FR3059201B1

FR3059201B1 - REINFORCED ISOLATED PRINTED CIRCUIT AND METHOD OF MANUFACTURE

Info

Publication number: FR3059201B1
Application number: FR1661234A
Authority: FR
Inventors: Tony Lhommeau; Patrice CHETANNEAU
Original assignee: Safran Electronics and Defense SAS; Safran Electrical and Power SAS
Current assignee: Safran Electronics and Defense SAS; Safran Electrical and Power SAS
Priority date: 2016-11-18
Filing date: 2016-11-18
Publication date: 2019-06-21
Anticipated expiration: 2036-11-18
Also published as: FR3059201A1

Abstract

L'invention concerne un circuit imprimé (100) comportant au moins une première (1) et une deuxième (2) couche conductrice séparées l'une de l'autre par une première couche diélectrique (5) réalisée dans un premier matériau composite de type préimprégné pour circuit imprimé, le circuit imprimé étant caractérisé en ce qu'il comporte en outre : - au moins une deuxième couche diélectrique (6) réalisée dans un deuxième matériau, le deuxième matériau étant un polymère présentant une rigidité diélectrique supérieure à celle du premier matériau; - au moins une troisième couche diélectrique (7) réalisée dans le premier matériau composite de type préimprégné pour circuit imprimé; la première (5) et la troisième (7) couche diélectrique entourant la deuxième couche diélectrique (6), et la première (1) et la deuxième (2) couche conductrice étant séparées l'une de l'autre par l'empilement (4) formé par les première (5), deuxième (6) et troisième (7) couches diélectriques.The invention relates to a printed circuit (100) comprising at least a first (1) and a second (2) conductive layer separated from each other by a first dielectric layer (5) made of a first composite material of the type prepreg for a printed circuit, the printed circuit being characterized in that it further comprises: at least one second dielectric layer (6) made of a second material, the second material being a polymer having a dielectric strength greater than that of the first material; at least one third dielectric layer (7) made in the first printed prepreg type composite material; the first (5) and the third (7) dielectric layer surrounding the second dielectric layer (6), and the first (1) and the second (2) conductive layer being separated from one another by the stack ( 4) formed by the first (5), second (6) and third (7) dielectric layers.

Description

REINFORCED ISOLATION PRINTED CIRCUIT AND METHOD OF MANUFACTURE

DESCRIPTION

TECHNICAL AREA

The field of the invention is that of printed circuits and in particular their use for the conversion of energy.

STATE OF THE PRIOR ART

The realization of electrical and electronic equipment, particularly in mechatronic systems, electronic systems and critical software in aeronautics, leads to the use of technologies allowing a high flexibility of design, as well as a high level of integration with a maturity and proven reliability.

Machined printed circuit board (PCU) technology meets these expectations. Printed circuits are also known to the skilled person under the term PCB for "printed circuit board" and are sometimes referred to as "printed circuit board". It is recalled that a printed circuit is a multilayer comprising at least two conductive layers (generally copper) and at least one dielectric layer disposed between the conductive layers. When the printed circuit comprises only two conductive layers, it is called double-sided printed circuit; the printed circuit is multilayer when it comprises more than two conductive layers. The dielectric layer or layers used to separate the conductive layers are generally a fiberglass laminate having an epoxy resin matrix, or even polyimide, PET or PTFE.

It is specified that in the context of the present application, a conductive layer is an electrically conductive layer, for example a metal layer.

Being widely used in all industrial fields, machined printed circuits are extremely well known and important research work is carried out to push their limits. One of these limits is their use for voltage levels above 500 V. On the other hand, discrete power components (such as MOSFETs, IGBTs, etc.) capable of performing a switching function of the type static switch, achieve high levels of performance and integration. Their implementation on a CIU-type substrate can limit this integration by several factors: - the increase of the integration volume: the recommended internal insulation distance between two potentials is important (500 μm for a voltage of 500 V). This distance is related to the capacity of the insulating material (non-homogeneous in the case of a composite of the laminate type) to hold, without degrading, a large field of several thousand volts per millimeter (V / mm); - The decrease in electrical performance: a significant distance between two potentials generates a significant parasitic inductance. If these two potentials are subject to rapid switching, in particular by the use of fast switches such as MOS type technologies or new GaN or SiC type technologies, a significant surge can occur, which degrades the electrical performance of the switches, it may even lead to the destruction of the semiconductors of the switches by an electrical breakdown effect.

