Solder Limits: Updates for the Age of Surface Mount

Estimated reading time: 8 minutes

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

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.

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