WO2000018203A1

WO2000018203A1 - Method and means for manufacturing printed wiring boards

Info

Publication number: WO2000018203A1
Application number: PCT/SE1999/001588
Authority: WO
Inventors: Leif Bergstedt
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 1998-09-18
Filing date: 1999-09-10
Publication date: 2000-03-30
Also published as: SE9803181D0; SE9803181L; AU6377899A; SE514949C2

Abstract

The present invention relates to methods and means for using a layer of a radiation reflecting material as a thermal mirror when bonding a polymer with a high bonding temperature on a printed wiring board comprising polymers with a low working temperature. A copper foil is first applied (101) on the core-laminate. A layer of Benzocyclobuten (BCB) is then applied (102) on the copper layer. The BCB is bonded (103) by exposing the BCB to infrared radiation which radiates on and through the BCB. The infrared radiation that radiates through the BCB strikes the copper layer which reflects the radiation back into the BCB. This prevents the core-laminate, arranged under the copper layer, from heating. The copper layer acts as a 'thermal mirror' and the radiation which is reflected back into the BCB generates some additional heat in the BCB.

Description

Method and means for printed wiring boards

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to the field of manufacturing of printed wiring boards and, in particular, to methods and means for using a layer of a radiation reflecting material as a thermal mirror when bonding a material with a high bonding temperature on a printed wiring board.

DESCRIPTION OF RELATED ART

A common way of manufacturing printed wiring boards is to use a carrier, e.g. a core-laminate, on which one or more layers of conductors, lacquer, polymers and/or dielectrics are applied in a multi-layer structure.

Polymers are commonly used in core-laminates and as dielectric layers on printed wiring boards (P B) .

A polymer that can be bonded at a temperature equal to or below 200°C or has a melting temperature equal to or below 200°C, is referred to herein as a "low temperature polymer".

A polymer that has to be bonded at a temperature above 200 °C or has a melting temperature above 200 °C, is referred to herein as a "high temperature polymer".

Thermosetting resins and thermoplastic resins, are examples of polymers that are commonly used in core-laminates and as dielectric layers on the printed wiring boards. There are thermosetting and thermoplastic resins that are low temperature polymers and there are thermosetting and thermoplastic resins that are high temperature polymers.

Epoxy is an example of a thermosetting resin that is a low temperature polymer. Benzocyclobuten (BCB) is an example of a thermosetting resin that is a high temperature polymer. In many situations it is desirable to use high temperature polymers, e.g. BCB, as dielectric layers on core-laminates comprising low temperature polymers, e.g. glass-epoxy. Said combination of polymers have better electrical and opti- electrical performance, maintains the infrastructure of the conventional packaging system and allows for constructions of large dimensions compared to the combination of low temperature polymers or ceramics in both dielectric layers and core- laminates .

A significant problem with existing methods of manufacturing printed wiring boards occurs if a high temperature polymer is applied and bonded on a printed wiring board comprising a core- laminate made of a low temperature polymer, e.g. glass-epoxy. The core-laminate starts to out-gas and disintegrate with a resultant breakdown of performance.

The cause of this problem is the high bonding temperature. The working temperature of the low temperature polymer in the core- laminate is considerably lower than the bonding temperature of the high temperature polymer. The low temperature polymer in the core-laminate changes its characteristics when exposed to temperatures higher than its working temperature.

Obviously, there is a need to improve the manufacturing methods so that the core-laminates are less affected by the heat needed to bond the BCB or any other high temperature polymer applied on the printed wiring boards.

The article "MCM Technology Based on Cu/Benzocyclobuten Thin- Film Multilayered Structure", K. atsui et al.,Nec R&D, Vol. 38, No.2, p.158-165, April 1997, describes a method for producing Multichip Modules (MCM) on a Printed Wiring Board (PWB) . A copper/benzocyclobuten thin-film multilayer structure is fabricated on a glass-epoxy Printed Wiring Board (PWB) with several ground and power plane layers . Thin layers of benzocyclobuten (BCB) and copper are applied on the PWB one layer at the time. In each copper layer a number of copper conductors are formed by electroplating and etching. This creates a number of levels with thin conductors separated by the benzocyclobuten layers in a multilayer structure.

As will be seen herein, the method disclosed in the above mentioned article is of a different type than the method and means of the present invention.

SUMMARY

The present invention overcomes a problem related to manufacturing of printed wiring boards.

The problem occurs when a material which needs a high temperature to be bonded is applied on a printed wiring board including a core-laminate comprising a material with a working temperature that is lower than the high temperature needed for bonding.

A primary object of the present invention is therefore to provide methods and means to prevent the material of the core- laminate in the printed wiring board from being exposed to the high temperature that is used when a high temperature material is applied and bonded.