Today, the standard used to define these isolation distances between two potentials is often IPC 2221, which defines the isolation distance internally (e) of a CIU according to the following rule: - if 301 <U <500 V, e = 250 μm;

if U> 500 V, e = 250pm + (U-500) x 2.5 pm / V

For example, if U = 600 V, the recommended thickness (e) between two layers is 500 μm and if U = 1000 V, the recommended thickness between two layers is 1500 μm.

Knowing that a standard UIC has a thickness between 1.6 and 2.4 mm and is generally limited to 3.2 mm to meet industrial constraints, the number of layers of a UIC is strongly limited which, for voltages greater than 500 V, is a strong constraint to the use of UIC technology for energy conversion.

As noted above, the dielectric insulation used in UICs is essentially an epoxy resin (or exceptionally polyimide, PET or PTFE) polymer matrix composite material reinforced with woven glass fibers, all of which is forming a laminate. The dielectric properties of this composite material can be influenced by the following factors: the fact that the composite material is non-homogeneous; the possible presence of bubbles during the polymerization process of the polymer matrix; a tolerance on the quantity of polymer used in the matrix, which leads to a tolerance on the thickness of the layer of composite material thus obtained; - various contaminations (dust, etc.).

The inventors have set themselves the goal of reinforcing the electrical insulation of a printed circuit to make it usable for energy conversion and especially for use at a voltage greater than or equal to 500 V.

STATEMENT OF THE INVENTION

This object is achieved by a printed circuit comprising at least a first and a second conductive layer separated from each other by a first dielectric layer made of a first composite material of prepreg type for a printed circuit, the printed circuit being characterized in that it further comprises: at least one second dielectric layer made of a second material, the second material being a polymer having a higher dielectric strength than the first material; at least one third dielectric layer made in the first printed prepreg type composite material; the first and third dielectric layers surrounding the second dielectric layer, and the first and second conductive layers being separated from each other by the stack formed by the first, second and third dielectric layers.

It is understood above and in the remainder of the description by "prepreg type composite material for printed circuit board" the standard prepreg composite materials used to form the insulating layers of printed circuits. Thus, such a "prepreg type composite material for printed circuit" is generally selected from the group comprising matrix composites epoxy resin, polyimide, PTE or PTFE reinforced with glass fibers, including laminates of the type FR4 for "flame resistant 4 "in English.

Preimpregnated dielectric layers are electrically insulating adhesive layers, which are better known by the term "prepreg". A prepreg is composed of a thermosetting resin, usually an epoxy resin, which impregnates a reinforcement, usually a fabric.

The principle of the invention is based on the integration of a dielectric layer of polymer (homogeneous) between two dielectric layers of composite material (non-homogeneous). This makes it possible to increase the robustness of the assembly with respect to the electrical stress and brings the following gains: - an insulation which does not depend on the manufacturing process of the composite material layers (independent of the quality of the process of polymerization, the possible presence of foreign bodies, bubbles, etc.), since the dielectric layer of polymer is a homogeneous material; a decrease in the integration volume and therefore a densification of the printed circuit by the choice of the material of the dielectric layer of polymer; by choosing a polymer having excellent dielectric strength properties (especially a high dielectric strength), the thickness of the dielectric layers between the conductive layers of the printed circuit can be reduced; a control of the dimensioning of the durations with respect to the aging of the dielectric insulators of the printed circuit subjected to an electric field; a control of the dielectric properties of the dielectric layers, in particular for high frequency or high voltage variation applications (high dV / dt); an integrated insulation solution having dielectric properties superior to those of a standard printed circuit formed of two conductive layers separated by a single dielectric layer of fiberglass-reinforced resin composite.

