Microphones are a part of our daily life. They are seen in many applications such as telephones, hearing aids, public address systems, sound recording, two-way radios, megaphones, radio and television broadcasting. Too widely use to the point where we’re not paying attention to their existence anymore.
As widely used as microphones are, the manufacturing process is very complicate. Let’s dive in and see how these little helpers are made and make our world a better one.
The image below helps you to have an overall understanding first.
Here comes the detailed explanation of the manufacturing process of microphones.
1. Wafer foundry
A wafer (also known as a slice or substrate) is a thin sheet of semiconductor material, such as a crystalline silicon.
The wafer are used as the substrate for microelectronic devices built into and out of the wafer, which is made by many microfabrication process steps such as doping or ion implantation, cutting, barreling, slicing, chamfering, polishing, and laser engraving
2. Wafer testing
In this step, the functional defects of all individual integrated circuits present on the wafer are tested by applying a special test pattern to the wafer before sending the wafer to die for preparation.
Wafer testing is done by a test device called a wafer prober. There are several referable ways for wafer testing: Wafer Final Test (WFT), Electronic Die Sort (EDS) and Circuit Probe (CP).
Surface-mount technology (SMT) is a method for fabricating electronic circuits in which leadless or short lead surface mount component (SMC/SMD) are mounted or placed directly onto the surface of printed circuit boards (PCBs) or other substrate.
It’s a circuit assembly technology by reflow soldering or dip soldering. SMT process consists of the following parts: older pads printing, pasting, reflow soldering, installing, and cleaning.
3.2 Adhesive bonding
Adhesive bonding (also referred to as gluing or glue bonding) refers to the technology of bonding the surfaces of homogeneous or heterogeneous objects together with adhesives which is a class of organic or inorganic, natural or synthetic substance with sufficient strength after curing. A few well-established adhesives such as SU-8, and benzocyclobutene (BCB), are specialized for MEMS or other electronic components.
3.3.1 Die attaching
Die-attaching is a key part of packaging process. It’s how the face of a die is attached to a substrate by a single joint. IC die adhesive is a room temperature curing epoxy resin adhesive, widely used in the bonding of electronic components. It has excellent bonding strength for package bonding among metal, ceramics, glass and hard plastic.
3.3.2 Wire bonding
Wire bonding is a method that weld fine metal wires to the substrate by heat, pressure and ultrasonic energy, which results in the electrical interconnection between chips and substrates and information interchange between chips. There are typically three wire bonding methods used in industry: hot-pressing wire bonding, tincture-tincture ultrasonic wire bonding and thermoacoustic wire bonding.
3.3.3 3-D microscopic examining
During this process, we use 3D microscopes to make sure the above steps are done accurately. For example, the cracks or dents are not allowed.
In this process, EMC (Epoxy Molding Compound) is used to encapsulate the finished products of wire bond to prevent the impact of the external environment. The main steps are:
- Lead frame is placed in the mold, each die is in cavity, and the mold is clamped.
- The block EMC is put in the middle hole of the mold.
- At high temperatures, EMC melts and flows along the track into cavity.
- Covering the chip starting from the bottom.
- Molding and curing after the coverage.
3.3.5 Laser Marking
Laser marking refers to a permanent mark leaving from the chemical reaction applied by laser that makes the surface material vaporize or undergo a color change. Laser marking can produce a variety of text, symbols and patterns, etc..
3.3.6 Post mold curing
Post curing is the process of heating the coating and keeping it at constant temperature for a period of time after the adhesive has cured at room temperature, which will expedite the cross-linking process and properly align the polymer’s molecules.
Depaneling is a process step in high-volume electronics assembly production. In order to increase the throughput of printed circuit board (PCB) manufacturing and surface mount (SMT) lines, PCBs are often designed so that they consist of many smaller individual PCBs that will be used in the final product. This PCB cluster is called a panel or multiblock. The large panel is broken up or “depaneled” as a certain step in the process2．
Test items of microphone are: frequency response in-axis and off-axis, sensitivity, distortion, signal-to-noise ratio, detection of audible imperfections, directivity, polar plot, polarity.
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