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5 - Thick-film technology

5.3 - Manufacturing technology

Fixing of SMDs on Thick film substrates
Possibilities to fixing components: Mechanical fixing (out-of-date, only on flexible foils)
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Conductive adhesives:
  • Isotropic (most frequently applied)
  • Anisotropic (mainly for chips)
Soldering (in case of polimers only on polyimide), reflow or laser technology Picture Discrete components can be mounted on thick-film substrates by reflow soldering technology (known from SMT technology). Contact surfaces made by thick film technology have adverse wetting behavior therefore, conductive adhesives are often preferred.
The soldering technology can be applied only in the case of thermal stable polyimide foils, (Max. treatment temp. of polyimid ~370oC)
Mounting of discrete components with conducting adhesives: In this process the mechanical fixing and electrical contact of the discrete components is assured by adhesives with electrical conductor behavior. The conductive adhesives are built up from two main phases, which are the resin base and the suspended conductor phase. The conductor phase of the adhesive (silver, gold, or nickel grains) is mixed into the resin base (epoxy or polyimide). These grains conduct the electric current. The conductive adhesives can be distinguished according to various aspects (e.g. commercial form, composition, the type of the resin etc.). According to their conducting behavior, we distinguish isotropic and anisotropic conductive adhesives.
The conductive behavior of the isotropic adhesives is equal in all directions (x, y, z). The isotropic adhesives need thermal handling at 140 - 150 oC / 30 minutes; or at 130 oC / 60 minutes; or at 90 - 120 oC / up to 100 minutes. The available form of these adhesives is mainly paste (rarely film).
Anisotropic adhesives: have specific conductive properties, namely they conduct the electric current only in one direction (z). In this case special gold coated ceramic grains are also used as conductor phase. The form of anisotropic adhesives is generally a dry film. The dedicated conduction direction is reached by the controlled conductor grain distribution of the film. The grains are embedded in a matrix arrangement. The fixing of a flip-chip IC by anisotropic conductive adhesive can be seen in Figures 5 and 6. First the anisotropic adhesive film is placed on the surface of the circuit. After it the flip-chip is pressed into the adhesive, meanwhile the system is heated up above the melting point of the adhesive. The unnecessary amount of the resin is flowing out from the volume between the contact surfaces meantime the conduction grains are blocked between the contact surfaces. Electrical contact occurs only in the direction perpendicular to the substrate (z).
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