WO1995035015A1

WO1995035015A1 - A capsule-stiffening arrangement and a method for its manufacture

Info

Publication number: WO1995035015A1
Application number: PCT/SE1995/000686
Authority: WO
Inventors: Karl-Erik Leeb
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 1994-06-14
Filing date: 1995-06-08
Publication date: 1995-12-21
Also published as: SE504623C2; AU2757795A; SE9402086D0; SE9402086L

Abstract

The present invention relates to an arrangement for stiffening electronics-housing capsules. According to the invention, the interspace between the capsule (2) and the electronic circuit board (1) is filled with plastic foam (4). The invention also relates to a method of producing a capsule-stiffening arrangement, in which a cross-linked plastic foam insert (5) is compressed and placed in the capsule cavity and thereafter heated so as to expand and fill the cavity, the plastic foam therewith stiffening the capsule walls and avoiding microphony.

Description

ACAPSULE-STIFFENINGARRANGEMENTANDAMETHODFOR ITSMANUFAC¬ TURE

TECHNICAL FIELD

Thepresent invention relates to an arrangement for stiffening capsules that house electronics, wherein the arrangement stiffens the capsule mechanically in an inexpensive manner while providing a suitable dielectric capsule environment.

BACKGROUND ART

Capsules which house electronic devices often need to be protected mechanically against external influences. This protection is often affordedbymanufacturingthe capsule from sheet metal, or producing the casing in the form of a casting, or by injection-moulding the capsule from a plastic material. The capsule often has a screening or shielding function which prevents interaction through electromagnetic radiation.

One problem with such encapsulation is that the distance from the capsule to the electronic devices enclosed therein can influence the function of the electronics, and that acoustic oscillations in the capsule walls are able to modulate the signal in the electronics, a phenomenon referred to as microphony. More specifically, flat metal walls or plastic plates that include electrically conductive inserts are easily vibrated, particularly when the frequency of ambient sound coincides with the resonance frequencies of the plates.

SUMMARY OF THE INVENTION

The present arrangement relates to a capsule-stiffening arrangement and a method of manufacturing the arrangement, with the intention of solving the problems associated with established technics. The inventive arrangement is comprised of an insert made of thermoplastic foam which fills the space between the elec¬ tronic device or devices and the walls of the capsule. The inventive arrangement is manufactured by compressing a piece of cross-linked plastic foam whose dimensions are slightly larger than the space to be filled to a volume which is smaller than the volume of said space, at a temperature above the glass transition point of the plastic material, and thereafter cooling said piece of plastic foam in its com- pressed state, so as to enable said material to be placed easily in the aforesaid interspace. The arrangement is then heated to a temperature above the glass transition point of the plastic foam, which then strives to expand to its original size. The plastic foam is very soft and will therefore not subject the electronic device or devices to harmful forces. When the arrangement is thereafter cooled to the temperature commensurate with the normal operating temperature of the electronic device of devices and lower than the glass tran¬ sition point of the plastic foam, the plastic foam will expand to fill the space between the electronic device or devices and the capsule housing, and also all nooks and crannies in the capsule, to form a light, acoustically dampening filling of appropriate hardness.

The invention will now be described in more detail with reference to a preferred embodiment thereof and also with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a cross-sectional view of the arrangement in its final state.

Figure 2 is a cross-sectional view of the arrangement and shows the plastic foam insert in its compressed state, in accordance with its method of manufacture. BEST MODE OF CARRYING OUT THE INVENTION

Figure 1 illustrates an electronic circuit board 1 enclosed in a metal casing 2. The earth plane of the electronic circuit board 1 is coupled to the metal casing 2 by means of a schema- tically illustrated conductor 3. The space between the electronic circuit board 1 and the metal casing 2 is filled with thermoplastic, cross-linked plastic foam 4.

One example of suchplastic foam is foamed polyethylenewhich, after foaming, has been irradiated with 3-rays create cross- links between themolecule chains. Polyethylene is a non-polar plastic material which is suitable for use in the present case, owing to its low dielectric losses. The plastic foam filling 4 is functional in preventing vibrations and distance variations between the electric circuit board 1 and the metal casing 2.

Figure 2 illustrates the same arrangement as that shown in Figure 1, but with the difference that Figure 2 shows the plastic foam insert 5 in its compressed state, in which the insert can be easily applied between the electronic circuit board 1 and the metal casing 2, in accordance with the method. The insert 5 is initially larger than the space to be filled, and is first heated to a temperature which lies above the glass transition point of the plastic material. The insert is then compressed to a size smaller than the size of said interspace and then cooled in its compressed state, so that it can be easily placed in the interspace. The insert 5 is then heated to a temperature which is higher than the glass transition point of the plastic foam, wherewith the insert strives to expand to its original size. In so doing, the insertwill fill the interspacebetween the electronic circuit board and the encapsulating casing, including all nooks and crannies, to provide a light, acoustically dampening filling of appropriate hardness, as shown in Figure 1. The plastic foam is very soft and will not therefore subject the elec¬ tronic device, or devices, to harmful forces.

It will be understood that the invention is not restricted to the aforedescribed and illustrated exemplifying embodiment thereof and that modifications can be made within the scope of the following Claims.

Claims

1. An arrangement for stiffening a capsule which houses an electronic circuit board, characterized in that the arrange¬ ment comprises a plastic foam insert (4) which fills the interspace between the capsule (2) and the electronic circuit board (1) .

2. An arrangement according to Claim 1, characterized in that the capsule (2) is comprised of metal and is connected electrically (3) to the electronic circuit board (1).

3. An arrangement according to Claim 1, characterized in that the plastic foam insert (4) is comprised of cross-linked polyethylene foam.

4. A method of producing a capsule-stiffening arrangement according to Claim l, characterized by cross-linking the molecular chains of a piece of plastic foam (5) whose dimen¬ sions exceed the dimensions of the space in the capsule (2) in the original state of said plastic, and thereafterheating the plastic foam to a temperature higher than the glass transition point of the plastic, compressing the plastic at said tempera- ture to a size smaller than the size of the space in the capsule (2) and cooling the plastic in its compressed state, placing the compressed piece of plastic (5) into the space in said capsule, and heating the plastic (5) to a temperature above its glass transition point, wherewith the piece of plastic foam (5) expands and strives to return to its original size and therewith fills said space in the capsule (2) .