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

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.

Wire bonding is an integral part of microelectronics assembly. It comes into play after attaching a die to a PCB; ceramic, glass, or aluminum substrate; or a rigid, flex, or combination rigid-flex circuit. Then, it’s wire-bonded using fine wires.

Protecting a bare die and its associated wire bonds is critical to ensure mechanical sturdiness and avoid and eliminate moisture. Consider that microelectronics assembly involves very delicate, fine wire—usually gold. Typical wire gauge is one, two, three, or five mils. Five-mil wire is usually used for high-current applications. Most often, wire gauge used is one mil; in some cases, sub-mil, like 7/10 of a mil, is also used.

Based on these highly delicate wires, both the die and the wire bonding itself must be protected from assembly processes, including handling, testing, and fixturing. When humans or machines even slightly and inadvertently touch this fine wire, the chances are great that it will be broken and destroyed. Also, the die and wire bonding must be protected against moisture. It can sneak in and create corrosion and oxidation on the die or substrate surface finish.

Two Protection Methods

There are two distinct sealing compound methodologies for protecting the die and wire bonding. One falls under a broad description of epoxy glob top encapsulation (Figure 1) with an auxiliary one called dam and fill as part of the glob top methodology. The second encapsulating protected method is lid and cover.

Zulki_Fig1.jpg

Figure 1: Glob top encapsulation.

Glob top epoxy material characteristics include non-conductivity, thermally conductive, low coefficient of thermal expansion (CTE), and good adhesion. These are important because when a die is encapsulated after assembly, you don’t want it electrically conductive. If it’s conductive, it’s going to short all the wires, and the assembly is destroyed.

However, at the same time, electrically, it should be isolated. But thermally, it should be conductive wiring because the heat is needed that’s generated by the wire bonding to be radiated to the ambient. After encapsulating the die and wire bonds, they are cured. Once cured, wire bond and die are protected.

Furthermore, as far as low CTE and good adhesion, OEMs should consider some key points. They want assurances that the low CTE allows the epoxy material to expand and contract on a small percentage in very low numbers. In other words, they don’t want too much degradation on the epoxy or the wires because they’re thermally expanding and cooling off rapidly. Also, there must be the utmost adhesive to the substrate material, whether it’s glass, copper, aluminum, or the PCB.

Dam and fill, which is part of the glob top methodology, is described as the name implies. The first step involves creating the dam or wall around the die and associated wire bonding by using a high-viscosity material. Then, the middle or cavity surrounded by the dam is filled with a low-viscosity epoxy. Thus, the high- and low-viscosity materials act as a highly effective protector of the die and wire bonding.

Lid and Cover

The lid and cover method is the other way of encapsulation. It can be a ceramic, plastic, or glass lid, depending on customer specifications and application. Such a lid can be soldered onto the substrate if the material is aluminum, nickel, gold, or hot air solder leveling (HASL).

In some cases, a specialized lid with B-staged epoxy is provided. Most likely, it is custom-made with epoxy already applied on the lid or cover. In this case, all that is needed is to cure it and then apply it around the die and wire bonds. While the lid-and-cover protection method isn’t as widespread as glob top, the lid-and-cover approach is used to some degree, depending on specialized PCB applications.

The main reason for not being as widespread as glob top is the fact a specialized lid and cover has to be tailor-made for a specific application. Plus, it has to be cured at a certain temperature. At times, a specialized curing methodology or machine is required, and sometimes, it needs specialized lighting for curing and meeting the UV light requirements.

On the plus side, the lid-and-cover approach using solder is more sealable compared to glob top epoxy. Thus, the lid-and-cover method is more solidly hermetically sealed, specifically if it is soldered on the PCB or a substrate. The solder doesn’t degrade over time, unlike the epoxy of glob top, which does degrade over time.

On the negative side, the lid-and-cover are sometimes custom-made, meaning it costs more money and takes more time. As a result, the lid-and-cover protection method isn’t as simple as performing the epoxy glob top method.

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

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2019

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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PCB Hybrid Manufacturing: Coming Your Way

05-15-2019

Today, PCB manufacturing is rapidly evolving to include SMT as well as microelectronics, such as chip-on-board (CoB) installation, flip-chip assembly, wire bonding, and die attach. SMT merged with microelectronics is also known as hybrid manufacturing. This is occurring because PCBs have begun shrinking at a faster pace in recent years due to the introduction of wearables, IoT devices, and portables demanding smaller circuit boards.

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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.

View Story

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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