Another object of the present invention is to provide methods and means to prevent out-gasing from and breakdown of the printed wiring board when exposed to the high temperature.

Yet another object of the present invention is to provide methods and means to reduce the time for bonding the high temperature material applied on the printed wiring board.

In a method according to the present invention, a radiation reflecting layer is applied on the printed wiring board prior to applying the material to be bonded, such that an incident radiation is reflected back into the material to be bonded.

According to an embodiment of the method the radiation reflecting layer is applied on the core-laminate of the printed wiring board. The material to be bonded is applied and dried on the radiation reflecting layer. The material to be bonded is then exposed to IR-radiation that generates the high temperature that is needed for the material to be bonded. The IR-radiation that radiates through the material to be bonded is reflected away from the core-laminate and back into the material to be bonded by the radiation reflecting layer.

The inventive method is therewith characterised as it appears from the appended claim 1.

An arrangement for utilising the method according to the present invention is characterised as it appears from the appended claim 13.

Embodiments of the present invention are characterised as it appears from the subclaims.

An advantage with the present invention is that a high temperature polymer can be applied and bonded on a printed wiring board with minimum affect on any low temperature polymer in/on the printed wiring board.

Another advantage with the present invention is that it prevents out-gasing from the core-laminate in the printed wiring board.

Yet another advantage with the present invention is that it prevents breakdown of the printed wiring board.

A further advantage with the present invention is that the bonding time is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a flow chart of an embodiment of a method according to the present invention.

Figures 2a-g illustrate cross-sections of a printed wiring board during certain steps of the method according to figure 1.

Figure 3 illustrates a cross-section of a printed wiring board.

Figure 4 illustrates a top view of a radiation reflecting layer according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention relates to methods and means for using a layer of a radiation reflecting material as a thermal mirror when bonding a material with a high bonding temperature on a printed wiring board.

Radiation is the transmission of electromagnetic rays through space. Examples of radiation are radio waves that are invisible and infrared rays that occur between light and radar waves. When e.g. infrared rays strike the surface of an object, the rays are absorbed and heat is produced in the object. If the rays are fully reflected from the object, no heat is produced in the object.

The emissivity [%] is the rate at which radiation is given off from an object. Absorption of radiation of an object is proportional to the absorptivity factor of its surface that is reciprocal of its emissivity. The lower emissivity and absorption the higher reflection is obtained from an object/material. Aluminium is an example of a material with a very low emissivity. Aluminium has an emissivity of 5% i.e. an aluminium object reflects 95% of the radiation that strikes the object. Other examples of materials, e.g. metal, with low emissivity are copper, gold and palladium.

A printed wiring board often comprises a core-laminate made of ceramics or low temperature polymers. Examples of low temperature polymers used in core-laminates are glass-epoxy

(which is a thermosetting resin) and polyester (which is a thermoplastic resin) .

One or more layers of conductors, e.g. a pattern of thin copper conductors, may be arranged on/in the core-laminates. As previously mentioned Benzocyclobuten (BCB) is an example of a high temperature polymer that can be used as a dielectric layer on printed wiring boards. The high temperature polymers require a high bonding temperature.

Figure 1 illustrates a flow chart of an embodiment of a method according to the present invention for using a layer of a radiation reflecting material as a thermal mirror on a printed wiring board. The printed wiring board has a core-laminate of glass-epoxy. According to a step 101 in figure 1, a layer of copper, e.g. 1- 50μm, is applied on a first side of the core-laminate as a radiation reflecting layer. The copper layer can be applied as a foil, by plating or sputtering. The copper layer covers the whole first side of the core-laminate. If needed, one or more narrow through holes are arranged in the copper layer.

The copper layer can as an alternative be a surface treated copper foil where the copper foil has been coated with another metal. The surface metal could for example be aluminum, palladium or gold. One advantage with such a surface treated metal layer is that it can be optimized in view of emissivity, conductivity and cost.

The surface metal could as an example be applied on the copper layer in an additional step after the copper layer has been applied on the core-laminate.

According to a step 102, a layer of BCB (Benzocyclobuten), e.g. l-15μm, is applied on the copper layer. The process of applying BCB on a printed wiring board is well known in the art.

The BCB can for example be applied by spinning or screen printing.

According to a step 103, the BCB is dried. The BCB can as an example be dried in a hot-air furnace, The process of drying the BCB in a hot-air furnace is well known in the art. According to a step 104, the method continues with a step 105 if a new layer of BCB is needed on the radiation reflecting layer. Otherwise the method continues with a step 106.

According to step 105, a new layer of BCB is applied on the radiation reflecting layer. The new layer of BCB is then dried according to step 103.

