Zulki’s PCB Nuggets: Putting the Heat on for Thermal Profiling

Zulki_650-0320.jpgA unique thermal profile is designed for each PCB job undergoing conventional SMT assembly, as virtually every PCB assembly professional knows. But what about a PCB assembly project involving both conventional rigid board and an extraordinarily small rigid or rigid-flex circuit undergoing microelectronics assembly? In short, that’s called PCB hybrid assembly, which requires two separate, unique, and distinctly different thermal profiles.

Here’s where extensive experience dealing with these unique thermal profiles comes into play. Combining SMT and microelectronics assembly with separate thermal profiles for each is of particular importance to OEMs designing advanced wearable, IoT, and other portable products. It’s important for these and other OEMs to get a handle on the number of factors going into developing the right thermal profiles for SMT and microelectronics assembly. Failing or falling short on any of the steps in creating the right thermal profile for either or both PCB assembly processes can prove costly in dollars and time to revenue to an OEM.

Let’s first do a quick review of the various aspects of a thermal profile for SMT assembly. Thermal profiling is a major part of the SMT manufacturing process and is basically a recipe for preparing an SMT component-populated PCB for the IR reflow oven.

Three steps are involved. First, solder paste is applied to the PCB by screen printing it through a stencil, otherwise known as stencil printing. Next, a pick-and-place machine takes components from a reel, tube, or tray and automatically places them on the board. Third, the PCB goes through either an IR reflow oven or through vapor phase.

When creating a thermal profile, a solder sample board is used and is designed based on the solder paste specifications of a particular manufacturer. Thermocouples are attached to different parts of the PCB. The idea here is to record temperatures at different spots to create a custom thermal profile, which includes three different area segments; those are the soak period, peak temperature, and cool-off period. As shown in Figure 1, zones 3–5 are soak periods, zones 6– 8 are peak temperatures, and zones 9–10 are cool-off periods.

Zulki_Fig_1.jpg 

Figure 1: A unique thermal profile graph, showing zones 1–10 at the bottom.

A thermal profile also depends on a few things to be in place. It must factor in the number of copper layers on the board, especially power and ground. It must also take into account component density at the top and bottom layer of the board. Plus, it has to account for whether it’s a leaded or lead-free profile.

Solder paste melting temperature for leaded and lead-free are vastly different, and that must be taken into account. A unique thermal profile is thus created for each assembly by keeping into account factors like the number of components and their types, the number of layers, and the board thickness.

Thermal Profiles for Microelectronics

There are several factors that must be considered in the development of a thermal profile for PCB microelectronics assembly. First, there are the thermal profile requirements associated with the particular die being used. Then, there is the die’s moisture sensitivity level that must be accounted for to assure it is preserved at the optimal level without moisture seeping into the package. Sometimes, these dies need to be baked at certain temperature profiles for a specific period of time.

You also have to look at the substrate’s thermal profile and glass transition temperature (Tg) requirements to determine if PCB material as FR-4, Rogers, or Megtron 6 are used. Now, we have to check out the optimal epoxy to be used in a particular project. The question is whether or not it is going to be conductive or non-conductive. The right selection is made based on the nature of the project and type of die to be used.

Lastly, all Tg and thermal profiles are important because these profiles, and Tg levels need to be compatible as far as operating temperature ranges and heat transfer levels so that die attach and wire bonding are properly performed during microelectronics assembly.

Zulki Khan is the president and founder of NexLogic Technologies Inc.

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2020

Zulki’s PCB Nuggets: Putting the Heat on for Thermal Profiling

03-11-2020

A unique thermal profile is designed for each PCB job undergoing conventional SMT assembly, as virtually every PCB assembly professional knows. But what about a PCB assembly project involving both conventional rigid board and an extraordinarily small rigid or rigid-flex circuit undergoing microelectronics assembly? Zulki Khan covers PCB hybrid assembly, which requires two separate, unique, and distinctly different thermal profiles.

