Browse Prior Art Database

HIGH CONTRAST SOLDER PASTE

IP.com Disclosure Number: IPCOM000008918D
Original Publication Date: 1999-Jan-01
Included in the Prior Art Database: 2002-Jul-24
Document File: 2 page(s) / 125K

Publishing Venue

Motorola

Related People

Martin Mason: AUTHOR

Abstract

Solder Paste printing is one of the basic process- es which is used to assemble Printed Circuit Board assemblies. This takes the form of a screen-printing process using a metal screen and solder in the form of a paste. The solder paste is a complex formula- tion made from tiny spheres of a eutectic mix of tin and lead which is combined with fluxes and solvents to produce a thick gray paste suitable for the screen print process.

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M-LA Technical Developments

HIGH CONTRAST SOLDE~R PASTE

by Martin Mason

INTRODUCTION

  Solder Paste printing is one of the basic process- es which is used to assemble Printed Circuit Board assemblies. This takes the form of a screen-printing process using a metal screen and solder in the form of a paste. The solder paste is a complex formula- tion made from tiny spheres of a eutectic mix of tin and lead which is combined with fluxes and solvents to produce a thick gray paste suitable for the screen print process.

MANUFACTURING INSPECTION METHODS

  To minimize the occurrence of manufacturing quality defects found on soldered assemblies, it is important to inspect the screen printed assembly prior to component placement and reflow. Each component pad should have an even deposition of solder paste correctly align on top of the pad. This is achieved using one of the following three methods.

1. Visual Inspection

  This is the human eye type of inspection that a manufacturing operator/inspector would perform after the assembly is screen printed. Due to the small size of the features to be inspected (due to the ever decreasing small size of electronic compo- nents) and the lack of contrast between the gray sol- der paste, the silver surface finish on the board and the dark green laminate / solder mask material of the board, it can be very difficult to assess the quality to any reasonable degree.

2. Machine Inspection- Grey Scale Systems

  The automatic inspection systems that are avail- able on most screen printing machines and most stand-alone inspection machines are based around a

visual system. The machine's camera is used to look at a screen printed assembly and assess by use of a grey level scale, the position and area over which the solder paste has been deposited. As there is very little difference between the colors of the paste and board materials, once viewed through the black and white TV type image, the system finds it difficult to distinguish between the gray paste, the board and the surface fi...