GB2325092A - Preventing distortion during the manufacture of flexible circuit assemblies - Google Patents

Abstract

In a process for assembling components to flexible copper-laminated polyester circuit boards (A), high temperature-resistant adhesive tape (C) is placed around the printed circuitry in order to prevent distortion of the boards during a soldering stage. Certain components may also need to be held by retention tools which exert a force in the downward direction only (Figure 3, not shown).

Description

CONTROLLED REFLOW OF FLEXIBLE COPPER LAMINATED POLYESTER CIRCUIT ASSEMBLIES This invention relates to a production process enabling the high volume attachment of electronic and mechanical components to flexible copper laminated polyester circuits.

Flexible copper laminated polyester electronic circuits are available commercially and comprise of copper foil bonded to polyester. The use of these circuits is currently restricted to inter-connect jumpers in electronic assemblies and circuits with hand soldered electronic components attached (which results in a very labour intensive procedure not suitable for high volume mass production) . When component carrying flexible electronic circuits are required, a more expensive material is generally utilized, this material has been used for several decades and has a higher temperature capability, thus allowing the use of conventional re-flow solder processes to be used for component connection.

An object of this invention is to provide a process that will enable the high volume attachment of electronic components to polyester substrates using conventional reflow soldering technology.

Accordingly this invention provides a process that will enable volume manufacturing to be utilized for the automated re-flow of conventional solder paste, producing a conventional metal alloy joint between component termination and copper current carrying tracks laminated to the polyester substrate, without unacceptable distortion to the polyester.

Most electronic components can be attached such as Surface Mount Chip Components as small as 0402 packages, QFP, SOIC and PLCC packages.

Some fine pitch components may require a secondary reflow with the use of a fixture to apply pressure during the liquidus stage. Through Hole components can also be attached with the introduction of special positioning tooling.

A specific embodiment of the invention will now be described by example with reference to the accompanying drawings in which: Figure 1 shows the copper laminated polyester circuit array precisely positioned on the custom designed and manufactured carrier and also details the placement of the retaining tape required to hold the copper laminated circuit array in position.

Figure 2 shows a typical re-flow oven profile when re-flowing conventional tin-lead solder paste on copper laminated polyester.

Figure 3 shows a typical reflow jig for applying pressure during the soldering process when attaching fine pitch components (0.5mm pitch).

As shown in Figure 1 the copper laminated polyester circuit arrays [a] require precision placement onto a carrier lb] which is suited to the assembly line and manufactured to within + or - 0.1mm in all directions. The layout and number of circuits on an array having been pre-determined at the design stage to allow the minimum of tolerance build-up and maximum solder paste printing accuracy.

The accurate placement of the circuit array may require the assistance of an optical alignment jig to allow the repeatable positioning of the fiducial marks.

The high temperature retaining tape [c] (a recommended tape is polyester with silicone adhesive) is then strategically positioned to provide the maximum flatness of the circuit arrays [a] and in addition permit even movement of the squeegee throughout the printing process.

Conventional tin-lead solder paste is then printed onto the circuits and placement of the components can now take place, accurate positioning of each component is essential.

For fine pitch components such as Quad Flat Pack Integrated Circuits with a minimum lead pitch of 0.5mm, special retention tooling, as illustrated in Fig. 3, may be required to eliminate unevenness of the substrate and component co-planarity concerns by exerting sufficient downward pressure to ensure the electrical and mechanical integrity of the joint This tooling is designed such that only a force is exerted in the Z axis none in the X or Y axis (this would lead to component shift and electrical shorts).

The carrier [b] complete with circuit array [a] is then fed into a multi-zoned reflow oven with a profile set up specifically for that array. The profile will vary according to the circuits, pallet and jig construction and the mass of the components placed on the circuits.

A recommended profile, as shown in Fig. 2 would be to ramp up to a maximum of 230"C ensuring the temperature of the substrate does not exceed 200"C and that the circuit is subjected to at least 15 seconds over the liquidus temperature of the solder paste in the reflow zone.

The array is then reflowed and allowed to cool until the polyester substrate has stabilized.

In certain circumstances fine pitch components and circuit design may dictate that a second reflow is necessary. This is achieved by applying a liquid noclean flux and passing the assembly through the oven again using the reflow jig to maintain even pressure and ensure good joint integrity.

The circuit arrays are then retained on their carrier for in-circuit and functional testing purposes, then removed from the carrier to allow circuit separation and profile cutting.

Claims (3)

1. A process utilizing restraining high temperature adhesive tape, fixtures and exacting process control enabling all surface mount components, down to package sizes of at least 0402 and lead pitches of at least 0.5mm, to be electrically and mechanically attached to a copper laminated polyester electronic circuit without unacceptable distortion to the polyester substrate utilizing conventional tin-lead solder and traditional printed circuit board assembly equipment without hand soldering.

2 Most Through Hole [Leaded Components], using a process as in claim 1, can be electrically and mechanically attached to a copper laminated polyester electronic circuit incorporating special retention tooling without unacceptable distortion to the polyester substrate utilizing conventional tin-lead solder and traditional printed circuit board assembly equipment 3. Most Wire Leaded components can be electrically and mechanically attached to a copper laminated polyester electronic circuit, using a process as in claim 1, incorporating special retention tooling without unacceptable distortion to the polyester substrate utilizing conventional tin-lead solder and traditional printed circuit board assembly equipment Amendments to the claims have been filed as follows CLAIMS 1. A process and method permitting precise positioning and retaining of Copper Laminated Polyester Flexible Circuit arrays flat onto carriers/pallets, permitting the attachment of Surface Mount Components by means of tin-lead solder, producing conventional metal alloy joints between component termination and current carrying tracks on the circuit, wherein the circuit arrays are held down by means of polyester silicone adhesive tape thus allowing stencil printing or dispensing of tin-lead solder paste and automated or manual pick & placement of the Surface Mount Components prior to re-flowing the solder paste by conveying through a specifically pre-programmed multi-zoned re-flow oven.

2. A process and method according to claim 1, permitting the attachment of Through Hole Components [Fixed Leaded Components].

3. A process and method according to claim 1, permitting the attachment of Wire Leaded Components [Flexible Wire Leaded Components].