WO2009135997A1

WO2009135997A1 - Method and arrangement comprising printed wiring board

Info

Publication number: WO2009135997A1
Application number: PCT/FI2009/050360
Authority: WO
Inventors: Dewei Tian; Mikko Silvennoinen; Esa Muukkonen
Original assignee: Perlos Oyj
Priority date: 2008-05-07
Filing date: 2009-05-05
Publication date: 2009-11-12
Also published as: FI20085418A0; FI20085418L

Abstract

A method of integrating a printed wiring board (PWB) into a plastic item, and an arrangement. The arrangement comprises a printed wiring board (PWB) and soft thermoplastic (2) attached to a first side (3) of the PWB (1) and on at least some locations of edges (4) of the PWB (1). The soft thermoplastic (2) establishes a first interface (5) at or underneath the plane (7) of a second side (6) of the PWB (1). The hard thermoplastic (8) forms a second interface (9) with the second side (6) of the PWB (1), the hard thermoplastic (8) being unattached to the PWB (1) but adhered to the soft thermoplastic (2) at the first interface (5).

Description

METHOD AND ARRANGEMENT COMPRISING PRINTED WIRING BOARD

FIELD OF THE INVENTION

The present invention relates to a method of integrating a printed wiring board (PWB) into a hard plastic item. The present invention further relates to an arrangement comprising a printed wiring board (PWB).

BACKGROUND OF THE INVENTION

There is a growing need to integrate printed wiring boards (PWB) into plastic items, such as covers of electronic devices, e.g. covers of mobile terminals and/or phones.

One of the disadvantages associated with the above arrangement is so called "banana" phenomenon, caused by material shrinkage and mismatch of coefficient of thermal extension (CTE). As a result, the PWB and the plastic item integrated into it has not an intended shape but more or less bent and/or twisted shape.

BRIEF DESCRIPTION [DISCLOSURE] OF THE INVENTION

An object of the present invention is to provide a method and an arrangement so as to alleviate the above disadvantages.

The method of the invention is characterized by accommodating the PWB into an injection moulding mould, injecting a soft thermoplastic capable to attach to the PWB into a first mould cavity formed by the mould and the PWB, on a first side of the PWB and on at least some locations of edges of the PWB by said PWB such that soft plastic reaches the plane of a second side of the PWB, the soft plastic forming a first interface, providing a second mould cavity formed by a mould, the PWB and the soft plastic attached to said PWB, injecting hard thermoplastic incapable to adhere to the PWB and capable to adhere to the soft thermoplastic into the second mould cavity such that it adheres to the soft thermoplastic at the a first interface, the hard thermoplastic forming the plastic item, so that the second side of the PWB and the hard thermoplastic form a second interface, wherein the first interface is at the same level with or underneath as the second interface.

The arrangement of the invention is characterized in that the arrangement further comprises: soft thermoplastic attached to the PWB on a first side of the PWB and on at least some locations of edges of the PWB, the soft thermoplastic establishing a first interface at or underneath the plane of a second side of the PWB, the hard thermoplastic forming a second interface with the second side of the PWB, the hard thermoplastic being not adhesive to the PWB but adhesive to the soft thermoplastic at the first interface. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on the idea of establishing stress release structures between the PWB and the hard plastic item, the stress release structures allowing PWB to move or slide in respect with the hard plastic item. BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached [accompanying] drawings, in which

Figure 1 is a schematic representation of an embodiment of the ar- rangement of the invention as a cutaway diagram;

Figure 2 is a schematic representation of another embodiment of the arrangement of the invention as a cutaway diagram; and

Figure 3 is a schematic representation of still another embodiment of the arrangement of the invention as a cutaway diagram. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 is a schematic representation of an embodiment of the arrangement of the invention as a cutaway diagram. The arrangement comprises a printed wiring board (PWB) 1 which is known as such and, therefore, not described more detailed way in this description. A layer of soft thermoplastic 2 is attached to a first side 3 of the

PWB 1. The soft thermoplastic 2 has low elastic modulus, preferably 10 - 100 MPa at a temperature range of 125 to -40 ⁰C. The bonding strength of the soft thermoplastic 2 to PWB 1 and its components is preferably at least 1 N/mm or 15 MPa. Similarly, the bonding strength of the soft thermoplastic 2 to hard thermoplastics, such as PC/ABS (polycarbonate/acryl-butadiene-styrene) blend, is preferably at least 1 N/mm or 15 MPa.

Furthermore, the soft thermoplastic 2 may have a low coefficient of thermal extension, preferably 150 ppm/K or lower. The strength of the soft thermoplastic 2 is preferably about 20 MPa or higher. The conductivity of the soft thermoplastic 2 is preferably 3 W/MK or better. The dielectric constant of the soft thermoplastic 2 is preferably not more than 4.5 and loss tangent not more than 0.01. The water absorption rate of the soft thermoplastic 2 is preferably under 0.3 wt %.

