surface mount technology Archives

Across the Belt Uniformity and Reflow Profiling

October 13, 2009 by Profiling Guru Leave a Comment

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).

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.

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).

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/

Thermocouple Attachment Results are in!

October 12, 2009 by Profiling Guru Leave a Comment

The Rochester Institute of Technology under the guidance of Dr. S. Manian Ramkumar Ph.D. just conducted (October 2009) the most comprehensive study to date on thermocouple attachment methods. Part I of II was to determine the most accurate and reliable method of thermocouple attachment. Part II that has yet to be released is to determine the best attachment methods for BGAs, with the goal of seeing if there are reliable non-destructive methodologies, so stay tuned.

Results in a nutshell:

Aluminum Tape out performed all materials even Kapton! In an ideal word, the best attachment method of a thermocouple to a component is what I like to call a naked TC. Aluminum double sided conductive tape was the closest thing to having nothing at all to attach the thermocouple. Kapton tape is less responsive (deflecting and insulating heat), never mind if you have ever seen a saw-tooth TC plotted on a profile you know it has a very hard time staying in place on your PCB. Additionally, High Temperature Solder which I have always considered the gold standard, is the least accurate or responsive. When you get to the critical peak temperature of your profile, high temperature solder is sluggish to respond to the rapid change in temperatures, thus distorting your readings. As Phil Zarrow and Jim Hall discuss in Board Talk, “mass” on your thermocouple is not your friend. Phil Zarrow:

any measurement method, the key element is to get the thermocouple in good contact with what you are trying to measure and to do it in a way that does not modify the area with a lot of extra mass or material that is going to give you an inaccurate reading….

Bingo! This is actually what this study shows, now with the numbers to back it up.

Study Methodology:

The study looked at:

Aluminum Tape
Kapton Tape
Chemtronics – CircuitWorks CW2400- Two Part Epoxy
High Temperature Solder
Loctite – 382 Instant Adhesive

The study used a KIC Explorer with standard type K thermocouples. Multiple runs of a substrate coupon (62 mils thick plain copper coated with silver) was routed into 12 uniform 0.24″ isolated sections.

Three identical test coupons were used and run multiple times. KIC’s air-TC was utilized as the control to which each thermocouple was measured as the coupon traveled through all heated zones.

A total of three boards were used, running each board through twice, allowing the internal temperature of the KIC device to drop below 40 degrees C before rerunning the profile.

The tape attach methods were measured uniformly for each RTD connection, using a dial caliper, while the high temperature solder and epoxy quantities for attach were found to be visually uniform.

This graph indicates the mean temperature differential that was noticed within the oven for the various attach methods. The readings are based upon the complete profile starting at room temperature and ending at the peak temperature. The data from the cool down zone was eliminated from the analysis.

The graph shows the mean differential and the 95% confidence interval for each attach method. The Aluminum tape had the least differential (-0.48) followed by Kapton Tape, Loctite Adhesive, CW-2400 and then HT-Solder. The Confidence intervals among most of the attach methods do not overlap except Kapton and Loctite, indicating that the means of the attach methods are significant. Significant differences exist between the methods except between Kapton and Loctite as there is overlap. Clearly Aluminum tape outperforms all of the other methods.

The thermocouples seem to behave similarly within each of the zones of the oven. Zone 6, where the soldering takes place or the peak temperature is reached, the thermocouple attach methods show a much higher temperature than the air temperature, indicating that the PCBs have attained much higher temperatures than the air. A closer examination of ZONE 6 reinforces the selection of Loctite or Aluminum Tape for Phase III of this project.

Conclusion:

When considering accuracy, repeatability and responsiveness, Aluminum Tape is a winner. There are of course advantages and disadvantages to each material. For example one can argue you can re profile a PCB set up with high temperature solder, but considering that the mass of the solder distorts your readings, this study even brings into question this bedrock of thermocouple attachment. Never mind high temp solder destroys your PCB as well as there is little control over the size of the blog from TC to TC and board to board. Also don’t forget every time you profile the same board again it loses some mass, which will be the focus of more blogs to come.

Are you profiling bare boards or bricks?

October 11, 2009 by Profiling Guru Leave a Comment

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:

A large mass will behave differently than a production board.
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.
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.

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/

SMT related Links to know

September 30, 2009 by Profiling Guru Leave a Comment

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

Blogs:

Circuits Assembly

SMT Editorial Blogspot

Forums:

SMTA on LinkedIn

SMTnet

Twitter:

Circuit Assembly

Global SMT

SMT Magazine

Save 15% in 15 minutes on your reflow’s energy consumption

September 21, 2009 by Profiling Guru Leave a Comment

Topic: 15% electricity savings in 15 minutes

Date: Friday, September 25, 2009
Time: 1:00 pm, Pacific Daylight Time (GMT -07:00, San Francisco)
Meeting Number: 336 789 255
Meeting Password: (This meeting does not require a password.)

To join the online meeting
1. Go to https://kicthermal.webex.com/kicthermal/j.php?ED=107946437&UID=1013415122
2. Enter your name and email address.
3. Enter the meeting password: (This meeting does not require a password.)
4. Click “Join Now”.
5. Follow the instructions that appear on your screen.

Topic: 15% electricity savings in 15 minutes
Date: Friday, September 25, 2009
Time: 1:00 pm, Pacific Daylight Time (GMT -07:00, San Francisco)
Meeting Number: 336 789 255
Meeting Password: (This meeting does not require a password.)

——————————————————-
To join the online meeting

——————————————————-
1. Go to https://kicthermal.webex.com/kicthermal/j.php?ED=107946437&UID=1013415122
2. Enter your name and email address.
3. Enter the meeting password: (This meeting does not require a password.)
4. Click “Join Now”.
5. Follow the instructions that appear on your screen.

« Previous Page