Solder Limits: Updates for the Age of Surface Mount

Emma Hudson | 02-01-2018

Solder Limits Defined

Solder limits are one of the fundamental parameters used when evaluating the PCB, solder resists, and metal-clad base materials for safety under the UL Recognition programme.  

Solder limits are designed to represent the soldering processes the PCB will be exposed to during component assembly operations and take into consideration any time that is spent over 100°C or the maximum operating temperature (MOT) of the PCB, whichever is greater. The only exception is hand soldering operations that do not need to be captured. Solder limits can be a single time and temperature, such as 288°C for 20 seconds, or multiple solder limits (MSL) that include multiple times and temperatures, including ambient periods to represent time between different soldering operations. Solder limits are used as part of the thermal shock procedure prior to many of the tests employed to evaluate the PCB for safety.

Interpreting Solder Limits

As previously mentioned, solder limits consider any time spent over 100°C or the maximum operating temperature (MOT) of the PCB, whichever is greater. To determine if the Recognized Solder Limits are being exceeded, we need to understand the soldering operations the PCB will be exposed to and the thermal profiles associated with these. We then use these thermal profiles to measure the time above the critical temperature.

Figure 1 shows a generic surface mount (SMT) type soldering profile; we can use this to determine the time spent over either 100°C or the MOT Recognized for the PCB. If the board has a Flame-Only Recognition then we determine any time spent over the 100°C line. For a Full Recognition PCB, it is any time spent over the MOT line.

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Figure 1: Generic SMT profile showing how to measure the time for the solder limits parameter.

When you look at the total time above the line you can see why using multiple solder limits with different times at different temperatures becomes more appropriate for soldering profiles of this type, otherwise you would be looking at a long period of time at a high temperature.

The way we deem compliance with the solder limits can be seen in Figure 2; this provides an example for a Flame-Only Recognized PCB with solder limits of 288°C for 20 seconds. If the t1 to t2 period is greater than 20 seconds, then the solder limits have been exceeded and the UL Recognition of the PCB has been invalidated. In Figure 3, we can see the same evaluation being made but for a PCB Recognized with an MOT of 130°C.

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Figure 2: Determining compliance of a Flame-Only Recognized PCB with solder limits of 288°C for 20 seconds.

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Figure 3: Determining compliance of a Full Recognition PCB with a 130°C MOT and with solder limits of 288°C for 20 seconds.

If a PCB is Recognized with multiple solder limits (MSL), an example shown in Figure 4, then the PCB can be exposed to temperatures between 100°C and the temperatures detailed for the time shown; these are in addition to each other and not alternatives to one another. For the example shown, this means 10800 seconds between >100°C and ≤180°C plus 80 seconds between >100°C and ≤230°C plus 10 seconds between >100°C and ≤260°C plus a minimum of 300 seconds at ambient plus 10 seconds between >100°C and ≤260°C.  

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Figure 4: Example of a Flame-Only Recognized PCB with multiple solder limits.

Why are the Solder Limits So Important?

The IPC D-32 Thermal Stress task group have conducted research that has shown that PCBs that pass a solder float test can fail during surface mount assembly soldering operations, which is not surprising to those of us in the PCB industry. We have been aware for a long time that the more severe the soldering operations are the greater the degradation of the PCB. The increased degradation impacts not just the reliability properties of the PCB but also the properties we evaluate for safety.

For UL to conduct an accurate safety assessment of the PCB we must use solder limits that are representative for the actual soldering processes the PCB will see during assembly operations; if we use inadequate solder limits during the testing than are intended for use in production then it invalidates the safety testing that has been conducted on that PCB, as we cannot be confident that the PCB will behave the same after being exposed to these more severe conditions.

What UL is Doing to Help the Industry

One of the main things we get told at UL, when it comes to the PCB manufacturer selecting the solder limits they want to use for Recognizing their PCB in combination with, is that they do not know what soldering profiles their customers will use. Coming to UL from a Tier 1 automotive electronics manufacturer, I completely appreciate this point. I never told my PCB suppliers what soldering profiles I was using on my boards and I was just one of many customers to that PCB company. What UL is doing is offering some standardised soldering profiles for the PCB manufacturers to use for Recognition purposes.

We are taking the IPC-TM650 2.6.27 T230 and T260 soldering profiles (Figures 5 and 6) and offering these as an option for the PCB manufacturers to use. These will only ever be optional and we will not force anyone to go down a specific route. We are also happy to use any other reflow profile a PCB manufacturer requests to be used.

UL is recommending a minimum of three reflow cycles, but the PCB manufacturer needs to understand the maximum number of cycles their customers may need. Recent suggestions of six cycles have been made by one contract assembler!

We are endeavouring to add these IPC T230 and T260 soldering profiles to UL 796—the standard used to assess PCBs—but this is not as a requirement for them to be used by manufacturers; it would only be as a more accessible guideline/option. UL does not control what goes into the UL standards; this is done through a consensus process where UL has only a single vote on the Standards Technical Panel (STP); we have been trying to add these profiles to the standard for some time but hope that the STP will see the value in doing this now and that they will enter the standard shortly.

UL is very open to having other standardised reflow profiles added to the standard and are happy to receive suggestions on this. It is not intended that UL will create its own reflow profiles.