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Zulki’s PCB Nuggets: 7 Steps to Successful Assembly for Medical Devices Using Microelectronics

02-12-2020

Seven major steps need to be taken to achieve successful SMT and microelectronics assembly for medical electronic devices. Zulki Khan explains how these key steps take on special significance for newly emerging implantable and ingestible medical devices and result in medical devices that are robust, smaller, highly reliable, more powerful, and lighter.

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Zulki’s PCB Nuggets: Successful PCB Microelectronics Assembly

01-15-2020

In addition to coverage of PCB microelectronics subjects, Zulki Khan addresses one of the most crucial areas: PCB fabrication that creates the circuit board undergoing microelectronics assembly. The burning question is, “Why is fabrication vitally important when it comes to successful microelectronics assembly?”

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2019

Zulki’s PCB Nuggets: Vital Details for Implantable Medical Devices

12-04-2019

In addition to smart pills and smart cameras, which Zulki Khan covered in a previous column, another segment of the medical electronics devices market is rapidly growing, as well: implantable medical devices, which medical personnel surgically or otherwise insert into various parts of the human body. Zulki explains the extra measures required for these devices.

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Zulki's PCB Nuggets: Multi-tier Wire Bonding—Diving Into PCB Microelectronics

11-07-2019

As the name implies, multi-tier wire bonding involves several levels of wire bonding beyond the single level of wire bonding, which is traditionally used in semiconductor and/or PCB microelectronics assembly. Here, you have two, three, and four levels of wire bonding, in some cases, called stacked wire bonding. Also, multi-tier wire bonding offers OEMs a solution when the number of inputs/outputs (I/Os) are far beyond the traditional ones that are used in the single wire-bonding application.

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Zulki’s PCB Nuggets: Smart Pills & Cameras—The Next Frontier for PCB Microelectronics

10-23-2019

"Take two aspirin and call me in the morning," is the proverbial, jovial, and often-cited elixir that doctors have prescribed over the years for whatever ails you. Today, medical electronics are adopting the same concept but with new technologies. Now, the phrase, "Take two aspirin," takes on new meaning, as medical electronics move into new frontiers of inspecting a human’s gastrointestinal tract with new, revolutionary ingestible smart pills and "pill cams."

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Zulki's PCB Nuggets: A Better Grasp of Glob Top Epoxy Factors

09-25-2019

In my last column, I cited important aspects of glob top epoxies, calling attention to the fact there are different epoxy manufacturers. In this column, I will continue to emphasize six other important factors of glob top epoxies.

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Zulki’s PCB Nuggets: Get a Handle on Glob Top Epoxies

09-12-2019

Most often, glob top is the prevalent method EMS providers use today. However, the most important point to be made about glob top is the fact that multiple manufacturers are producing different glob top epoxies. And within each manufacturer, there are numerous types of epoxies being produced. Another key point is that EMS providers and contract manufacturers generally are the ones deciding on the kind of epoxy to use. This column will further describe how you can get a handle on glob top epoxies.

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Zulki’s PCB Nuggets: Protect the Die and Wire Bonding for Effective PCB Microelectronics Assembly

07-31-2019

Protecting bare dies on a PCB or substrate is a major process of microelectronics assembly. As we’ve said before, microelectronics assembly and manufacturing work in tandem with traditional SMT manufacturing for complete PCB hybrid manufacturing of today’s smaller form factor products, including IoT, wearables, and portable devices.

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Zulki’s PCB Nuggets: PCB Microelectronics—Inspection and Calibration

07-18-2019

Microelectronics manufacturing is the companion of SMT manufacturing and forms PCB hybrid manufacturing. Tools for SMT manufacturing have been around for a long time and have proven their value. Now, with microelectronics, new and different types of high-powered laser microscopes are populating the microelectronics assembly and manufacturing area to provide highly effective inspection and calibration.

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Zulki’s PCB Nuggets: Three Die Attach Methods for Microelectronics Manufacturing

06-27-2019

Die attach technology is increasingly being applied in PCB hybrid manufacturing (i.e., combining traditional SMT manufacturing with microelectronics) to comply with the requirements of small PCBs, especially rigid, flex, and combination rigid-flex circuit boards. These smaller boards are used in a variety of IoT, wearable, and portable applications.

