Stop Destroying PCBs in profiling your reflow process
2009 Presentation at SMTA Long Island on how to use software tools that avoid destroying your PCBs during the profiling process.
To view the complete video series (click here).
To subscribe to my Podcast for iTunes (click here).
Podcast: Play in new window | Download (Duration: 4:00 — 26.9MB)
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).
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: http://profilingguru.com/reflow/profiling-bga-webinar/
Why are you replacing BGAs?
There is a great post today in Circuitnet titled “BGA Replacement Limits,” that can be found under Circuitmart. Panelists answer the following question:
How many times can a BGA component be replaced at the same location on the same PCB and retain reliability?
Mark McMeen of STI Electronics suggests that the answer may be as little as two times!
…most companies err on the cautious side and only replace twice at the same location after the initial build which is normally 2 thermal cycles for top and bottomside reflow thermal cycles.
I think a broader question needs to be asked, why are you replacing BGAs in the first place? In my experience, often the answer is due to poor reflow profiling. Often there is nothing wrong with the oven, PCB or BGA. Why is it so hard to properly profile a BGA? I believe the reason is most folks don’t have the option of placing a thermocouple underneath the BGA nor sacrificing a board in drilling a hole on the underside for TC placement. In the old days, you could get away with snaking a TCs under the BGA, but with micro BGAs this is just not an option. So what do people do? They stick a TC on top of the BGA or along side it. Many do nothing at all which is kind of scary and wind up asking question like how many times can I redo my board.
To go to show how hot of topic this is, I held a series of webinars a couple months ago with a turnout in the hundreds. I shared some ideas, here is an abridged 8 min version of the session for those of you that missed it. Part of the answer is proper TC attachment which by the way is currently under study at RIT to see the most reliable method as well as determine if there is a non destructive methods that is both valid and repeatable.
The other part of the equation is profiling your PCB not only for your BGAs but also those components that cannot tolerate as high of temperatures. I’ve seen plenty of manufacturers so focused on a $500 BGA, ignoring pretty much what else is going on with other components on their PCB. Certainly having the ability to define separate specifications, for example a peak temp for a DIP while addressing the special needs of your BGAs will lead to fewer BGAs having to be reworked in the first place.
After all, which is better, to treat the symthoms or the root cause?
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.
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Podcast: Play in new window | Download (Duration: 8:00 — 50.7MB)
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)
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…..
….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.
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.
Profiling guidelines for reflow of solder bump flip chip attach to organic and/or ceramic packages
This post is in response to a suggested topic on this blog. The following answer was provided by Brian J. Toleno, Ph.D., Director Technical Service at Henkel:
When profiling a flip-chip to an organic substrate you typically want the delta T across the component to be as low as possible in order to minimize stresses and warpage. Of course, like any good profile, making sure that the solder bumps and solder paste reach liquidus, stay above the liquidus temperature for the recommended time, and have a controlled cool down as possible are key. In addition, if the flip chip device is going to be underfilled, then it is important that when using a no-clean flux solder paste that the solder paste is fully activated in order to minimize any possible flux/underfill interactions. So making sure you measure the temperature at the flip-chip bump/solder paste/solder pad area is important. You also want to make sure you measure the temperature at the centre and the corners (making sure they track close together). While there typically is not as great of a chance of a large delta T, like there would be in a BGA or CSP device – when one does occur it can be more catastrophic.
Is Go-No-Go profiling good enough?
What is go-no-go profiling? Well basically you get a pass/fail, green light/red light indication at the end of your profile
letting you know whether you are in spec or not.
How often are most of us, when we get the “go” or green light at the end of the profile indicating that we finally got the profile in spec, ready to button it up and turn the thermal process over to production?
If I have and in-spec profile does this mean I’m ready for production?
Well, if you think about it, if you are just relying on a go-no-go indication for your profile results you are never sure if your process is at the very limits or somewhere deep within spec. In the case of the former, depending on the demands of your production on your thermal process you could be operating at an out of spec condition during production, even though your profile gave you a green light!
