Implementation of Low Power Diode Laser for Soldering by FPC ( Fibre Push Connection) Method

Abstract

The advances in miniaturisation and ever increasing complexity of integrated circuits frequently mean an increase in the number of connections to a component with simultaneous reduction in pitch. For these emerging smaller contact geometries, micro‐laser connection technologies are required. The reliability of the connection plays a decisive rôle. The implementation and reproducibility of laser connections technology in micro‐electronics depend on good thermal contact between the two parts and high quality absorption of the material surface used. Laser energy can cause local melting due to overheating of the lead because of the low distance between lead and bump. This effect influences the reproducibility of the contacts. Even the slightest interruption in the thermal contact of the parts can cause non‐reproducibility of the contacts. Materials with a higher quality of absorption, for example Sn(32% ), can be soldered with a good level of reproducibility. This clearly differs from gold (4% ) or copper(7% ) surfaces. Due to the low absorption of these materials it is necessary to use a laser with a higher intensity to produce the same energy. Irregularities in the quality of absorption, laser instability and thermal contact can not guarantee reproducibility of the interconnections with this high laser intensity. The FPC (fibre push connection) system offers several solutions to the problems mentioned. This system enables the laser to be transported by fibre to the contact parts. The end piece of the fibre serves at the same time as a pushing unit. The advantage of this system is that the attenuation heat of the fibre end surface is also available for the connection. This improves the use of laser energy. As part of the laser energy at the end surface of the fibre is transformed into thermal energy, independently of the absorption quality of the material used, connection of a gold‐plated contact part is possible. By pressing the connecting parts with the tip of the fibre, optimal coupling is achieved. The reproducibility of different metallisations and the reliability of connections with a pitch below 100 μm are presented as well as further applications of this system.

Fluxless Flip‐chip Bonding on Flexible Substrates: A Comparison between Adhesive Bonding and Soldering*

“…In order to avoid heating the flexible circuit with the thermode during collective FC assembly, the use of a laser as an alternative joining method was investigated. The FPC (fibre push connection) is a suitable laser connection method, G. Azdasht et al 13, 14 showed that the FPC offers several solutions with regard to the problems mentioned. In this system the laser beam is transported by a glass fibre to the contact parts.…”

“…Laser soldering using an Nd‐YAG laser or diode lasers has been successfully applied for TAB inner lead and outer lead bonding 14, 20. A new approach in using laser energy developed by IZM‐Berlin consists in coupling laser energy through the flexible circuit to the soldering interface.…”

“…FC bonding with Au bumps to Sn or Sn/Pb‐substrate pads can be performed without flux14 and at lower temperatures of 180‐250°C according to the phase diagram between gold and tin. However, the metallurgy in the Sn‐rich part of the Au‐Sn system is more critical.…”