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:  http://profilingguru.com/reflow/profiling-bga-webinar/

Share

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.  http://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.  http://profilingguru.com/category/reflow/automation/

Share

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!

Share

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.

Share

Characterize your Thermocouples

characterizetcsTCs are sensitive instruments of measurement. TCs are abused mainly because they are misunderstood. First, go out and purchase a set of TCs with the proper rating for your process with the least amount of variability.  Look at the thermocouples that you are using.  Organize the TCs according to observed temperature values by plugging them into your profiler. For now, we are not going to enter into a discussion about electromotive force, thermometric sensitivity, linear vs. non-linear, TC conductor length, weld type, or even conductor material. What we are looking for here is that we are dealing with Type “K” thermocouples.

To characterize the thermocouples, you will need a few items.  First, get a new set of thermocouples, where the welded bead has been pre-formed by the same procedures. The length of the conductors (the length of the TC) should be more or less the same for each TC, with the method of TC attachment remaining constant.  My preferred method is Aluminum Tape (see TC attachment in Chapter 7 – Thermocouple Attachment Methodology and Materials).

Here are some suggested materials to use for characterization:

1. Three (or more) sets of Thermocouples  (Type “K”, all from the same manufacturer).

2. One stainless steel plate (if the same plate is used for each run, it can be made of any material as long as it is capable of withstanding the designated highest temperature for extended periods of time).  I recommend 1 or 2 mm. stainless steel, 12 inches long and 8 inches wide.

3. Squares of Aluminum tape to attach the TCs to the plate. Squares should be of equal size, with method of attachment consistent from TC to TC.  Remember, we are looking for reproducibility.

Run the plate and profiler through the reflow oven. Be certain to document as much data as possible for the sake of the experiment and for reproducibility, especially with respect to oven set points, conveyor speed, TC attachment, distance of the profiler from the plate, length of the TCs, internal profiler temperature, and any other data point that varies, having an effect on the reflow process in the future.

Repeat the run several times. Like most data, the bigger the sample, the better the results. Take your time to set up this effort.  If you are not getting similar results each time, then there is something wrong with the set-up of your experiment.  Go back and check the repeatability of each step.

Here is an example of a run where I dumped my TC readings into a spreadsheet.

characterizationtcexcel

I had over 300 samples for seven TCs.  The average TC reading for each TC is shown in the last row.  The spread from the lowest value of 149.5°C to the highest value of 150.6 °C, shows a variation of 1.1°C, which is within the +/- 1.2°C  rating for type K thermocouples.  Since I went to all this effort, I decided to pick from my study: TC#2, #4 and #7 for profiling my next PCB, knowing that my variability is no longer 1.1°C, but now .3°C, which is the difference of 150°C and 149.7°C.

The point is this, it may seem like you are splitting hairs, but 1°C here and 2°C there has a cumulative impact.  If you had a process that only allowed for, perhaps, a 5°C process window, which is not unusual today and you could cut out half of your variability due to your facility, equipment and TCs, wouldn’t it be worth it?

Share

Characterize your Oven

reflowovenA well-defined calibration program according to ISO or NIST (National Institute of Standards and Technology) is recommended or, in most instances, required. Having the oven and the profiler calibrated ensures that the data collected by the profile, and in the interest of scientific methodology, is valid and repeatable.

In addition to the calibration program, a random check of the oven, in relationship to the controller and feedback loop, can be performed using a profiler or recorder. Simply recording the observed temperatures in a zone and comparing those to the zone set point will provide the desired data.

Share

Characterize your Facility

characterization

Processes are often set-up on one continent and expected to be a carbon copy set-up on another continent. What do you think is going on when the end result doesn’t turn out this way? The answer: environmental factors. Play the detective and characterize your facility!

What is Going On with your Exhaust?

At first glance, the exhaust stack on the reflow oven looks simple enough. Its purpose is to remove excess heat and the occasional vapor that escapes from the reflow chamber.  What happens when the CFM or CMM is set higher than that of the designed setting? How stable is the flow of exhaust? Does it increase or decrease when other equipment in the facility is added, started or stopped?

I bet you have no idea and how could you?  Most facilities do not install site gauges. Most facilities add equipment monthly. Changes occur to the barometric pressure outside the facility and fans accumulate all kinds of crud.  I have seen 6 inch exhaust ducts blocked more than 4 inches.

This is not to make you tear apart every duct, but be aware that many factors can change the setting of the exhaust. It is not likely that a single variable will crash your process.  You should, however, check the exhaust flow on occasion and certainly after changes are made to the system during maintenance.

Share