Assembly Operations

Medical Applications

The use of adhesives in medical applications was for a long time restricted to the manufacture of self-adhesive bandages (plasters, self-adhesive strips of fabric, etc.). The first pressure sensitive adhesives used for this were based on natural rubber. This was in part later superseded by synthetic rubbers (e.g. polyisoprene, polyisobu­tylene). In the middle of the 20th century, pressure sensitive adhesives based on polyacrylic acid esters became increasingly important, both for general use and for bandage materials.

Adhesives are today employed in diverse areas of medicine, replacing traditional methods with “friendlier” processes. In many cases, for example, stitches can be avoided by applying special cyano­acrylate adhesives to quickly close skin wounds. An advantage here is that the whole wound can be covered, so largely suppressing secondary bleed­ing and the risk of infection. Cyanoacrylic acid butyl ester is normally preferred over the methyl and ethyl esters because it cures more slowly and the polymerisation produces less heat; it also causes less tissue irritation. By and large, this adhesive is only used for relatively small wounds and occasionally in vascular surgery.

Consider the transdermal patch…where the drug delivery mechanism to the blood stream is through the skin. Adhesives enable a more efficient method of drug delivery rather than prescribing a high-ingestion dosage (which is subsequently flushed out of the body by the liver). Transdermal patch technology is highly effective because the adhesive that sticks the patch to your skin – made of products of chemistry like acrylic, acrylic-rubber hybrid, polyisobutylene and styrenic rubber solution – actually controls the rate at which the drug enters the body. This ensures that the drug dosage is continuously and evenly administered throughout the day, without the spikes and falls associated with medicines taken orally.

The applications are many…smoke cessation, hormone replacement, and cardiovascular aid (I.e. nitroglycerin delivery) are commonplace. New transdermal patches hit the market every day with more items like pain cessation becoming a reality. Other innovative products such as foot care and cosmetic patches, and nasal dilator strips have hit the market in recent years all possible because of the unique functional properties of the pressure sensitive adhesives integral to the product.

One of the newest bioadhesives on the market enables drugs to be delivered through the inside of the mouth, nasal passages and other mucus membranes instead of just through skin. It adheres extremely well to the soft, wet mucus membranes of the body because of adhesives made from starch-polyacrylic acid blends, which then completely erode and disappear. Drug makers are able to put their medicine into tablet, film or powder form, and the patient is able to attach the product directly to a mucus membrane, providing a means for controlled delivery of drugs to specific areas of the body or systemically (throughout the body).

In heart surgery, fibrin (made from fibrinogen), a soluble protein recovered from blood, is a key sealing agent having a haemostatic effect. Compared to cyano­acrylates, fibrin is gentler to body tissue but before use it must undergo a special treatment to prevent germs being spread. The use of methacrylate based adhesives has been a great success in orthopaedics for anchoring hip socket implants to the bone (see figure below). There are currently no other types of adhesive used for this application. The adhesive products comprise a.) a powder component (a mixture of polymethyl methacrylate and a polymeri­sation initiator) and b.) a liquid component (whose main components are methyl methacrylate and a polymerisation accelerator). Although this type of adhesive puts a not inconsiderable stress on bone and tissue due to the strong heat development, hip and knee implants anchored using this adhesive are in 90 percent of cases functional for about 15 years.

In dentistry, fillings based on UV curing acrylates have largely replaced traditional filling materials such as amalgam. The products have a long open time (the period during which they can be used after mixing) and bond in just a minute or so when exposed to UV light.

Electronics Applications

For purposes of this website, the Electronics Market is defined as anything containing a microprocessor chip, including but not limited to:

Cell Phones and PDAs TVs, Radios, VCRs, DVDs Anything containing a Printed Circuit Board

Five specific areas discussed in more detail below are:

  1. Things Powered by Electric Motors
    (note: you may find that other reference sources categorize "electric motors" under Assembly (Non-Rigid) since the product which the motor is powering is in and of itself a durable "assembled" rigid product, such as a kitchen appliance. For simplicity, we have grouped "electric motors" in with "electronics.")
  2. Electronic Circuitry
  3. Passport Chips
  4. DVDs
  5. Cell Phones

Things Powered by Electric Motors...

A whole host of electrical devices, from kitchen appliances to CD-players and video cameras, contain electric motors. These mass-produced motors are expected to have precise synchron­isation and a long lifetime. Adhesives are nowadays indispensable in motor assembly, for example for connecting the armature and shaft, for connecting the commutator and shaft, for attaching ball-bearings and for securing screws in position. The preferred type of adhesive for this is an anaerobically curing acrylate adhesive, namely an acrylate adhesive that cures when oxygen is excluded.

Electronic Circuits

From the very outset, the rapid developments in the area of electronics have involved ever smaller components and ever greater circuit complexity. The continuing miniaturisation has not only resulted in smaller components but also components that are more sensitive to heat. For cost reasons, it was also necessary to reconsider the methods used to assemble printed circuit boards (PCBs). The solution to these problems was advanced bondingtechnology. Adhesives have hence taken over many tasks in the assembly of electronic circuits. Besides being used for mechanical attachment, they also function as for example electrical conductors and insulators. Today, many electronic components are fixed in their intended positions with adhesives, prior to soldering.