If you are like most production facilities being in spec during production not only matters but having an indication of how far within spec is your process is just as important since depth of being in spec is the only way to ensure continuous quality production. A common tool used for not only determining whether you are in or out of spec as well as how far in or how far out (depth) is using PWI (process window index).
See the blog entry PWI.
http://profilingguru.com/process-window-index-pwi/
Maximize Throughput | Profiling Software
Many of you will have an issue with “bottlenecks” in your process. This can happen at any point in the SMT Reflow process. Depending on the product that you are manufacturing, it is also likely that the “bottleneck” will jump from equipment set to equipment set. For our purposes, let’s look at the reflow process when it is identified as your bottleneck.
I have seen several methods that address a reflow bottleneck. The obvious solution is to increase the conveyor speed of the reflow oven. This is a task that requires a bit of skill. Your profiler becomes the single most effective tool to improve the Throughput Time (TPT) of the reflow process.
Let’s look at the fundamental changes of your profile with an increase in conveyor speed. First, the PCB will spend less time in each zone. Also, your process will move toward the shorter end of your spec as defined in seconds. For example, if soak time is defined as 30 to 90 seconds, your actual process will be perhaps in the 30-50 second range as opposed to a comfortable 50-70 second range that was established at slower conveyor speeds.
The longer the oven, the more wiggle room you have for increasing conveyor speed without having to make significant changes to your profile. Having a longer oven suggests that the PCB stays in the reflow process for a longer period of time, but keep in mind that it really comes down to how many products per minute exit the oven. Whether the oven is 10 feet or 30 feet in length, a higher conveyor speed will increase the number of products that exit the oven per minute.
Work In Process (WIP) is determined from the time the lot enters the SMT process to the time it is ready for shipment as a finished product. This duration is stated normally in hours and can add up to a few days to weeks, depending on the product. Having a longer oven does not mean increased TPT nor does it violate your WIP objectives.
It is tricky working with shorter ovens with fewer zones since they do require higher temperatures per zone to reach the desired specifications, as compared to longer ovens. I have found that using solder paste that uses a Ramp to Spike (RTS) profile works better in ovens with fewer zones. The RTS pays less attention to the soak and more to the overall length of the profile (the soak, of course, is often not listed in the solder spec for an RTS profile). Also, shorter ovens impact the slope in the RTS profile. For example, if you have a 5 zone oven, the first zones will need to be set at a fairly high set point in order to process the rest of the profile. At this point, the slope becomes very steep as the PCB moves from ambient to 160°C. 160°C could be the set point of the first zone! Also, in a distance of just a few feet, the product will need to rise in temperature to greater than 217°C in PB Free and above 183°C in eutectic solder. Many specifications will call for a peak of 200 to 240°C, which puts further demands on your shorter oven. In this instance, it is desirable to calculate the slope over the entire profile, setting it from 130°C to 180°C over a shorter period of time. Again, you need to look at how long (in seconds) the PCB stays in each area of the profile when designing your spec.
Now that we have looked at oven length when considering an increased throughput, how do we develop a profile that will remain within specification at higher conveyor speeds? Some profiling software will allow you to make a desired change to conveyor speed and then return a predicted profile. This typically can be completed in seconds, before even running your first profile.
A clever feature of some profiling software is that you can set a range of allowable conveyor speeds while maintaining acceptable limits to your process window. For example, when factoring the variability (drift) in your oven, you can comfortably run your process using only up to 70% of your available process window. In practice, anything over 70% is risky due to drift that can push your process out of spec. To eliminate this concern, run a “what if” scenario, where you define your minimum and maximum range for conveyor speed and maximum allowable PWI (in this case, set to 70%).
The profiling software will literally search billions of possible combinations, giving you the maximum possible conveyor speed without violating your 70% PWI threshold. Of course, you may find the conveyor speed to be too slow still. What can you do? Bump up your allowable process window or buy a new oven. In either case, you are in control of your predictive modeling (this sure beats hundreds of hours of trial and error and possible board destruction). This process is similar to the prior section on Getting your Product Deeper in Spec and it will take as long as it takes to heat up or cool down your oven and re-profile if verification of your new predicted profile is required. In practice, I find that it takes less than one hour.
