SMT Solver: Developing a Reflow Profile

Developing a reflow profile for electronic assemblies is like trying to figure out the time and temperature (bake profile) to bake turkey, chicken, and shrimp for the same length of time at the same temperature in the same oven without undercooking the turkey or overcooking the shrimp. The difference, however, is that using a bad reflow profile for electronic products is much more consequential than an undercooked turkey or overcooked shrimp and some disap-pointed guests you invited for dinner.
 
The basic idea of developing a good profile is to make sure all solder joints reach the minimum temperature to achieve good solder joints but don’t exceed maximum temperature to prevent damage to components or to solder joints. This is not an easy task, especially when you have a board with components of different sizes and thermal masses: BGAs, sockets, finepitch, BTCs, and 0402/0201, similar to the previous example.
 
The good news is that we have the tools, technologies, and processes to deal with the profiling challenge. What I plan to do in this and a few follow-up columns is to provide specific guidelines and rules for developing a unique profile for each product without any damage and warpage to components and boards and with minimum possible profile-related defects. More specifically, I will try to:
  • Provide an overview of various types of thermal profiles, purpose, key requirements, and challenges in developing thermal profiles.
  • Explain the importance of different soldering zones—such as preheat, soak, reflow, and cooling—and their impact on solder quality.
  • Discuss the details of how and where to attach thermocouples to achieve the desired soldering temperatures in various soldering zones.
  • Emphasize the importance and difference between time above liquidus (TAL) and true TAL and their impact on the quality of solder joints, especially on head-on-pillow.
  • Provide specific recommendations for thermal profiles for most commonly used
  • Pb-free (SAC and low-temp), Sn-Pb, and mixed alloys.
  • Discuss conflicting requirements of different types of packages and alloys on the same board and show some examples of profile related defects.
What Is a Thermal Profile?
A thermal profile is a unique temperature vs. time plot for each fully populated printed wiring board assembly (PWBA) using thermocou-ples attached to the solder joints with high-temperature solder, copper, or aluminum tapes to selected representative components on the board as the board travels at a given belt speed through various temperature zones of an oven or soldering system. There is a lot in that one sentence, but it is worth paying attention to some of the things I mentioned.
 
For example, you need to use a fully populated board and definitely not a bare board or another board that possibly looks like the board but a fully populated board that comes as close as possible to the thermal mass of the product you are profiling. You are using thermocouples that are attached with high-temperature solder or copper or aluminum tapes and not Kapton tapes used by many people. The location of attachment is generally the solder joint itself (unless you’re specifically monitoring other spots, such as temperature-sensitive body components or the bare board itself, when using low-Tg PCBs.) And even the length (about a meter) of the thermocouple wires and their gauges (about 36 gauge) is very important so that you don’t get the incorrect temperature.
 
Editor's note: To continue reading this column which appeared in the May issue of SMT007 Magazine, click here or download the PDF for future reference.
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2020

SMT Solver: Developing a Reflow Profile

06-03-2020

Developing a reflow profile involves ensuring all solder joints reach the minimum temperature to achieve good solder joints but don’t exceed maximum temperature to prevent damage to components or to solder joints. This is not an easy task. Ray Prasad provides specific guidelines and rules for developing a unique profile for each product without any damage and warpage to components and boards and with minimum possible profile related defects.

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SMT Solver: Developments in BTC Guidelines: IPC-7093A, Pt. 1

04-15-2020

As the chair of this IPC committee, let me share the latest developments in bottom-terminated component (BTC) design and assembly guidelines in this three-part series. In this first column, I will give you an overview of this technology and standard. In my upcoming columns, I will take an in-depth look at the design, assembly, quality, and reliability issues in BTC technology that have been incorporated in this latest IPC-7093A revision.

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SMT Solver: Choosing the Right Defect

01-17-2020

Ray Prasad addresses some key issues that are important for all of us to be aware of and learn about, especially for managers in SMT assembly and engineers who aspire to be future managers. Topics covered include choosing the right defect and developing a DFM and process recipe.

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2019

SMT Solver: Today’s Soldering Options

11-22-2019

If you have to deal with mixed-assembly boards with both surface-mount and through-hole components—as is the case today for more than 95% of electronic products—the selection of a soldering process becomes more complex, especially if you use both tin-lead and lead-free components on the same board.

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SMT Solver: How Standards Impact You and Your Company

11-06-2019

Standardization is one of the key issues in promoting any new technology, but it is almost mandatory for SMT because of the need for automation to promote consistency in quality. Standards make the market grow faster than it would without them. A good standard benefits both users and suppliers. For example, if the package size tolerances are tightly controlled (within the requirement of the standard), the user can properly design the land pattern and use the same design for all suppliers of that package.

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SMT Solver: Would You Prefer Shorts or Opens in Your Products?

07-29-2019

Would you prefer shorts or opens in your products? Of course, neither. But what if you do have to choose? Ray Prasad says he would choose a more desirable defect, if there is such a thing. But what is a desirable defect? A defect that would never escape inspection and test and would be caught before shipping the product to the customer. Read on why.

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SMT Solver: Benchmarking Defect Levels in Your Products

06-17-2019

In this column, Ray Prasad discusses why zero defects may be a desirable goal but not a realistic one. He also shares some industry data as proof, which you can also use to benchmark defect levels in your products. Finally, he also addresses the choices when selecting components that have a big impact on the level of defects you should expect.

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SMT Solver: Assemblers Can Help Customers Reduce Cost, Improve Reliability

05-08-2019

It is commonly assumed that the level of defects is primarily dependent on how the assemblers control their manufacturing processes. This sort of mistaken belief will cause you to never find the root cause of the problem. Hence, the problem will persist forever. And just because defects are discovered in manufacturing does not mean that they were created in manufacturing. Find out why.

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2014

Flux Classification

02-15-2014

In the previous column, I discussed flux functions and general considerations in their selection. In my next three columns, I will review various types of fluxes.

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Major Types of Fluxes

01-20-2014

Organic acid (OA) fluxes are stronger than rosin fluxes but weaker than inorganic fluxes.

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