Browse Prior Art Database

PROGRAMMABLE REFLOW SYSTEM

IP.com Disclosure Number: IPCOM000005830D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2001-Nov-09
Document File: 4 page(s) / 195K

Publishing Venue

Motorola

Related People

John Thome: AUTHOR

Abstract

A system is described for a closed loop reflow system which allows the same profile to be used for different board types, This is accomplished by the use of a programmable heat source which provides local preheat to the portions of the board having high thermal ballast.

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m M-ROLA Technical Developments Volume 10 March 1990

PROGRAMMABLE REFLOW SYSTEM

by John Thome

Abstract

   A system is described for a closed loop reflow system which allows the same profile to be used for different board types, This is accomplished by the use of a programmable heat source which provides local preheat to the portions of the board having high thermal ballast.

Introduction

   Assembly of present high circuit density printed circuit boards (PCB's) is most effectively accomplished by a proc- ess known as reflow soldering. This process involves the application of solder as a paste to the printed circuit board, then placing electrical components into the paste, and finally causing the entire assembly to be electrically joined by heating the assembly in an oven until the solder melts and wets the metallized areas on both the PCB and the com- ponents. In production operations, this oven uses infrared (IR) heaters and has a tunnel configuration with a conveyor for passing boards through the heated area as is depicted in Fig. 1. A problem encountered in the reflow soldering of printed circuit boards is the necessity for changing the oven settings in order to compensate for differences in thermal mass of different PC boards. A second problem is caused by differences in thermal mass of different regions within a PCB. This requires the incorporation of a long stabilization time in the heating of the board to allow the higher mass regions of the board to "catch up" to the lower mass regions. The stabilization period is followed by atemperature "spike" designed to raise the temperature of the board above the soldering temperature and quickly cool the assembly to minimize thermal damage to components. In a conveyor oven, this stabilization zone adds significantly to the cost of the oven and to the time required to perform the soldering process. The lighter portions of the PCB experience high temperatures for extended periods of time causing a degradation of the reliability of the solder joints because of the formation of undesirable intermetallic compounds. For these reasons, it is desirable to execute the soldering process in the minimum time required to accomplish wetting of the metallic surfaces.

Similar problems exist for the preheating of PCB's for solder wave soldering. Differences in localized thermal ballast result in localized non-uniformities which in turn, result in inconsistencies in the soldering performance or reliability.

   Numerous measures are applied which are directed at addressing these problems. These measures improve the thermal transfer to the PCB, thereby reducing the time required to accomplish the reflow or preheating process. Most notable, and also most effective at minimizing overheating of lighter areas of the board is vapor phase soldering. This method utilizes the latent heat of condensation to transfer heat to the PCB, thus the board temperature never exceeds the boiling temperature of the heat transfer liq...