According to step 106, a via-pattern is generated in the BCB layer. The process of generating via-patterns in BCB is well known in the art.

According to a step 107, the BCB is bonded by exposing the BCB to infrared radiation. The infrared radiation is used to achieve the required bonding temperature of the BCB. BCB and other thermosetting resins are normally cured during the bonding process. The process of exposing the BCB to infrared radiation is well known in the art. The BCB can as an example be exposed to infrared radiation in an infra-red furnace.

The infrared radiation radiates on and through the BCB. The infrared radiation that radiates through the BCB strikes the copper layer that reflects the radiation back into the BCB. This prevents the core-laminate, arranged under the copper layer, from heating. The copper layer acts as a "thermal mirror" and the radiation that is reflected back into the BCB generates some additional heat in the BCB.

According to a step 108, a conductor pattern is generated on the BCB if a conductor pattern is needed, i.e. a new layer of conductors is arranged on the BCB layer applied in step 102 and step 105. The process of generating conductor patterns in BCB is well known in the art.

According to a step 109, the method continues with step 105 if a new layer of BCB is needed. Otherwise the method ends. The BCB layer can as an alternative be applied in a number of very thin BCB layers, e.g. l-15μm per layer, in a multi-BCB- layer structure on the radiation reflecting layer, e.g. the copper layer. Each thin BCB layer is dried before the next layer of BCB is applied. Hence, step 104, 105 and 103 according to figure 1 are repeated a number of times until the required thickness of the BCB has been obtained before the method continues to step 106. The bonding time for each thin BCB layer is shorter compared to the bonding time needed when only one (thicker) layer of BCB is applied. The number of thin BCB layers applied in the multi-BCB layer-structure can as an example be 2-

4.

The copper applied on the core-laminate as a radiation reflecting layer according to step 101 in figure 1 is just one example of a material that can be used in the present invention. Any material that has a low emissivity, i.e. a good reflecting characteristics, and a melting temperature higher than the bonding temperature of the high temperature polymer can be used. Preferably a material with a good conductivity such as metals, e.g. aluminum, palladium, copper or gold.

Figures 2a-g illustrate cross-sections of a printed wiring board 200 during the steps of the method according to figure 1 where a printed wiring board is manufactured according to the present invention. References are made to both figures 2a-g and to the corresponding steps in figure 1.

Figure 2a illustrates the printed wiring board 200 with a core- laminate 201 and a first (top) and a second (bottom) side 202, 203 respectively. A first layer of conductors 204 is arranged on the second side 203 and a second layer of conductors 205 is arranged within the core-laminate 201. The first side 202 has no conductors at all. A first via 206 connects the first and second layer of conductors with each other. A second 207 and a third 208 via, extending from the second layer 205 to the first side 202 of the core-laminate, are arranged in the core- laminate 201.

In figure 2b, a layer of copper 209 has been applied on the first side 202 of the core-laminate according to step 101. Two through holes 210a-b are arranged in the copper layer 209 above the second and third via 207, 208 respectively. These through holes should be as narrow as possible. If no vias are needed no through holes are made.

In figure 2c, a layer of BCB 211 has been applied on the copper layer 209 according to step 102. The BCB layer 211 covers the entire first side 202 of the core-laminate 201.

According to figure 2d, a pattern film 213 is arranged above the BCB layer 211 to be able to generate the via-pattern after the BCB layer 211 is dried according to step 103.

In figure 2e, the via-pattern comprising two through holes 215a-b (for vias) has been generated according to step 106. The through holes 215a-b have been arranged in the BCB layer 211 above the through holes 210a-b in the copper layer 209.

In figure 2f, the BCB layer 211 is bonded according to step 107 by exposing the BCB to infrared radiation 212.

The arrows 212 in figure 2f illustrate how the infrared rays radiate through the BCB layer 211 and reflect, from the copper layer 209, back into the BCB layer.

This means that heat generation in the core-laminate 201 is minimised when the BCB layer is bonded due to the high emissivity of the copper layer 209 which reflects the infrared radiation away from the core-laminate 201.

In figure 2g, a conductor pattern 216 has been generated on the BCB layer 211 according to step 108.

The radiation reflecting layer can as an alternative be applied on both sides of the core-laminate 201. Figure 3 illustrates a cross-section of a printed wiring board 300 where a first and a second side 302, 303 respectively of a core-laminate 301 have been covered by a copper layer 304a-b as a radiation reflecting layer. Each of the copper layers 304a-b have then been covered by a BCB layer 305a-b. The arrows 312 illustrate how the infrared rays radiates through the respective BCB layer 305a-b and reflects, from the respective copper layer 304a-b, back into the respective BCB layer 305a-b.