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Zulki’s PCB Nuggets: Consider the Integrity of Wire Bonding

06-12-2019

While reliability and integrity can be regarded as synonymous as far as PCB manufacturing with microelectronics assemblies is concerned, the integrity of wire bonding—the methodology of interconnecting the wire to the bond pad—takes on other reliability-associated process qualities. Here are three factors that need to be implemented to create the integrity of wire bonding.

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Zulki’s PCB Nuggets: Avoid PCB Wire-bond Loop Failures

05-30-2019

Today, hybrid PCB manufacturing is making greater inroads into our industry, which is the marriage of traditional SMT manufacturing together with microelectronics and wire bonding. In many cases, the OEM working with EMS providers doesn’t fully understand the nuances of effective wire bonding and related failures.

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2014

Tighter Scrutiny Needed for PCB Cleaning Agents

05-13-2014

PCB cleanliness on the assembly floor is now getting more attention, due to tiny residues and contaminants being left on assemblies after new, advanced assembly processes. Cleaning methodologies, testing, analysis, and special chemistries are being taken to a new level to assure customers of ultraclean boards to avoid costly latent issues.

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Zulki's PCB Nuggets: Tighter Scrutiny Needed for PCB Cleaning Agents

05-13-2014

PCB cleanliness on the assembly floor is now getting more attention, due to tiny residues and contaminants being left on assemblies after new, advanced assembly processes. Cleaning methodologies, testing, analysis, and special chemistries are being taken to a new level to assure customers of ultraclean boards to avoid costly latent issues.

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Uncovering Assembly Problems of High-Speed PCBs

03-12-2014

The high-speed board may be perfect when it comes to BGA assembly. All the balls properly collapse; all the thermal profiles are accurately determined and performed. All soak temperatures, pre-heat, soak, and cool-off periods fall within manufacturer limits and ranges. Yet, this high-speed board fails at high speed at the time of system functional level testing in the system.

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Zulki's PCB Nuggets: Uncovering Assembly Problems of High-Speed PCBs

03-12-2014

The high-speed board may be perfect when it comes to BGA assembly. All the balls properly collapse; all the thermal profiles are accurately determined and performed. All soak temperatures, pre-heat, soak, and cool-off periods fall within manufacturer limits and ranges. Yet, this high-speed board fails at high speed at the time of system functional level testing in the system.

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EMS Discovers Mature IC Technologies

01-14-2014

Columnist Zulki Khan asks, "Did you know that really new, up-to-the-moment PCB technologies are nesting on the doorstep of PCB assemblers?" In fact, he says some of these technologies are very mature, but they're completely new to the assembly side of things.

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Zulki's PCB Nuggets: EMS Discovers Mature IC Technologies

01-14-2014

Columnist Zulki Khan asks, "Did you know that really new, up-to-the-moment PCB technologies are nesting on the doorstep of PCB assemblers?" In fact, he says some of these technologies are very mature, but they're completely new to the assembly side of things.

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2013

Another Look at AOI

11-13-2013

PCB inspection is taking on greater significance as boards and packaging become increasingly smaller, with greater functionality. Automated optical inspection (AOI) and its backup associate, X-ray, team up to catch a variety of board assembly problems. But it's AOI that's at the forefront of this process.

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Zulki's PCB Nuggets: Another Look at AOI

11-13-2013

PCB inspection is taking on greater significance as boards and packaging become increasingly smaller, with greater functionality. Automated optical inspection (AOI) and its backup associate, X-ray, team up to catch a variety of board assembly problems. But it's AOI that's at the forefront of this process.

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Zulki's PCB Nuggets: ECOs Reviewed - The Importance of Accuracy

09-11-2013

Designers can perfectly layout a design and, in theory, follow written specifications to the letter, but when one factors in the practicality of that design, virtually everything associated with it has its limitations--ranging from the material used to make the board to assembly, machine tolerances, and process limitations.

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