Zulki’s PCB Nuggets: Successful PCB Microelectronics Assembly

So far, I’ve discussed a variety of PCB microelectronics subjects. However, I haven’t covered one of the most crucial areas: PCB fabrication that creates the circuit board undergoing microelectronics assembly. OEMs need to carefully consider fabrication for their products that are assembled via the PCB microelectronics route.

The burning question is, “Why is fabrication vitally important when it comes to successful microelectronics assembly?” Keep in mind that PCBs going through microelectronics assembly are considerably more special due to the intricacies involved. That includes the utmost precision required for placing a bare die on the circuit board and the critical cleanliness of the board before and after a bare die is attached.

In effect, fabrication is vital because it serves as the base on which the total and successful microelectronic assembly process depends. For starters, a fabrication shop producing PCBs for microelectronics assembly should, at the minimum, have ISO 9001 certification, so it has the infrastructure in place to produce quality PCBs. And it’s all the better if the fabricator has additional qualifications like ISO 13485, AS9100, and/or some mil/aero certifications.

Aside from these certifications, fabricating PCBs for microelectronics assembly demands excellence in three major areas: board and substrate co-planarity, process flow and control, and verification. In simple terms, co-planarity refers to the PCB or substrate surface being 100% uniform or non-planar. This is where surface finishes come into the picture to determine which ones are completely uniform or not for microelectronics assembly. Candidates include leaded or lead-free hot air solder leveling (HASL), Entek, also known as organic solderability preservative (OSP), immersion silver, electroless nickel immersion (ENIG), and electroless nickel/electroless palladium/immersion gold (ENEPIG).

Leaded or lead-free HASL is out the door right away because it is the most uneven surface finish. Entek, or OSP, poses two major issues: corrosion, and it’s not suitable for proper microelectronics assembly because it does not have peel strength to properly hold wire bond pads in place. Likewise, immersion silver is prone to corrosion.

ENIG and ENEPIG are the two surface finishes that are acceptable; of the two, ENEPIG is the absolute best surface for microelectronics assembly. OEMs can live with ENIG. Ideally speaking, however, ENEPIG is the better of the two because it’s very co-planar with gold-to-gold contact. Also, there’s excellent conductivity between gold wire to the gold surface finish. Plus, there’s heat transfer rate, and adhesion is robust and occurs quickly.

Next is process flow and control efficiency and how they impact the success of microelectronics assembly. The PCB layout or IC design engineer creates the necessary specifications for pad definitions with appropriate tolerances listed; he or she then provides them in the form of fabrication notes. These notes define the best ways to fabricate a board.

Take fabrication tolerances, for example. Figure 1 shows the inconsistencies of two to three pads that should be equal in dimensions but are not. As shown, Pad #3 is only 0.060 mm while Pad #2 is 0.069 mm, a difference of over 12% in pad dimensions.

Zulki_Figure_1-650.jpg

In this case, the PCB layout engineer didn’t spec out the exact tolerances, which is routinely done in regular PCB manufacturing. Regular SMT manufacturing requires a ±1 mil, which is typically very tight tolerance-wise. The absolute maximum is the standard ±3 mils.

In microelectronics, tolerances are measured in microns rather than in mils, which is standard for SMT manufacturing. In microelectronics assembly, 1–2 mils could be too large of a tolerance to optimally perform these bonds. It also means avoiding the solder mask from encroaching on the bond pad, which is absolutely prohibitive. Laser direct imaging (LDI) is a methodology that is used for assuring that the solder mask is properly aligned with the bond pad. This allows die attach and wire bonding to be correctly performed.

Also, fiducial marks are of utmost importance for both SMT and microelectronics assembly. They are crucial for defining data point locations for component placement as part of the PCB SMT assembly. For microelectronics, a bare die should have fiducial marks because they allow the programmer or process engineer to perform the die-attach programming. This programming is based on a bond-pad diagram the OEM customer provides.

Precision in the pick-and-place operation is another point to make in defining process control. Consider specific areas from which to pick and place a die. For example, there can be a lens, sensor, or perhaps a camera function on a bare die. Hence, there is no discretion as to picking up that particular die. If it gets picked up from the lens or sensor area, the pick-and-place operation can damage the complete die even if highly precise tools are used. In those cases, those particularly sensitive areas on a die must be clearly defined in assembly descriptions and notes to avoid die damages.

Moreover, in the process flow and control area, an EMS provider must have in place an incoming fabrication quality control check. That is important because the QC check immediately determines the good from the bad PCBs. There can be a range of deficiencies and problems, including marginal boards in terms of pad size, solder mask issues, co-planarity, and/or contaminations, among others.

Lastly, after completing the microelectronics assembly, including die attach and wire bonding, there is a verification process. This is the critical step to ensure that assembly was properly performed. Highly advanced and sophisticated tools, like a Keyence digital microscope, are ideal for their very precise measuring capabilities, inspection, and verification for die-attach and wire-bond applications. These verification features are necessary to detect virtually any defect in the whole manufacturing process that is not visible using regular inspection tools.

Therefore, EMS providers engaged in PCB microelectronics must have on their assembly lines verification tools like this. High-caliber verification tools and disciplined inspection methodologies enhance the overall capabilities for performing successful microelectronics assembly.

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

 

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2020

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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