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Zulki's PCB Nuggets
By Zulki Khan
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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.
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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