Jaakko Lenkkeri

Thermal fatigue and metallurgical reactions in solder joints of LTCC modules

Abstract The fatigue behaviour of solder joints in ceramic LTCC modules on FR-4 boards was investigated using thermal fatigue experiments at temperature intervals of 0 – 100°C and −40 – 125°C, and non-destructive and SEM/EDS investigations of thermally cycled modules. Dissolution reactions and formation of tin-based intermetallics were encountered in the joints and found to cause degradation in them. A high peak reflow temperature in the fabrication of modules had a detrimental effect on performance. Vias filled with silver located under the central regions of the solder pads of the LTCC modules enhanced the reliability of the solder joints.

Laser-assisted direct writing of strain sensitive silicon resistors

Abstract The laser-induced deposition of doped silicon lines from mixtures of silane and diborane using an argon-ion laser is reported. The piezoresistive properties of the lines deposited on different substrate materials, such as sapphire and silicon-on-sapphire (SOS), are investigated because of their potential use as pressure sensor elements. The lines deposited on sapphire, using a gas mixture that contains 1% diborane in silane, exhibit polycrystalline characteristics with gauge factors less than about 20. The lines deposited on SOS substrates, on the other hand, show anisotropic characteristics in their piezoresistivity properties. The maximum gauge factor in this case is of the order of 44. Still higher gauge factors up to 77 are obtained using a gas mixture with 0.1% diborane in silane. The use of this technology for pressure sensor development is being studied by the construction and testing of diaphragm-type pressure sensor prototypes.

Reliability of adhesive joints in dual interface smart cards

In dual interface smart cards the adhesive joints between the micro module and card body are critical points for the reliability. There are many issues, which affect the reliability of the joints. The choice of the electrically conductive adhesive is one of the key aspects. In this study three different anisotropically conductive adhesive films and one isotropically conductive adhesive paste has been tested by several methods including e.g. bending, torsion, thermal and humidity tests. The Matlab model has been developed for studying the factors influencing the reliability of adhesive joints in smart cards for thermal cycling and cyclic bending type deformations. Assuming viscoelastic behaviour for the adhesive material and Coffin Manson type failure criterium for the joints the model is able to give information about the effects of different parameters on the durability of the adhesive joints. The experiments carried out show that there are very large differences in reliability between different types of adhesives. For anisotropically conductive adhesives the type of connection pads in the card body is also very important. The connections made with isotropically conductive adhesive are very resistant for bending deformations of the card.

Nd: YAG Laser Processing for Circuit Modification and Direct Writing of Silicon Conductors

The use of focused laser beams makes possible delicate and flexible fabrication and modification of integrated circuits and components. This has clear benefits for small-scale production of circuits and for prototype fabrication. It also makes it possible to do trouble-shooting and to repair circuits and it improves the ability to test large circuits. The laser can be used to cut conductors on integrated circuits using either a normal air atmosphere [1,2] or an etching gaseous or liquid medium [3,4]. Using a laser pulse or pulses it is possible to destroy the thin insulating layer between two conducting layers and in this way accomplish an electrical contact between the conducting layers [5]. Other methods of making electrical connections with lasers have been reported [6–9]. The energy of the laser beam can also be used to deposit layers of materials through chemical reactions in laser induced chemical vapour deposition (LCVD) [10].

Laser-Induced CVD Of Doped Silicon Stripes On SOS And Their Characterization By Piezoresistivity Measurements

An argon-ion laser with the visible cw radiation was used to deposit highly doped p-type silicon stripes on (1102) sapphire substrates covered by a 1 μm thick epitaxial silicon layer (SOS). The resistivity of the stripes deposited in an atmosphere of silane with 1% diborane was 0.5 to 2.5 mΩcm. The laser-induced pyrolytic deposition of silicon on SOS was analyzed using numerical simulations of the temperature distribution caused by a focused laser beam. Comparisons were made between experimental and calculated cross-sections of deposited stripes. The measurements of piezoresistivity were carried out by controlled bending of samples of rectangular shape upon which the stripes had been deposited. The stripes deposited along various directions on the substrate revealed anisotropic behaviour in their piezoresistivity properties. The results show that the stripes tend to adopt the same epitaxial crystalline orientation as the SOS layer. The longitudinal gauge factor along the [110] direction of silicon, which direction is normally used in strain sensor applications, varied between 15 and 44 depending on the laser power applied. A local temperature near the melting point of silicon seems to be needed to assure the epitaxial growth.