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Figure 5: IPC TM-650 2.6.27 T230 Soldering Profile

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Figure 6: IPC TM-650 2.6.27 T260 Soldering Profile

How to Update Solder Limits

The solder limits are used as part of the test procedure for the three primary tests of bond strength, delamination, and flammability and, as such, increasing the severity of the solder limits for an existing PCB type will involve testing using these revised limits. They are used in some other construction-specific type tests too, such as conductive paste adhesion, so these would also need to be considered, but for the vast majority of boards Recognized with UL it will be the three primary tests that need to be considered.

Figures 7 and 8 summarise the test requirements for updating the solder limits for a standard rigid multilayer construction.

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Figure 7: Assessing the testing needed to increase solder limits for a Recognized PCB—metal-clad base materials.

HudsonFig8.JPG Figure 8: Assessing the testing needed to increase solder limits for a Recognized PCB—solder resists.

Each UL/ANSI grade of base material would need to be assessed, so if a board has both FR-4.0 and FR-4.1 materials detailed for use the complete testing would need to be done on each UL/ANSI grade before we could look to apply the CCIL/MCIL program.

One problem that PCB manufacturers are likely to face is that the vast majority of base materials and solder resists have not been Recognized in combination with solder limits suitable for SMT reflow profiles, which in turn means that the CCIL/MCIL program cannot be used for the base materials and the Permanent Coating program for the solder resists, as the solder limits of the material have to be equal or more severe than the PCB it is being added to for these reduced test/no-test programmes to be considered. If the solder limits of the materials are not suitable, each base material would need to be evaluated for bond strength, delamination, and flammability and each solder resist for flammability.

UL will endeavour to communicate the meaning of the solder limits to all relevant parties but the PCB manufacturers need to help us and work with their suppliers to insure they are using the appropriate solder limits when Recognizing their materials such that the CCIL program and Permanent Coating program can be routinely used to minimise the testing required for the PCB manufacturer.

We strongly recommend any new PCB be Recognized with solder limits suitable for SMT reflow soldering, unless the PCB manufacturer is 100% confident that the PCB will never be exposed to soldering of this type.

What will happen moving forward?

UL intends to actively communicate this message to the relevant parties: OEMs with UL-listed products, Recognized PCB assemblers and PCB manufacturers, and Recognized material manufacturers supplying the PCB industry. The intent is to send out a bulletin to all parties to ensure everyone understands what solder limits are and that anyone who has a requirement to use a Recognized PCB must ensure that the solder limits of the PCB are not exceeded during the soldering processes for the Recognition to still be considered valid.

UL are also attempting to have the PCB standard—UL 796—updated with the standardised IPC TM-650 2.6.27 T230 and T260 soldering profiles, as mentioned previously. It is not a requirement to have these thermal profiles in the standard for the PCB manufacturer to request to use these but we feel it will make it easier for the industry to request them if they are presented as an option.

From the start of 2018, UL’s Follow-Up Service (FUS) inspectors will receive refresher training about what solder limits are and how to interpret them when inspecting the PCB assemblers and OEM. The inspectors will be asking to see evidence of the soldering profiles any Recognized PCBs have been exposed to during any assembly operations and they will check that these have not exceeded the Recognized solder limits for the PCB being used.

Any time a Recognized PCB is found to have exceeded the Recognized solder limits for that board it will be deemed to not be in compliance and a Variation Notice will be raised and further action will be needed to resolve the matter.

Ideally, the assembler and/or OEM will let the PCB manufacturer know what solder limits they need their UL-Recognized PCB to have but we know this happens very infrequently. So it is important for the PCB manufacturer to take the initiative and start Recognizing their PCBs in combination with soldering profiles suitable for the world of multiple surface mount soldering operations.

Summary

The solder limits for the vast majority of Recognized PCBs are not representative of the surface mount soldering operations that are commonplace within the assembly industry today and this has to be fixed. The PCB industry has been aware for many years that the more severe the soldering processes the greater the degradation of the PCB. Typically, we consider the degradation with regard to the reliability of the PCB but it is just as valid when considering the safety elements. For the safety assessment to be valid, the PCBs must have been evaluated in combination with Solder Limits that represent the actual soldering processes the board will be exposed to in production. The traditional solder float test is not valid for a PCB that will be exposed to SMT soldering operations.

UL want to make it easier for the industry to be able to assess their PCBs and their materials in combination with soldering profiles for the SMT age and are offering the IPC TM-650 2.6.27 T230 and T260 as an option for the safety evaluation. The industry can select any other profile they wish but we often hear that the PCB manufacturer does not know the reflow profile that will be needed, so we hope by offering some industry standard profiles this will help all parties in doing the right thing.

Testing is going to be needed to bring PCBs already Recognized with single time/temperature solder limits up to these SMT soldering profiles but this could be minimised if laminate and solder resist manufacturers can be convinced to also bring their Recognized solder limits up to meet these. We would certainly recommend all new PCB types being evaluated use SMT style solder limits. The industry must address this; the solder limits have to represent the soldering processes the PCB will be exposed to.

The final message I would like to leave you with is that we are here to help you with this! Any questions or concerns, any help needed, please contact me and I will do what I can to assist you through this process and make sure your PCBs are ready to be used for the SMT soldering profiles that are so commonplace in our industry today.

This article is based on a presentation Hudson made at the Institute of Circuit Technology Harrogate Seminar 2017 in Yorkshire, England, December 2017.

Emma Hudson is the UL PCB industry lead for EMEA+LA region. To contact Hudson, click here.