Emerald Group Publishing Limited
Copyright © 2000, MCB UP Limited
Pure silver flakes conduct electricity and heat
Pure silver flakes conduct electricity and heat
Keywords: Loctite, Adhesives
New electrically conductive adhesives to replace solder on temperature-sensitive components and impossible- or difficult-to-solder substrates are now available. Although they can be used for conventional board production, they have been primarily developed to facilitate assembly of the latest flexible circuitry and chip-on-board (COB) and chipon-glass (COG) technologies.
There are five isotropic (i.e. conductive in any plane) and two anisotropic (i.e. conduct current in the Z-axis only, whilst remaining insulators in the X-Y plane) grades.
All isotropic grades are silver-filled epoxy adhesives with a common thermal conductivity of >1.5 W/moC, and the standard advantages of substrate compatibility, low-temperature/no-clean processing, freedom from lead, high fatigue-resistance, good electrical conductivity, and a wide operating-temperature envelope. Each adhesive, however, has been formulated for specific bonding tasks and application methods of the electronics industry (Plates 8 and 9).
Loctite 3880, for instance, is a fast-curing, one-part product which can be dispensed by syringe or stencil, for applications where solder cannot be used (e.g. mounting of indicator diodes to flex indicators). Its heat-cure profile is flexible: typically, users have the options of ten mins @ 125°C, six mins @ 150°C or three mins @ 175°C. Fully cured, it has high tensile strength (circa 25MPa) and typical lap shear strengths (MPa) of 4.8, 3.1, and 2.8 on copper, glass and aluminium, respectively. Volume resistivity is <0.002 Ohm-cm, and operating temperature ranges from minus 60°C to 150°C.
Plate 8Electrical Conductivity of anistropic adhesives is achieved by contact of the adhesive with substrate and bumps on flip chip or flex circuit, after heat and pressure have been applied to remove the insulting epoxy matrix from conductive particles
Plate 9Loctite's silver-filled isotrophic thermally and electrically-conductive adhesives facilitate the assembly of temperature-sensitive components, flexiable circuits and difficult-to-bond substrates
Loctite 3882 is a very flexible conductive adhesive designed for applications, where high flexibility is required. Compatible substrates include metals, ceramics, glass, laminates, and moulded plastics. It can be cured at room temperature or by relatively low heat schedules (typically two hours at 65°C or one hour at 110°C); and applied by syringe, stencil, screen-printing, or - for modifications and repairs - manually. Its volume resistivity is < 0.001 Ohm-cm, and operating temperature extends from minus 60 to 110°C.
Loctite 3883, distinguished by a low CTE (20ppm/°C) and low ionic content, is for stencil or screen-print die attach applications. Heat cure requires two hours at 100°C or two to three hours at 60°C. Volume resistivity is < 0.0009 Ohm-cm; operating temperature is from minus 60 to 150°C.
Loctite 3887 although specifically developed for the strong bonding of gold-plated parts in electronic interconnect applications, also provides strong, durable bonds on many different substrates, among them ceramics, glass, laminates and moulded plastics. A notable property of this adhesive is its ability to absorb stress caused by thermal mismatch between dissimilar substrates. It can be dispensed by syringe and stencil (for which its long "open" life makes it particularly suitable) and when fully cured (one hour at 125°C or 30 minutes at 150°C) provides good mechanical properties. Volume resistivity is < 0.0003 Ohm-cm, and operating temperature is minus 60 to 135°C.
Loctite 3888 was developed for re-work, repairs, modifications and prototypes. Applied manually, it has a pot life once mixed or thawed of 90 minutes at 20°C, and cures at room temperature (or three hours at 65°C). It has a multi-substrate compatibility, and produces bonds with good mechanical properties (e.g. aluminium to aluminium lap shear strength of 12.4 MPa.). Volume resistivity is 0.0003 Ohm-cm, operating temperature range is minus 60 to 125°C.
The two new anisotropic adhesives, Loctite 3440 and 3445, have been produced for flip chip interconnection (e.g. COB and smart card), TAB Interconnections, and high-density flexible printed circuits. 3440 is recommended for LCD assembly and can also be used on glass substrate, whereas 3445 cannot be used on glass. Their ability to conduct electrical current solely in the Z-axis is achieved by carefully controlling the level of electrically conductive filler in an epoxy matrix. In 3440 the filler comprises mono-dispersed gold-coated polymer spheres; 3445 uses SnBi (tin/bismuth) fusible particles. Electrical conductivity is achieved by the continuous contact of the fillers with the bumps and pads of opposing substrates, after heat and pressure have been applied to crush the entrapped spheres and particles. The epoxy matrix surrounding the free-floating, uncrushed filler elements acts as an insulator, preventing current flow in any other direction.
Loctite 3440 and 3445 share many advantages: both can be applied by stencil and screen-printing, have stable electrical contact resistance when subjected to humidity and thermal cycling, provide strong, easily-repairable bonds, facilitate fast processing and easy assembly, and can be printed to a fine pitch. During extensive field trials, pitches of 100 micron (90 micron bumps with ten micron separation) without short circuits became a test norm (though liner pitches can be achieved).
Cure requirements for the two new anisotropic adhesives differ in the pressure needed: the recommended bond-line cure temperature for both is of 180°C for 60 seconds, but 3440 needs a pressure over the component area of 1.5 Nmm-2 and 3445, 0.7Nmm-2.
Shorter initial cure times can be used where the bond will be subjected to a post-cure treatment. Higher cure temperatures will also reduce cure time.
In addition to its new electrically adhesives, Loctite also has an existing range of fast-curing thermally conductive adhesives which provide high and medium strength, self-shimming bonds with thermal conductivities extending up to 1.65 W/moC.