RU2312474C2 - Precision flexible loop and method for high density assembly of electronic device using such loops - Google Patents

Abstract

FIELD: engineering of flexible multilayer electronic boards (loops) for assembly of microelectronic equipment.

SUBSTANCE: in accordance to the invention, as substrate material, polyimide, polyether and fluoroplastic films are used. Polyimide film substrates make it possible to achieve high thermal stability of loops and allow usage of different methods of assembly, including welding and soldering, and fluoroplastic films are most usable for making loops based on them for assembly of UHF circuits, due to low losses in UHF frequency range and possible constant wave resistance of loops which connect UHF modules and devices. Structure and placement of contact areas, meant for assembly of electronic components or connection of loop to other devices or electronic boards, and also materials and covers, used for forming aforementioned contact areas, must also provide for high density inter-cell connection in electronic equipment. To that end, contact areas are positioned on the other side of flexible substrate and connected to working conductors via through metallized apertures in the substrate. External sides of contact areas are tin-plated, for example, covered in layer of tin-bismuth alloy, onto the surface of contact areas which is adjacent to substrate, a layer of material is applied to prevent spreading of soldering alloy, chromium for example, and connection of contact areas of loop to contacts of electronic devices is realized by welding-soldering using double electrode tool for micro-mounting, brought to contact with the other surface of contact area via through aperture in the film positioned above the contact area.

EFFECT: manufacture of flexible electronic board, wherein the gap between adjacent conductors does not exceed 20 micrometers, making it possible to ensure placement density of components and contact areas for assembly thereof, approaching contact placement density of modern VLSI circuits.

2 cl, 2 tbl, 2 ex

Description

The invention relates to the field of creating flexible multilayer printed circuit boards (loops) for the installation of microelectronic equipment.

The technical problem to be solved is the manufacture of a flexible printed circuit board in which the gap between adjacent conductors does not exceed 20 μm, which allows for a density of components and contact pads for their installation, approaching the terminal density of modern VLSI terminals.

1. Known technical solutions, for example, a method of manufacturing multilayer printed circuit boards (RF patent No. 95108852), in accordance with which two substrates are used: foil and flexible foil polyimide. On their surfaces form a pattern of conductors and pads. Through holes for etching are etched in the polyimide substrate, filled with solder, through which both substrates are connected. The method is quite simple, does not require highly qualified personnel, but does not allow for tight installation, since the use of soldering does not exclude the spreading of solder and does not allow to obtain a gap between conductors of less than 150 microns.

2. The closest analogue of the proposed loop design can be described in the patent WO 2004/066694 design and method of manufacturing a printed circuit board. The method provides the manufacture of a printed circuit board having a flexible film substrate with a precision pattern of conductors, and provides for connection to the substrate of the board on the side of its conductors using an intermediate organic layer of reinforcing sheet material, which can then be shaped so that the reinforcing material remains only on board-free areas of the board. The disadvantage of this design of the board and the method of its manufacture is a large number of operations, relatively low reliability and poor maintainability, since during the re-installation of electronic components, sections of the reinforcing material can easily be damaged.

To increase reliability, increase packing density, increase productivity in the manufacture of electronic devices, it is proposed to use flexible cables based on a substrate from a polyimide film and a system of conductors with contact pads located on the substrate.

The technical requirements for connecting loops for installation include the following:

- minimum pitch between conductors and pads;

- ensuring minimum signal delay by reducing the length of the connecting conductors;

- ensuring the identity and stability of parasitic capacitances, inductances and wave impedance of the conductor system by increasing the accuracy of maintaining the size and location of the conductor system and pads during manufacture;

- ensuring the reliability and durability of the joints formed by one of the known methods of soldering, welding or others;

- resistance to repeated excesses;

- the possibility of using automated design and manufacturing methods;

- elimination of the possibility of short circuits due to the exclusion of the spreading of solder during the installation of electronic devices using cables;

- high maintainability, easy access to individual findings during adjustment and control;

- stability of linear dimensions during production and operation, high mechanical strength, tensile strength in a wide temperature range.

The stated technical problem is solved by the fact that polyimide, polyester and fluoroplastic films are used as the substrate material.

Polyimide film substrates can achieve high thermal stability of the cables and allow the use of various installation methods, including welding and soldering, and fluoroplastic films are most suitable for using cables based on them for mounting microwave circuits, since they have small losses in the microwave frequency range and make it possible to achieve constant wave impedance of loops connecting microwave modules and devices.

