Precision flexible loop and method for high density assembly of electronic device using such loops

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

 

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

Solved the technical problem lies in the manufacture of flexible printed circuit boards, in which the gap between adjacent conductors does not exceed 20 microns, which allows for density component and pads for mounting them, approaching the density of the conclusions of modern VLSI.

1. Known technical solutions, for example, a method of manufacturing a multilayer printed circuit Board (patent RF №95108852), according to which use two substrates: foil and flexible polyimide foil. On their surfaces form a pattern of conductors and pads. In the polyimide substrate is etched through-hole interconnects, fill them with solder, by which establishes connection of the two substrates. The way is simple, does not require highly skilled personnel, but does not allow you to get a tight installation, since the use of soldering does not exclude the solder spreading and allows you to get clearance between conductors is less than 150 microns.

2. The closest analogue of the proposed design loop can serve as described in the patent WO 2004/066694 design and a method of manufacturing a printed circuit Board. The method provides the t manufacturing a printed circuit Board, having a flexible film substrate with a precision pattern of conductors, and provides the connection with the substrate Board by conductors by means of the intermediate organic layer of strengthening sheet material, which then can be given the desired shape so that the reinforcing material was left on the free points of the mounting areas of the Board. The disadvantage of this design Board and method of its manufacture is a large number of operations, the relatively low reliability and poor maintainability, because when reassembling the electronic component sections of reinforcing material can be easily damaged.

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

To technical requirements of connecting trails for installation include the following:

- minimum step between the conductors and pads;

- ensuring minimum delay signals by reducing the length of the connecting wires;

- ensuring the identity and stability of the parasitic capacitances, inductances and waves the first resistance system of conductors by increasing the accuracy of keeping the size and location of the system conductors and pads during manufacture;

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

- resistance to repeated bending;

- the possibility of using automated methods of design and production;

- exclude the possibility of the emergence of Samorodok by eliminating the spreading of the solder when mounting electronic devices with plumes;

- high maintainability, easy access to certain conclusions when making adjustment and control;

- dimensional stability during production and operation, high mechanical strength, resistance to tearing in a wide temperature range.

The technical problem is solved by the fact that as the substrate material used polyimide, polyester and fluoropolymer film.

A substrate of polyimide film can achieve high thermal stability of plumes and allow the use of different installation methods, including welding and brazing, and fluoroplastic film most suitable for use plumes on their basis for the installation of microwave circuits, as they have small losses in the microwave frequency range and can achieve continuous wave resistance of the loops connecting the microwave modules and devices.

The design and placement of the pads, the designation is the R for mounting an electronic component or connection of the flat cable to other devices or printed circuit boards, and also used in their formation materials and coatings must also allow for high-density megacheese connection in electronic equipment.

To do this, the pads are placed on the other side of the flexible substrate and connected to the working conductors via through-plated holes in the substrate. The outer side of the pads obslujivaut, for example covered with a layer of alloy, tin-bismuth, adjacent to the substrate surface of the pads, put a layer of material that prevents the flow of solder, for example chromium, and the connection pads of the plume with the conclusions of electronic devices perform welding-soldering using a dual electrode tool for micromath supplied to the other surface of the contact pad through above it through the hole in the film.

Example 1.

To test the applicability of various polymeric materials for flexible substrates loops were fabricated and tested prototypes of such products. The results are given in table 1.

Table 1
Parameters filmsMaterial films
The polyethylene terephthalateTeflon The polyimide
Tensile strength, N/m21602001750
Elongation at break, %10017570
The modulus of elasticity3,5×1043,the l×l043×104
Operating temperature373500673
Melting point533553Destruction above 673 K
The relative dielectric constant3,52,13,2
The tangent of dielectric loss angle (at 1 kHz)3×10-21×l0-33×10-3
Electric strength295×106/M150×106B/m276×106/M

As can be seen from the table, most durable and heat-resistant material is polyimide, so he is mostly suitable for the manufacture of circuits for low-frequency instruments. For operation at high frequencies it is better to use Teflon because it has a lower dielectric constant and much lower losses.

Example 2.

For the choice of methods and modes of installation equipment using flexible circuits have been manufactured, assembled and tested samples in various methods of contact. The results are given in table 2

Table 2
Method of contactThe strength of the connectionThe number of cases to reduce the clearance between pins (zacharatos)
Breakout force, NTest method
The initial state of the pads plume0,89Shiftno
Unilateral0,25Gapno
welding dual electrode0,92Shift
Welding indirect0,25Gap5%
pulse heating0,95Shift
Micropayment0,27Gap10%
0,96Shift
Note: 1. Step conductors on the substrate was equal to 150 μm. On Ernest pads, adjacent to the substrate, thermal evaporation was applied a thin layer of chromium ARCH (TU 14-5-76-76).
2. Tensile strength was determined at an angle of 90aboutto the surface of the substrate.
3. The shear strength was determined at 45aboutto the surface of the substrate.

Studies and tests have shown that pulsed flux soldering welding gives the best results and eliminates the appearance of Samorodok compared with welding indirect pulse heating and installation using microbalance, thereby providing a more solid installation.

The scope of the design loop and method of installation with the help of such circuits is a PCB fabrication on a flexible substrate, hybrid integrated circuits, electronic modules for information and microwave systems, medical equipment (hearing AIDS, cardiac stimulants) and other equipment that require high density mounting and reliability.

1. Precision flexible flat cable containing a flexible substrate, for example, of polyimide film, the system is formed on the substrate conductors and associated contact pads, wherein the pads are placed on the other relative to the conductors of the substrate and are connected through conductors met lizirovania holes in the film, and over the pads are provided through holes in the substrate is smaller than the size of the pads, but sufficient for the passage of the tip of the tool to micromontage.

2. Method of mounting electronic devices with precision flexible loops containing a flexible substrate, for example, of polyimide film, the system is formed on the substrate conductors placed on the other side of the substrate contact pads associated with conductors through plated holes in the film, characterized in that the outer sides of the pads plume obslujivaut, for example, covered with a layer of alloy, tin-bismuth, adjacent to the substrate surface of the pads, put a layer of material that prevents the flow of solder, for example chromium, and connect the contact pads of the plume with the conclusions of electronic devices welding-soldering using a dual electrode tool for micromontage supplied to the other surface of the contact pad through above it through the hole in the film.



 

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2 ex

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