Getting your Product Deeper in Spec | Profiling Software
Q: What happens when defects occur when the thermal load on the oven increases? Do you slow down production? Do you change the oven set points by cranking up the heat to compensate for the increased load?
A: The answer is to establish a NEW profile that is deeper in spec., a profile that is able to better stand up to the daily variations of a reflow process.
Today, profiling software allows you to establish these new deep-in-spec profiles with relative ease. You can precisely define your specifications and run various predictive scenarios. For example, you know that you can’t slow down your conveyor speed, but you can change your oven set points. The profiling software can give you a predictive result that puts you as deep in spec as possible before ever having to run a profile.
In practice, how much work needs to be done to take this out-of-spec process and bring it within spec depends on a lot of factors. How far out of spec are you to start with? What inputs can be changed? How tight are your specs?
If your process is already taking up most of your process window or not far out of spec, then only minor changes will most likely be needed to bring your profile much deeper into spec. In this case, only one additional profile is likely required to bring your profile very deep within spec. In my experience, this profiling process takes about 30 minutes, most of which is waiting for the oven to cool. If your profile is far out-of-spec., you may need up to an hour to bring it within spec.
Each time you re-profile, it is an opportunity to further improve your profile, bringing it further into spec with each effort. Profiling software will tell you a possible scenario for improvement each time, which takes your excellent deep-in-spec profile still deeper within spec. Each one of these changes, on average, takes about 30 minutes.
A word of caution: having a profile in the center of the spec or at 0% PWI, is not always the optimal improvement. While “0%” PWI is statistically desirable, there are other factors to consider. For example, though 30% PWI indicates that you are only utilizing 30% of the allowable process window of your solder paste, in practice, when you find that a PWI of 65% produces a physically better connection, which is better? Specs are just that: specs. They have a range for a reason. In this case, at the upper end of the spec (opposed to the center of the range), a joint may solder better. Advantageous about most profiling software is that you can go back and re-define your specs to see what your new profile will look like without having to rerun the profile. The allowed range can be further narrowed to a particular spec, which results in a better joint. In essence, you are now re-defining your spec, and all future profiles will only consider this new range.
Reading your Profile via Profile software
Let’s take a close look at the profiling graph and the specifications used to create the graph.
The Solder Paste Library of your profiling software provides several choices:
Maximum Slope Between Temperatures
Maximum Rising Slope (Ramp Rate)
Maximum Falling Slope (Cooling Rate)
Preheat
Soak
Time Above Liquidus (TAL)
Reflow
Peak
Maximum Exit Temperature
For some inputs, such as slope, preheat, soak and TAL you can define multiples of the same input. For example, you might want to define more than one slope for your process.
Inputs and Segments of the Profile
These terms: specs, variables, segments, zones and inputs are thrown around and often used interchangeably. Unfortunately, some of these terms are used to describe completely different aspects of the reflow process, which leads to lots of confusion. For this guide, we use two terms to describe slightly different meanings. When I discuss setting up your profile, I use the term “input” to describe ramp, soak, slope, when I etc. I use the term “segments” to describe these same terms in relation to the profile graph.
Inputs each have their own specifications. This section explains how they are measured and the defects associated with each of these segments of the profile. Interchangeable terms are also used across the industry to describe these inputs/segments. Here, I list them all. For instance, maximum rising slope and ramp are used interchangeably with the same meaning.
Maximum Slope Between Temperatures
Three important parameters make up this specification:
1. First, at what point in the profile do you want to know the slope? Rather than looking at the whole profile, this specification will look at a specific temperature range, for example, what is the slope between 150°C and 200°C?
2. Next what is the ideal slope range? From 0 to 4°C/sec?
3. And third, how many seconds would you like to calculate the slope over? A default value is typically set to 20 seconds, more on this in a moment.