Preferably, the second material has a dielectric strength at least twice that of the first material. It is understood that this magnitude ratio of the dielectric strengths of the first and second materials is for dielectric rigidities measured with the same measurement apparatus and under the same operating conditions. Advantageously, the first material has a dielectric strength of at least 400 V / mm.

Preferably, the second material has a dielectric strength of at least 1000 V / mm.

According to a first variant of the invention, the stack separating the first and the second conductive layer consists solely of the first, second and third dielectric layers.

According to a second variant of the invention, the stack separating the first and the second conductive layer comprises, in addition to the first, second and third dielectric layers, a fourth dielectric layer made in the second material and a fifth dielectric layer made in the first prepreg type composite material for printed circuit board, the fourth dielectric layer being disposed between the third and fifth dielectric layers.

The first printed prepreg composite material may be a composite of curable resin and fibrous reinforcement. Advantageously, the first printed prepreg type composite material is a composite of epoxy resin reinforced with glass fibers, preferably type FR-4.

Advantageously, the second material is chosen from thermosetting polymers and thermoplastic polymers, preferably chosen from polypropylene (PP), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), polyetheretherketone (PEEK), polyetherketoneketone (PEKK) , a polyetheretherketoneetherketone (PEEKEK), a polyimide (P1) and a liquid crystal polymer (LCP).

Preferably, the dielectric layers of the first material have the same thickness.

Preferably, the printed circuit according to the invention is intended for use in energy conversion for nominal voltages greater than 120 V, preferably greater than 500 V. The invention also relates to a method of manufacturing a circuit board. printed circuit, characterized in that it comprises the steps of: -providing a second dielectric layer made of a second material, the second material being a polymer having a higher dielectric strength than the first material; providing a first and a third dielectric layer made of a first printed circuit prepreg composite material, the first and third dielectric layers surrounding the second dielectric layer; forming a first and a second conductive layer, the first and second conductive layers being separated from each other by the stack formed by the first, second and third dielectric layers and thus also being separated from one another; other by a first dielectric layer.

BRIEF DESCRIPTION OF THE DRAWINGS Other aspects, objects, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made in reference to the accompanying drawings in which: - Figure 1 shows an example, in a sectional view, of a standard double-sided printed circuit; - Figure 2 shows, in a sectional view, a first embodiment of a double-sided printed circuit according to the invention; - Figure 3 shows, in a sectional view, a second embodiment of a double-sided printed circuit according to the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

For technological reasons, the thicknesses of the prepreg-type dielectric insulating layers ("prepreg") are greater than or equal to 35 μm. The standard thicknesses are approximately 50, 90, 70,100,120,150 and 200 μm.

For reasons of ease of implementation, the dielectric layers made of polymer that make it possible to ensure the dielectric strength must have a thickness greater than 10 μm in order to be easily implemented between the layers of resin-impregnated glass fabrics ("prepreg"). ) and to be able to withstand the pressing process used during the manufacture of the printed circuit (high pressures of several MPa, at temperatures generally between 150 and 180 ° C under high vacuum).

The polymer dielectric layer (s) must meet specific technical constraints, such as having excellent dielectric strengths (at least a steady-state resistance of 1000 V / mm), for indicative purposes, the IPC 2221 standard defines a uniform FR4 epoxy resin-glass "prepreg" composite at 400 V / mm) to ensure good resistance to environmental stresses such as moisture uptake and temperature.

To illustrate the invention, we will compare the standard insulation distances (thickness e) as a function of the voltage according to the IPC 2221 standard and according to the invention. The results are summarized in the table below.

It is recalled that the abbreviations used mean: FR4: technical acronym for designating a composite having an epoxy resin matrix reinforced with a woven glass fiber fabric; BOPP: bi-oriented polypropylene.

In Figure 1 is shown an example of a standard printed circuit solution 10. The printed circuit 10 is a multilayer formed of two layers

Conductors 1 and 2 (for example, made of copper) sandwiching an insulating layer of dielectric material 3 of the "prepreg" type (for example, FR4). For a voltage of 600 V and according to the IPC 2221 standard, the dielectric material layer 3 of the "prepreg" type (FR4) must have a thickness (e) of 500 μm.