Figure 4 illustrates a top view of an embodiment of a radiation reflecting layer 400 according to the present invention. The radiation reflecting layer 400 is a copper foil comprising a number of through holes for connections between conductive layers arranged under and above the radiation reflecting layer 400.

The radiation reflecting layer in the present invention can be used as a conductive layer, e.g. a ground plane, in a printed wiring board.

As mentioned before copper is just one example of a material suitable for the radiation reflecting layer in the present invention, e.g. the radiation reflecting layer 400. Any material that has a low emissivity, i.e. good reflecting characteristics, and a melting temperature that is higher than the bonding temperature of the high temperature polymer can be used. Preferably a material with a good conductivity such as metals, e.g. aluminum, palladium, copper or gold.

As previously mentioned BCB is an example of a high temperature polymer that can be used as a dielectric layer on printed wiring boards. Another group of polymers that can be used by the present invention instead of BCB is PTFE-polymers (Poly Tetra Fluoro Ethylene) .

Claims

1. A method for using a layer of a radiation reflecting material (209,304a-b) as a thermal mirror on a core-laminate

(201,301) when a polymer (211,305a-b) is applied and bonded on said core-laminate (201,301), where said polymer (211,305a-b ) is exposed to a bonding temperature higher than the working temperature for said core-laminate (201,301), and where said core-laminate (201,301) has a first (202,302) and a second (203,303) side, c h a r a c t e r i s e d in:

a) applying (101) said layer of radiation reflecting material (209,304a-b) on at least one of said sides (202,203,302,303) of said core-laminate (201,301);

b) applying (102) said polymer (211,305a-b) on said radiation reflecting material (209, 304a-b) ;

c) exposing (103) said polymer (211,305a-b) to radiation (212) to achieve the required bonding temperature in such a manner that said radiation (212) is incident on and radiating through said polymer, and so that said radiation which radiates through said polymer (211,305a-b) is reflected back into said polymer.

2. The method as claimed in claim 1, c h a r a c t e r i s e d in that said core-laminate (201,301) is a printed wiring board (200,300) with a pattern of conductors (204,205) in at least one layer.

3. The method as claimed in claim 1 or 2, c h a r a c t e r i s e d in that said core-laminate (201,301) comprises thermosetting resin.

4. The method as claimed in claim 1 or 2, c h a r a c t e r i s e d in that said core-laminate (201,301) comprises thermoplastic resin.

5. The method as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that said radiation reflecting material (209,304a-b) is applied in such a way that said radiation reflecting material minimises the heating of said core-laminate (201,301) from said radiation (212,312).

β. The method as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that said radiation reflecting material (209,304a-b) is applied in such a way that said radiation reflecting material prevents out-gasing from said core-laminate (201, 301) .

7. The method as claimed in any one of claims 1-6, c h a r a c t e r i s e d in that said radiation reflecting material (209,304a-b) is a metallic layer.

8. The method as claimed in claim 7, c h a r a c t e r i s e d in that said metallic layer (209,304a- b) is a surface treated metallic layer where at least one surface of said metallic layer has been coated with a material with a low emissivity factor.

9. The method as claimed in any one of claims 1-8, c h a r a c t e r i s e d in that said radiation reflecting material (209,304a-b) is used as an electrical conductive layer.

10. The method as claimed in any one of claims 1-9, c h a r a c t e r i s e d in that said polymer (211,305a-b) is a high temperature polymer.

11. The method as claimed in claim 10, c h a r a c t e r i s e d in that said high temperature polymer (211,305a-b) is Benzocyclobuten.

12. The method as claimed in any one of claims 1-11, c h a r a c t e r i s e d in that said radiation (212,312) is infrared radiation.

13. An arrangement for reflecting radiation between a core- laminate (201,301) and a polymer (211, 305a-b) , c h a r a c t e r i s e d in that said arrangement comprises a radiation reflecting material (209, 304a-b) , where said radiation reflecting material is so arranged that radiation (212,312) which radiates through said polymer (211,305a-b) and strikes said radiation reflecting material is reflected back through said polymer (211, 305a-b) .

14. The arrangement as claimed in claim 13, c h a r a c t e r i s e d in that said radiation reflecting material (209,304a-b) comprises at least one through hole (210a- b) .

15. The arrangement as claimed in claim 13 or 14, c h a r a c t e r i s e d in that said radiation reflecting material (209,304a-b) is a metallic layer.

16. The arrangement as claimed in claim 15, c h a r a c t e r i s e d in that said metallic layer (209,304a- b) is a surface treated metallic layer where at least one surface of said metallic layer has been coated with a material with a low emissivity factor.

17. The arrangement as claimed in one of claims 13-16, c h a r a c t e r i s e d in that said radiation reflecting material (209,304a-b) is used as an electrical conductive layer.