you want to maximize conveyor speed, it will search for that one profile that gives you the highest throughput without violating any of your parameters.  As it is crunching through these possible combinations, there are millions of other profiles in spec that you don’t see because they are not the optimal profile for a given request. This is not to say that they are any worse.  When you reload a recorded profile to determined fixed oven set points, there may very well be profiles that are still deep within spec that can be run under your new Reflow oven configuration. Perhaps, instead of changing your oven over 4-5 times a day, it now only needs to be changed 1-2 times.

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