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EMS Partners Use Thermal Profiles to Improve Quality, Costs

When original equipment manufacturers (OEMs) evaluate electronics manufacturing services (EMS) partners for printed circuit board assemblies, the manufacturer's solder processes deserve close attention. Small differences in process, such as the use of thermal profiles, can have a big impact on yields, quality, and cost.

As boards go through solder reflow or wave solder, the EMS must ensure that every area of the board reaches the right temperature for soldering and remains there for the right duration. They must also control the rate of heating: Heating too quickly can damage components or boil the flux, leaving splatter and solder balls; while heating too slowly can cause the flux to dry out and become inactive before the solder forms a good joint.

A printed circuit board's thermal profile describes its temperature over time during soldering. The profile is affected by the number of layers: More layers mean more copper and a greater amount of heat absorbed. At Z-AXIS, boards range from two layers to 28 layers and there is a significant difference in thermal profiles over this range. The type of parts also affects the profile, with areas containing large or heavy parts such as BGAs heating up much more slowly than areas with smaller components.

Many high-mix EMS providers create three or four standard thermal profiles, each covering a range of values for the typical number of board layers and predominant component types they encounter. Then they apply one of these standard profiles to each new board design that enters manufacturing, assuming the standard profile will be close enough to the board's actual profile to give good results.

In our experience, a better approach is to measure, record, and optimize the actual thermal profile of each board design during pre-production, and use that unique profile in manufacturing.

Thermal profiling of a board is fairly easily done with a Datapaq, a device with several thermocouple sensors and data acquisition hardware in a heat-shield case. The temperature probes from the Datapaq are affixed to the bare circuit board in multiple places. Larger, heavier components that can have a significant impact on heating are modeled by adding an equivalent weight above the probe where these components will be placed.

The Datapaq is placed into its heat-shielding case and goes through the solder machine with the bare board, recording board temperatures the entire time. When removed, it gives us time vs. temperature graphs for different areas of the board, showing what temperatures were reached, for how long and at what point of the process. We analyze these graphs to ensure that:

  • All areas of the board reached the required reflow temperature and held it for the required time, as specified by the solder data sheet.
  • The board did not exceed its maximum rate of heating (typically 3°C/second).
  • No areas of the board exceeded the temperature ratings for the components to be used.

If adjustments are needed, we can change the rate of heating by independently adjusting several different temperature zones in the reflow oven, to within 1°C. It is occasionally necessary to adjust the speed of the conveyor, slowing it down for heavy boards. For wave solder we can control the frequency of the fluxing pumps as well.  With experienced technicians, we can usually make these solder process adjustments and get a board's thermal profile exactly right on the first or second try.

These settings are stored by board part number in the memory of the solder oven or wave solder machine. Each time the board comes up for a production run, we enter the board's part number to automatically adjust the solder machine settings and optimize soldering for that board.

Optimizing the unique thermal profile for each board typically takes less than an hour of work in pre-production for each board design. Compared to using a handful of “one-size-fits-most” standard thermal profiles, it gives us lower solder defect rates to improve quality while reducing costs for the customer.

17 comments on “EMS Partners Use Thermal Profiles to Improve Quality, Costs

  1. Eldredge
    February 11, 2014

    In the past, I have also used self-contained thermal data loggers similar to this to measure the thermal profile of a vacuum desposition process. We had to take some additional precautions to protect the electronics, but the data obtained proved to be very useful.

  2. Hailey Lynne McKeefry
    February 13, 2014

    As i sat in my first electronics class (way back in the good old days), the first thing the instructor said is “If you remember nothing else from this class, remember this: Heat is the enemy.”  Well, i try to always listen to my instuctors so i've remembered that (and perhaps a few other things) for 20 plus years.  What makes thermal profiles so challenging? I'm suprised we aren't more on top of this.

  3. Hailey Lynne McKeefry
    February 13, 2014

    @Eldredge, how did your organization end up using this data?

  4. Michael Allen
    February 13, 2014

    Hailey, the benefits of a good profile are enormous.  It can reduce defects and hence rework greatly.  Common defects that are affected by the profile are of course cold solder joints, but also tomb stoning,  bridging, solder balls, poor wetting,  parts cracking,  and voiding.  The real challenge comes when you have a board with a lot of large parts in one area and very few in another. 

