Across the Belt Uniformity and Reflow Profiling

I am often asked the question about how to handle components that are close to the outer edge of a PCB.   Today a question came in on Circuitnet to highlight this problem:

Title: Issues with BGA Components Near PCB Edges

What issues are we likely to see when we place BGA components very close to PCB edges?

What impact might it have on reliability?

Will equipment (screening, placement, reflow, etc.) require modification?

T. B.

I leave it to the screen printer, pick and place and reflow oven guys to answer the equipment part of the equation, but I can answer how one can determine with a profile if your BGA is getting what it needs as well as how other aspects of your PCB are impacted.

Across the Belt Uniformity:

There can be anywhere from a 2 – 5+C variation in temperature across the belt.  For example, BTU uses this homemade fixture to test for uniformity.  The idea is fairly simple.  With a set of type K calibrated thermocouples, you can easily monitor 6 TCs across the belt.  You want obviously to see the least amount of variation (if you were wondering the front TC is for measuring air temperature which is also used for automatic mapping of the profile to the oven zones with KIC2000 software).

BTU tool

Profiling for Reflow:

BGAs typically require more heat to reach their peak temperatures than smaller massed components like electrolytic capacitors.   For example, your BGA might have a peak temperature of 245C.

PCB2

While your electrolytic capacitors cannot tolerate as high as a peak temperature, therefore you want to set their maximum peak temperature lower, for example to 235C (this is just a relative example).

PCB3

With KIC2000 software, you can define each component in isolation.  If the BGA is off on the edge, I might need to bump up even further my peak temperature spec since in many reflow ovens, the outer edge near the rail is the coolest.  This is why you see some ovens with heat tape running along the rails!  Keep in mind of course as you crank up your oven to reach higher temps to reflow your outer edge BGAs, everything else on your board is also going to be impacted.   More the reason you better be hooking up thermocouples to temperature sensitive components to ensure they do not get fried while you are focusing your attention on your BGAs.  Profiling software that can “balance” the board is a must.  If there ever was a case where software can help solve complex problems in profiling, here you go!

I had a webinar back in July talking about BGA profiling.  There is also a video that illustrates what I explained above.  You can find this in an earlier posting:  https://profilingguru.com/reflow/profiling-bga-webinar/

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Are you profiling bare boards or bricks?

No one of course reflows bare boards, so why would you profile one?  For that same matter, no one sells bricks, so why do you put one through your reflow oven?

Profiling Bare Boards:

Today I came across CM doing exactly this.  They were processing networking boards.   They were  just too complex and expensive to profile, so the solution instead of finding a scrap board or some other reasonable substitute was to profile it as a bare board.  I guess the thinking was it is better than nothing, but can anyone honestly say that a bare board comes close to representing a true production board?  After all wouldn’t you agree profiling modern boards with mixed components, higher densities and micro-BGAs are already a challenge and to think profiling a bare board would yield any reliable results is a stretch?

Profiling Bricks:

So if this is such a terrible solution, what about putting a brick through your reflow oven?   A brick, come on Brian, who does this?  Well what do you think you are doing when you take one of the many fixtures available on the market that are used for characterizing an oven and using it to profile?    I bet if you melted them down (with profiler included) they aren’t far off in mass from a brick.   Consider the following attributes of a large mass:

  1. A large mass will behave differently than a production board.
  2. A large mass acts like a heat sink and will cause the oven to react differently compared to when a production board is run through the reflow oven.
  3. A large mass will result in a change to airflow due to its larger size as compared to the production board.

Now notice I included the profiler as part of the mass.  Many fixtures further add mass by adding a two pound weight to the fixture!   Now don’t get me wrong, these fixture do give you a relative measurement from week to week or month to month as to changes in the oven, but they do not tell you if your product is in spec nor provide a thermal profile.   Changes in the oven do not neatly correlate to changes in one’s profile.  After all how can they?   Chaos theory came out of the field of thermal dynamics, nothing neat about it.  Just like a bare board is no substitute to a populated PCB, a brick is also no substitute.

Don’t take my word for it, hear it from the oven manufacturers themselves.  https://profilingguru.com/reflow/standard-calibration-tool-for-reflow-process/

Here is a quote from Fred Dimock of 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.

….I have personally seen companies place unrealistic performance specifications on reflow oven testing with (fixtures) 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….

Both solutions profiling bare boards and bricks are inadequate.  Make matters worse if you do both such as I saw with this CM, the results are only compounded.  In other words, you are developing a profile based on an unpopulated board and afterwords taking measurements with a thermal mass that does not in anyway represent how your oven will in fact react to a production board.  This is classic garbage in, garbage out.autofocus

Now there are alternative solutions that don’t require the destruction of a production board in the process.   Many of the automated systems will create accurate virtual representations of production boards without the need to attach a single thermocouple.   There are also some brilliant software solutions that allow you to create accurate profiles without the need to run a profile.  https://profilingguru.com/category/reflow/automation/

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SMT related Links to know

RSS feeds, Tweets, blogs and newsletters, how do you keep up?   Well here is the latest on what’s available in the SMT industry.   I subscribe to all of these newsletters and regularly pick out areas of interest related to profiling for you.   I also comb the blogs though I only know of two, not including profilingguru, which is quite remarkable considering other industries have hundreds if not thousands.   The SMTA group forum on LinkedIn yields on occasion a nugget, but you need to build a profile to join.  SMTnet has always been a jewel.  Lastly, Twitter is a new phenomenon for many of us.   I am still trying to get the knack of it myself but it does have some value no doubt and will continue to grow.

On-line Newsletters:

Circuitnet

Electronics Production World

EMS Now

GlobalSMT

PCB Update

SMT Week

Blogs:

Circuits Assembly

Forums:

SMTA on LinkedIn

SMTnet

Twitter:

Circuit Assembly

Global SMT

SMT Magazine

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Running lead free and eutectic PCBs simultaneously on the same reflow oven

Surface Mount Technology ran a piece titled Parallel Processes: Simultaneous Lead and Lead-free Soldering with a Single Reflow System written by Hans Bell of Rehm Thermal Systems GmbH.  Hans details a study where by controlling conveyor speed of each lane of a dual-lane system, it is possible to run both a lead and lead free product simultaneously.

The devil of course is always in the details:

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 ability to develop process windows for each product leaving enough room for each to call upon the same oven zone set points is key and of course taking into account special temperature tolerant components on each board.  Hans’ idea is intriguing.  Based on my experience in a world were many PCBs manufacturers struggle to profile or perhaps do not profile at all,  this is certainly a tall order.  Nevertheless his idea is do’able for perhaps many processes, since changing just the conveyor speed to reduce product changeover on a single lane oven is being done today (click here for an excellent application note using KIC product’s to achieve this end).  Why this couldn’t be adopted to a dual lane system running both lead and lead free simultaneously has its merits.

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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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