UNIFLOW
®
2 PULSED THERMODE CONTROL
990-228
E-1
APPENDIX E
GUIDELINES FOR REFLOW SOLDERING
Overview
This Appendix covers the process definition and components used in the pulse heated reflow soldering
of flexible circuits to printed circuit boards. We will look in depth at the component part design criteria
necessary to achieve the optimum quality and consistency of a flex to PCB assembly, concentrating on
the most popular joint designs and component parts used in the process today.
Definition Of Process Technology
Pulse heated reflow soldering is a process where two pre-fluxed, solder coated parts are heated to a
temperature sufficient to cause the solder to melt, flow, and solidify, forming a permanent electro-
mechanical bond between the parts and solder. Pulse heated soldering differs from the traditional
soldering process in that the reflow of solder is accomplished using a heating element called a thermode
which is heated and cooled down for each connection. Pressure is applied during the entire cycle
including heating, reflow, and cooling. A pulse-heated control delivers energy to the thermode, which is
mounted on the reflow soldering head. A thermocouple, attached to the thermode, provides feedback to
the control for repeatable, consistent heat generation.
The soldering head brings the two parts into intimate contact. At a precise pressure the head signals the
control to begin the heating cycle of the thermode. The thermode conducts heat to the parts, and the
subsequent thermal transfer of heat melts the solder between the parts. The molten areas begin to flow
resulting in coalescence between the two solder masses. When the reflow cycle is terminated by the
Control, the parts continue to be held together during the cooling cycle such that the solder re-solidifies
and a joint is formed. A good solder joint is defined as one where the solder adequately joins both
surfaces and wetting (flow of solder) has occurred on both part surfaces.
Flex Components
The most common type of flex used in the pulse heated reflow soldering process is manufactured from
polyimide (also known under the trade name of Kapton). Two layers of polyimide encapsulate the
copper traces (normally 0.5 – 2 oz). The two most common copper conductors are rolled annealed (RA)
copper and electro-deposited copper (ED). ED is most cost effective and widely used. The thickness of
copper traces ranges from 0.0007 – 0.004 inches. The polyimide can have operating temperatures
ranging from 130 – 200
q
C and withstand soldering temperatures up to 300
q
C for a short time. The
temperature of the thermode is always higher than that of the parts that are heated by the thermode. A
temperature drop of 50-80ºC can occur between the thermode and joint, across the Kapton flex,
depending on the thickness. Thickness of the flex ranges from 0.001 – 0.0047 inches.