FIG. 2 shows an example of a solution with reinforced insulation for a voltage of 600 V. The printed circuit 100 is a multilayer formed of two conductive layers 1 and 2 (for example, copper) which sandwich a stack 4 formed two insulating layers 5 and 7 of "prepreg" type dielectric material (for example, FR4) and a dielectric layer 6 of polymer (for example, polypropylene (PP)), the dielectric layer 6 of polymer being disposed between the dielectric layers 5 and 7 (FR4). The total thickness (e) of the insulation layers 5, 6, 7 (thickness of the stack 4) is thus: two layers FR4 of 90 μm and a layer of PP of 20 μm, ie 200 μm.

It will be noted that for voltages below 500 V, the gain (which is 50 pm gain per standard insulator layer (ie a "prepreg" type dielectric layer) for a voltage of 500 V) is irrelevant. vis-à-vis the extra cost of the manufacturing process. However, this solution can provide technological justification for electrical isolation between two potentials, if a particular application requires it (for example, in the case of a requirement for double insulation or reinforced insulation according to IEC 61010) by the implementation of a multilayer insulating barrier consisting of two different dielectric materials for applications with lower voltages (U <500 V). For example, it may be desired to obtain enhanced isolation between a 115 V / 400 Hz network and a 28 V network in the case of a planar transformer integrated in a UIC.

FIG. 3 shows an example of a double-insulation solution for a voltage of 600 V. The printed circuit board 100 comprises two conductive layers 1 and 2 (for example made of copper) between which is placed a stack 4 of insulating layers comprising three insulating layers 5, 7, 9 of "prepreg" type dielectric material (for example, FR4) and two insulating layers 6 and 8 of polymer (for example, of PP), each insulating layer 6 and 8 of polymer (PP) being disposed between two insulating layers (5 and 7, 7 and 9) of "prepreg" type dielectric material (FR4). The total thickness (e) of the insulation layers is thus: three layers FR4 of 90 μm and two layers of BOPP of 20 μm, ie 310 μm.

Claims

A printed circuit board (100) having at least a first (1) and a second (2) conductive layer separated from each other by a first dielectric layer (5) made of a first composite prepreg type material for circuit printed circuit, the printed circuit being characterized in that it further comprises: -at least a second dielectric layer (6) made in a second material, the second material being a polymer having a higher dielectric strength than the first material; at least one third dielectric layer (7) made in the first composite material of prepreg type for a printed circuit; the first (5) and the third (7) dielectric layer surrounding the second dielectric layer (6), and the first (1) and the second (2) conductive layer being separated from one another by the stack ( 4) formed by the first (5), second (6) and third (7) dielectric layers and in whichthe stack (4) separating the first (1) and the second (2) conductive layer further comprises a fourth ( 8) a dielectric layer made in the second material and a fifth (9) dielectric layer made in the first composite material of printed circuit prepreg type, the fourth (8) dielectric layer being disposed between the third (7) and the fifth (9) ) dielectric layer.

The printed circuit according to claim 1, wherein the second material has a dielectric strength at least two times greater than that of the first material.

The printed circuit board of claim 2, wherein the first material has a dielectric strength of at least 400 V / mm.

The printed circuit according to claim 2 or claim 3, wherein the second material has a dielectric strength of at least 1000 V / mm.

A printed circuit according to any one of claims 1 to 4, wherein the first printed circuit prepreg type composite material is a fiberglass reinforced epoxy resin composite, preferably FR4 type.

6. Printed circuit according to any one of claims 1 to 5, wherein the second material is selected from thermosetting polymers thermopilastic polymers, preferably from a polypropylene, a polyethylene terephthalate, a polytetrafluoroethylene, a polyetheretherketone, a polyetherketoneketone, a polyetheretherketoneetherketone, polylmide and liquid crystal polymer.

7. Printed circuit according to any of claims 1 to 6, wherein the dielectric layers of the first material have the same thickness.