Single-component paste-like epoxy resin adhesives are used. These adhesives bond in 3 minutes at 120°C. Acrylate adhesives are used to a lesser extent. The latter are pre-cured by UV light to allow rapid fixing. Their final strength is then developed by applying heat.

Electrically conducting adhesives are normally epoxy resin adhesives containing a very small amount of ionic contaminants. To a lesser extent special polyimide adhesives are also used. In both cases these adhesives contain 70 to 80 weight percent of very fine silver powder. They are used when components cannot be soldered due to their sensitivity to heat, especially when repairs are being carried out. In addition, adhesives with heat conducting properties (e.g. adhesives containing high amounts of aluminium oxide) are growing in importance due to the increasing requirements on the heat management of circuits.

Chips in Your Passport

In addition to smart chips, today’s passports contain so-called Die Attach Adhesives. With their help, the chips containing the necessary biometric data, such as personal details and the digital visual image, are glued directly casing and then the electronic connection is made with wires. By connecting wirelessly to an antenna, the chip transfers the relevant data to a reader at passport control.

The capture, storage, and retrieval of personal data stored on travel documents, tickets for big events like the Super Bowl or business inventory is becoming increasingly important. A large amount of data can now be quickly recorded and processed. In particular the increasing need for security in international travel and in enterprise resource planning have increased this demand.

The existing technology in the chip-on-board (COB) process produced smart cards; however, these proved too elaborate and expensive for widespread application in disposable articles, such as tickets. But then the new "flip chip" Technology paved the way for mass market applications.

The chips were smaller and could be placed diretly on so-called smart labels (thin, containing plastic film antennas) and electrically contacted. The production costs are significantly lower than for plastic cards. Significantly, the chip stays on the plastci card and conductds electricity to allow the flow of data. This technology was realized with an adhesive that was specially developed for this application.

This utilizes two different types of contact: on one side a conducting, anisotropic adhesive and on the other side a non-conducting underfiller were developed. The high numbers achieved through the development of adhesives that harden in 4 to 10 seconds and the low production costs have opened new areas of application.

Manufacture of Digital Versatile Discs (DVD)

Unlike a Compact Disc (CD), a DVD has a sandwich construction and is namely a bonded object. Various designs are possible, with information storage on one or both sides. UV curing acrylate adhesives are used for bonding the DVD compo­nents. These adhesives bond under the influence of UV light in a few seconds and besides having high optical purity the bonds have very good long-term stability under warm humid conditions. Special hotmelts based on thermoplastic elastomers are also gaining in importance in this field.

Adhesives in Cell Phones: "Bonded Communications"

Some inventions become indispensable within a very short time. And the cell phone is one of them. How did we ever get along without them? Hardly anyone had one in the 1980s, but by the end of the 1990s they were everywhere. Now, one is always within easy reach. But nobody realizes that up to 21 different types of adhesive foil are used in cell phone production today.

The cell phone has been an indispensable tool of daily life since the late '90s. Business people on the go are reachable thanks to cell phones; if an accident happens, help is just a phone call away; if you are running late, a quick text message keeps your friends and family informed.

But why are cell phones getting cheaper, better and smaller? The answer is simple: Adhesives.

What modern adhesive films, composed of epoxy resin, silicone or polyurethane, can accomplish is amazing. You can produce electrical connections between two or more contact points or replace small screws and solder joints. The bond is flatter, less complex and cost-intensive. In comparison to solder joints, adhesive film is lead free and less vulnerable to heat, thereby reducing the risk of thermal damage.

There are two types of electrically conductive adhesive film:

  1. Isotropic conductive adhesive: In this type of adhesive film, the current can flow in all directions. This is how it works: Electrically conductive adhesive particles are pressed into the contacts during the drying process,allowing an electrical connection. The minimum distance between two contacts is only 0.035 mm. Incidentally, isotropic conductive adhesive film is used in EMI shielding (electromagnetic interference).
  2. Anisotropic conducting adhesive: This type of adhesive film allows power to flow in only certain directions.Micro-particles are embedded in the adhesive, which are compressed between two contact points, simultaneously generating insulation between the individual points.

Further adhesive film in cell phones protects against shock and prevents the permeation of moisture. And adhesive film protects the screen from unpleasant scratches, ensuring untarnished communication.

Smaller, Faster, More Powerful...

Without adhesives we would live in a world of electronic chaos. Conductive adhesives combine different materials quickly, safely and permanently -- even in small areas.

Micro-assemblies, applications made possible in ever-smaller electronic modules and components, are only possible with adhesive. Adhesives have always played a role in the documentation and dissemination of information. Their significance has been shown and proven through the millennia -- from papyrus of the ancient Egyptians to the navigation system, laptop and plasma screen TV of today.

Additional Resources

Electrical/Electronic End-User: Cost of Lead-Free Solder Materials (Article courtesy ASI magazine) European implementation of the Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (RoHS) directives, coupled with the adoption of lead-free soldering by Japan’s electronics industry, has created unavoidable costs for the global electronics industry. Following are examples of costs associated with the implementation of lead-free soldering. One-time costs include investment in new equipment, changing bill of materials, WEEE and RoHS licensing and compliance charges....Learn more.