The industry standard for temperature profiling gets a complete overhaul.

What is thermal profiling, and why does it play a critical role in determining quality of soldering joints?

Thermal profile is a unique temperature vs. time plot for each fully populated printed circuit board assembly (PWBA), using thermocouples attached with high-temperature solder or copper or aluminum tapes to selected representative components of a PCBA as it travels through an oven or soldering system through various temperature zones and at a given belt speed.

Although different products, based on their thermal mass, require different amounts of thermal input, all products must achieve the minimum temperature (temperature above liquidus) without exceeding the maximum temperature (without damage to any components) within a defined time period (thermal profile).
This is the key reason for developing a unique profile for each product.

The thermal input is determined by temperature/gas flow settings in each zone, the number of zones and the belt speed, which stays the same in each zone. The minimum and maximum temperatures and duration in a given zone are established to ensure formation of intermetallic bonding between the leads of the components and their corresponding footprint or land patterns on those pads.

The biggest challenge for the person responsible for developing the profile is that all components, even though their thermal masses are different, must meet the same minimum and maximum temperature requirements.

So developing a thermal profile of an assembly populated with very large thermal mass components (such as a large ball grid array, or BGA) and small thermal mass components (such as 0201 or smaller chip resistors and capacitors) is a balancing act. Even more complex, different heating and cooling rates have various effects on different types of defects. For example, a slower heating rate will help reduce voids in a BGA, but will increase the potential for HiP in the same BGA.

As a practical matter, minimum soldering temperature is somewhat (~25°C) above the liquidus temperature of the solder alloy, but some exceptions must be made for mixed alloys.

A new standard. A new document provides plenty of details on those and other issues related to machine soldering. That document, IPC-7530A, “Guidelines for Temperature Profiling for Mass Soldering Processes (Reflow & Wave),” is in its final stage of the industry balloting process. Barring any unforeseen situations, the document should be released at the IPC Apex Expo Conference in San Diego in February.

IPC-7530A is the first revision of the original IPC-7530. It is really not a revision, however, since nothing from the original document has been included. The original document was published a long time ago and was totally obsolete.

The focus of this document is on reflow profiling, but profiles for other soldering processes such as vapor phase, laser, selective soldering and wave soldering are also covered. Some historical information is provided to understand the reasons for the near disappearance (and now slow reemergence) of vapor phase in favor of convection soldering as a result of detailed thermal profile studies.

The document starts with basic terms and definitions relevant to thermal profiling, details the various elements of profiling, and concludes with a section dealing with defects related to thermal profiling. There are too many details to mention, but the basic aim of the document is give the reader background for the reasons behind the guidelines. Even small but very useful details such as the number and gauge of thermocouples, their locations and methods of attachment, are included.

The new document provides tables for temperature and time settings in each of the four profile zones, namely preheat, soak, reflow and cooling, for all kinds of assemblies, including SnPb, high and low temperature Pb-free alloys and alloys for mixed assemblies. The importance of TAL (time above liquidus) and true TAL, and the impact on head-in-pillow (HiP), is elaborated in great detail. Examples of actual profiles for the same board with different thermal masses are shown to drive home the importance of a unique profile for not only each assembly but each side of the same assembly.

The purpose of this standard is to provide useful and practical information to those responsible for developing thermal profiles to produce acceptable SnPb and Pb-free electronics assemblies. This standard is for managers, design and process engineers, and technicians who deal with mass soldering processes.

Maintaining control. Developing a good thermal profile is a balancing act on the part of the process engineer, who needs to ensure smaller and temperature-sensitive components do not get overheated or damaged, but at the same time the larger components such as processors, sockets and connectors reach their minimum soldering temperatures.

Reflow soldering requires controlled rates of heating and subsequent cooling; however, too rapid a heating rate can damage boards and components. Likewise, high cooling rates can damage components and result in temperature gradients of sufficient magnitude to warp PCBAs and larger components, and may fracture solder joints. It is for these reasons that appropriate temperature profiling is essential to high-quality solder joints.

Even though thermal profiling is one of the key variables that determines quality of solder joints and level of defects, and prominently features in any legal conflicts between users and suppliers, very few companies develop profiles correctly. Also amazing is many companies use bare boards to develop profiles. This is a waste of time and energy because the thermal mass of a fully loaded board is very different from the thermal mass of a bare board. Many companies use the same profile for single- and double-sided populated boards. If you wouldn’t use the same time and temperature settings to bake a 4 lb.  chicken as a 20 lb. turkey, why use the same profile for assemblies of different thermal masses?

This standard clears up all kinds of misunderstandings about thermal profiling and provides specific guidelines as to how to correctly develop thermal profiles. It has been my pleasure to chair this important and sorely needed task group on thermal profiling and to work with a dedicated group of engineers from a wide range of user and supplier companies.

Acknowledgments

Although many contributors made publication of this standard possible, and they are duly acknowledged in the document, some individuals deserve special mention for their substantial contributions. They are Rob Rowland of Axiom Electronics (vice chair), Dr Raiyomand Aspandiar and Dr. Dudi Amir of Intel, Paul Austen of ECD, and Vern Solberg, Solberg Technical Consulting. It is also worth acknowledging this document could not have been published without focused effort by Chris Jorgensen of IPC, who kept us focused on our task.

Ray Prasad is principle of Ray Prasad Consultancy Group (rayprasad.com) and chair of the IPC-7530 Task Group; smtsolver@rayprasad.com.

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