The design and placement of pads intended for mounting electronic components or connecting a cable to other devices or printed circuit boards, as well as the materials and coatings used in their formation, should also allow for high-density intercell connection in electronic equipment.

For this, the contact pads are placed on the other side of the flexible substrate and connected to the working conductors through the through metallized holes in the substrate. The outer sides of the contact pads are tin-coated, for example, coated with a tin-bismuth alloy layer, a layer of material preventing the spreading of solder, such as chromium, is applied to the surface of the contact pads adjacent to the substrate, and the connection of the contact pads of the cable with the terminals of electronic devices is carried out by soldering using a double electrode of the tool for micromotage supplied to the other surface of the contact area through a through hole in the film located above it.

Example 1

To test the applicability of various polymeric materials for flexible loop substrates, prototypes of such products were manufactured and investigated. The test results are shown in table 1.

Table 1 Film Options Film material Polyethylene terephthalate Ftoroplast Polyimide Tensile strength, N / m ² 160 200 1750 Elongation at break,% one hundred 175 70 Elastic modulus 3.5 × 10 ⁴ 3, l × l0 ⁴ 3 × 10 ⁴ Working temperature 373 K 500 K 673 K Melting temperature 533 K 553 K Destruction above 673 K Relative permittivity 3,5 2.1 3.2 Dielectric loss tangent (at 1 kHz) 3 × 10 ^-2 1 × l0 ^-3 3 × 10 ^-3 Dielectric strength 295 × 10 ⁶ V / m 150 × 10 ⁶ B / m 276 × 10 ⁶ V / m

As can be seen from the table, the most durable and heat-resistant material is polyimide, so it is most suitable for the manufacture of loops for low-frequency equipment. To work at high frequencies, it is better to use fluoroplastic, since it has a lower dielectric constant and significantly lower losses.

Example 2

To select the methods and modes of installation of the equipment using flexible cables, samples were manufactured, mounted and tested with various contacting methods. The test results are shown in table 2

table 2 Contact method Bond strength The number of cases of reducing the gap between the conclusions (shorts) The separation force, N Test method The initial state of the contact pads of the loop 0.89 Shift no Single sided 0.25 Separation no dual electrode resistance welding 0.92 Shift Indirect welding 0.25 Separation 5% pulse heating 0.95 Shift Micro soldering iron 0.27 Separation 10% 0.96 Shift Note: 1. The pitch of the conductors on the substrate was 150 μm. On the surface of the contact pads adjacent to the substrate, a thin layer of ERH chromium was applied by thermal evaporation (TU 14-5-76-76). 2. The peel strength was determined at an angle of ⁹⁰ ° to the substrate surface. 3. The shear strength was determined at an angle of ⁴⁵ ° to the substrate surface.

The studies and tests showed that flux-free pulsed soldering-welding gives better results and eliminates the occurrence of shorts in comparison with welding by indirect pulsed heating and mounting with a micro-soldering iron, thereby ensuring a denser installation.

The scope of the loop design and installation method using such loops is the manufacture of printed circuit boards on a flexible substrate, hybrid integrated circuits, electronic modules for information and microwave systems, medical equipment (hearing aids, cardiac stimulators) and other equipment where high density mounting is required and reliability.

Claims (2)

1. A precision flexible cable containing a flexible substrate, for example, of a polyimide film, a system of conductors formed on the substrate and connected by contact pads, characterized in that the contact pads are placed on another surface of the substrate relative to the conductors and are connected to the conductors through metallized holes in the film, and through the pads, through holes are provided in the substrate, the dimensions of which are smaller than the sizes of the pads, but sufficient for the passage of the tool tip for m installation. 2. The method of installation of electronic devices using precision flexible cables containing a flexible substrate, for example, from a polyimide film, a system of conductors formed on a substrate with contact pads located on the other side of the substrate connected to the conductors through metallized holes in the film, characterized in that the external the sides of the contact pads of the loop are tin-coated, for example, covered with a tin-bismuth alloy layer, a layer of material is applied to the surface of the contact pads adjacent to the substrate, preventing spreading solder, such as chromium, and connect the contact pads of the cable to the terminals of electronic devices by welding and soldering with the help of a double electrode of a micromount tool supplied to the other surface of the contact pad through the through hole located above it in the film.