Energy Consumption Reduction for Reflow | Better Thermal Management

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

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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)

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You can see a prior blog posting discussing RPI at:   https://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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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.

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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.

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Configuration of the Graph | Reflow Process

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The profile graph will tell you whether or not your process is within spec and that your reflow oven is creating the correct profile, nothing more. Double check your set-up again before diving into your profile. The information from the graph must be “true.”

There are dozens of software packages that mostly render the same information, with temperature on the Y axis and time across oven zones on the X axis. These software packages vary great with respect to how they manipulate this data or allow you to play around with the numbers.

Not all of your oven set points are going to match those of your profiling software. Profiling software can do this automatically but not all oven brands communicate with all profiling software brands. In the end, you might be left with a graph that appears to be in spec but, in reality, is meaningless due to differences in the set points in the oven and in the profiling software.

To be considered relevant, ALL profiles must follow these guidelines:

  • Profiles are generated by a specification from the solder paste manufacturer, component suppliers and substrate tolerances.
  • The set points of the oven heating zones are set.
  • The oven conveyor speed is set.
  • The profiler set points are the same as the oven heating zone set points.
  • The profiler oven conveyor speed is the same as the oven.
  • The reported number of heating zones are the same as the number of oven heating zones.
  • Any change in the set points of the oven MUST be changed to the profiler software.

Any violation of these guidelines will render your profile invalid and meaningless!

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Starting your Data Run

Profiling is no different than running an experiment, which relies on your ability to create reproducible profiles with your inputs, with each data sampling remaining constant.  If you start your data run on the bench and manually load the profile into the Reflow oven, it is unlikely that you will get the exact start and stop point for each run. Take care to start and stop in the same physical location each time.

profilerTo ensure repeatability, profiling software should, at a minimum, include both automatic distance measurements and temperature triggers, rather than manual start and stop points. The air thermocouple feature of the KIC system does away with the need to manually trigger (on/off) of the profile run, eliminating a step that can potentially invalidate your profile run.

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