Slope is important for both component and solder paste tolerances.
Maximum Rising Slope (Ramp Rate)
The Maximum Rising Slope, or Ramp Rate, looks at the whole profile, specifically the steepest slope over the entire profile and does not just look at a specific region. At this stage of the reflow process, the temperature rise from ambient to the first heating zone is of most interest since the greatest potential for component damage and solder ball spatter from a high ramp rate exists. The parameters are measured in degrees per second as temperatures increase.
To calculate slope, you will need to input a specified “duration” of time. The typical default value is 20 seconds. The more data points you have, the more accurate the calculation since this increases your sample set, and in the end, the validity of your data. However, not all processes will have a default value of 20 seconds. If the area in which you intend to calculate the slope over is small, your sample size will have to be measured in fewer seconds, perhaps adjusting the default value down to 10 seconds.
Maximum Falling Slope (Cooling Rate)
Properly cooling your product may be necessary for your process. Some specs call for rapid cooling. Depending on your profiling software, the maximum falling slope or cooling rate can be used to define the limit of the cooling rate or specify a certain decrease in degrees per second over a given time.
Soak
Preheat and Soak are typically listed as two separate inputs in most profiling software even though they call for, more or less, the same parameters. For some engineers the terms are distinguished by process type. Preheat being used for wave soldering and soak being used for reflow. More commonly the initial ramp from ambient is called preheat and the relatively flat section from that initial ramp to the reflow spike is called soak.
Some solder paste manufacturers will request that the profile use preheat and some will call for a soak period. These are similar inputs, if not one and the same. In some profiling software both terms are listed separately. Based on a review of many solder paste specifications, the soak specification is normally for a longer duration and the preheat is a shorter duration with a higher ramp rate. Again, this is defined by the solder paste manufacturer, who determines the desired specification for the intended performance of their solder paste. Component manufacturers can also call for specifications of preheat or soak with respect to their components.
Time Above Liquidus (TAL) (Reflow)
TAL and Reflow Process are both defined in terms of temperature over a period of time in seconds. Generally, the temperatures are ~183°C for eutectic solder and ~ 217°C for lead-free.
Of all the inputs, this is perhaps the most important since it can be the most troublesome, especially in the world of lead-free. Look very closely at the different package types and density of a given area of the PCB since these factor into the required time to bring a given bond pad to the desired temperature specification. Of course, we are talking about overall density, but not everything on your PCB is going to react the same to higher temperatures. While an exposure to the higher temperatures of TAL can be destructive over time, the duration necessary to achieve effective phase changes of the solder paste is, generally, not destructive. The key is to get in and out as quickly as possible to get the job done, while limiting the exposure to these higher temperatures. However, if the process is repeated several times, changes do occur in the PCB and destruction will begin. Many PCB’s will undergo both top-side and bottom-side reflow. Occasionally, a third reflow will be required to attach specific components and, of course, selective, wave soldering and rework may factor into the equation for the same PCB. This repeated combined exposure during the TAL segment can be destructive.
Peak
Why do we want to exceed the melting point of an alloy by a range of temperatures and duration of time? Ask your QA department since cold solder joints are one of the most common defects associated with inadequate peak temperature. The additional increase in temperature over liquidus guarantees that high density areas will have the opportunity to flow properly, ensuring a complete process. The solder paste manufacturer lists a peak spec but the component manufacturer’s specifications can be more important. The component manufacturer’s peak spec will be a “Do Not Exceed” value, in contrast to the solder paste manufacturer’s spec that calls for a peak range. Your job in developing the spec is to find a peak value that does not violate your component manufacturer’s tolerance still completing reflow to the satisfaction of your QA department.
Maximum Exit Temperature
This parameter has little to do with the solder paste manufacturer’s specification and more to do with a requirement of your specific process.
Two values are listed: temperature and distance. Temperature is the desired exit temperature and distance is determined by the location of the product in the oven or at the exit. The product board sensor will aid in determining how this value is calculated.