    Yes, heat is the enemy of components but applying just the right amount of heat at the right time can greatly improve the robustness of a process.

  5. Eldredge
    February 13, 2014

    @Hailey – The process we were evaluating involved depositing a cermet material for forming resistors. We were seeing a wide variability in Therrmal Coefficient of Resistviity (TCR) of the deposited film. It turned out that thermal variation across the deposition area affetced the final TCR of the film, so we learned to either even out the thermal profile of the process, or avoid populating product at the extremities of the deposition area.

  6. Daniel
    February 13, 2014

    “As boards go through solder reflow or wave solder, the EMS must ensure that every area of the board reaches the right temperature for soldering and remains there for the right duration. They must also control the rate of heating: Heating too quickly can damage components or boil the flux, leaving splatter and solder balls; while heating too slowly can cause the flux to dry out and become inactive before the solder forms a good joint.”

    Michael, in PLC controlled pcb soldering machines, I think all these factors are addressing properly, especially with wave type soldering.

  7. Hailey Lynne McKeefry
    February 14, 2014

    Thanks for the examples, Michael… It dovetails with another conversation i was having today about how a poor quality or reliablity part can take down a much more expensive larger whole. it's a good reminder that it's teh worth of the worth of the system, not the worth of the individual part, that needs to be considered when counting the cost of a problem or defect that might be created.

  8. Hailey Lynne McKeefry
    February 14, 2014

    @Eldredge, thanks for the detail…having a real-world example is really helpful.

  9. Daniel
    February 14, 2014

    “Yes, heat is the enemy of components but applying just the right amount of heat at the right time can greatly improve the robustness of a process.”

    Michael, you are right, especially in PTH (Pole Through Holes)PCB board, where wire jumpers are using.

  10. Daniel
    February 14, 2014

    “It dovetails with another conversation i was having today about how a poor quality or reliablity part can take down a much more expensive larger whole. it's a good reminder that it's teh worth of the worth of the system, not the worth of the individual part, that needs to be considered when counting the cost of a problem or defect that might be created.”

    Hailey, in quality analysis there is a saying 'quality of the product/device is measured on the least quality of its components'.

  11. Michael Allen
    February 19, 2014

    @Jacob, Yes, PLC-controlled soldering machines are programmed to achieve specific temperatures and timing. But the only way to be sure that the actual PCB is reaching those temperatures is to measure it. Two different boards going through solder using the same PLC “recipe” may result in very different temperature profiles. Or a section of a board with heavy copper might reach the correct temperatureeven though the rest of the board has. You may need to slow the down the conveyor or increase temperature in a specific zone. Adjusting the PLC program for each board design takes care of these board to board variations.

  12. Hailey Lynne McKeefry
    February 19, 2014

    @Jacob, it makes total sense…I had just never really thought about the complexity of the testing. this has been a very useful conversation.

  13. Daniel
    February 19, 2014

    “You may need to slow the down the conveyor or increase temperature in a specific zone. Adjusting the PLC program for each board design takes care of these board to board variations.”

    Michael, I think in PLC controlled system, adjusting the temperature for a while in running condition may be difficult.

  14. Daniel
    February 20, 2014

    “it makes total sense…I had just never really thought about the complexity of the testing. this has been a very useful conversation.”

    Hailey, one of the merits of community/social media discussion

  15. Eldredge
    February 27, 2014

    Or a chain is only as strong as it's weakest link.

  16. Eldredge
    February 27, 2014

    @Michael Allen – Exactly right – the temperature settings are only part of the puzzle. Different thermal masses will absorb and/or distribute heat differently. Sometimes there is a transition (loading) effect with the leading edge of product as it enters the different temperature zones. It is important to understand the thermal trnasitions that the workpiece experiences, which may be drastically different from the thermal transitions that the thermocouple for the heat controller experiences.

  17. Daniel
    March 3, 2014

    “Or a chain is only as strong as it's weakest link.”

    Eldredge, it's like the quality of a product depends up on the least quality of the components.

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