Finally, the soft thermoplastic 2 has preferably good scratch, wear and abrasion resistance as well as good processability characteristics: gap filling, low moulding temperature and pressure as well as low mould shrinkage.

Some examples of excellent soft thermoplastic materials are sty- rene-ethylene-butylene-styrene copolymer (SEBS) and polyurethane.

A layer of soft thermoplastic 2 is attached to a first side 3 of the PWB 1. The soft thermoplastic 2 attaches to the PWB 1 by chemical bonding and/or by mechanical bonding.

The soft thermoplastic 2 is also attached to edges 4 of the PWB 1 , at least on some sections of said edges 4 so that the soft thermoplastic 2 extends at the plane 7 of a second side 6 of the PWB 1. The soft thermoplastic 2 establishes at the plane 7 a first interface 5. The first interface 5 can encase all the edges 4 of the PWB, or only some selected sections of the edges 4.

A layer of hard thermoplastic 8 forms a second interface 9 with the second side 6 of the PWB 1. The hard thermoplastic 8 has preferably Young's modulus in range of 2 - 20 Gpa. Some examples of preferable hard thermo- plastic materials are polycarbonate (PC), polycarbonate-ABS blend (PC/ABS), optionally filled with glass fibers up to 20 wt%, and polyamides (PA), optionally filled with glass fibers up to 55 wt%.The hard thermoplastic 8 does not make an adhesion to the PWB 1 or components thereon. Therefore, as the hard plastic 8 shrinks due to its cooling, it is able to slide in respect with the PWB 1 at the second interface 9.

The hard thermoplastic 8 is adhered to the soft plastic 2 at the first interface 5. The soft plastic 2 functions as an intermediate element between the PWB 1 and the hard thermoplastic 8. The soft plastic 2 is flexible and resilient and therefore it enables the hard thermoplastic 8 to shrink or expand dif- ferently from the PWB 1 Thanks to this, the shrinking of the hard plastic 8 only cases very low stresses to the PWB 1. Therefore the PWB 1 will not bend or twist when the arrangement is taken away from a mould.

In an embodiment the upper surface of the PWB1 is parallel with the plane 7, as shown at the left edge of the PWB 1 in Figure 1. In another em- bodiment the upper surface of the PWB1 slopes away from the PWB 1 as shown at the right edge of the PWB 1 in Figure 1. An aperture 10 has been made through the PWB 1 prior to injection of the hard plastic 8. The hard plastic 8 has filled the aperture 10 during the injection. As the hard plastic 8 has cooled down a mechanical lock has built between the PWB 1 and the hard plastic 8. This mechanical lock is named here as a centre lock 10, because it establishes a point where the PWB 1 is bond to the hard plastic 8 without any substantial possibility to a mutual movement in the directions of the plane 7. It is to be noted, that the invention can also be realized without the centre lock 10. The centre lock 10 is especially practicable with long PWB's. The hard plastic 8 can be designated and shaped for various purposes. Basically, any electronics embedded or integrated in moulding plastic parts can be manufactured by the method of the invention. Some examples of such products are automobile electronics in instrument panel, under bonnet, and in bodywork. Further examples are apparatuses of consumer electronics, and apparatuses and equipment in industry. In an embodiment of the invention the hard plastic 8 is a cover, e.g. a cover of an electronic device. Said electronic device can be, for instance a portable electronic device, such as a mobile terminal or a laptop.

Next a method for attaching the PWB 1 to the soft thermoplastic 2 and the hard thermoplastic 8 is described.

Firstly, the PWB 1 is inserted into an injection moulding mould. The mould is not shown in figures because it is a common mould and known as such. The mould has a mould cavity and means for keeping the PWB 1 in the cavity. Next the mould is closed and a soft thermoplastic is injected into a first mould cavity that is defined by the mould cavity and the PWB 1. The first mould cavity has a form that shapes the injected soft plastic into the form of the soft thermoplastic 2.

The soft thermoplastic 2 is let to cool and solidified into an extent where a second mould cavity can be provided on the PWB 1. This can be done in principle in three different ways: replacing a mould-half of the mould by another half of mould, or transferring a movable core of the mould to another position, or transferring the PWB 1 and the soft thermoplastic 2 to another mould. These principles are commonly known and, therefore, they are not dis- cussed here in a detailed way.

The second mould cavity is formed by a mould, the PWB 1 and the soft plastic attached to the PWB 1. Next, a hard thermoplastic incapable to adhere to the PWB 1 but capable to attach to the soft thermoplastic 2 is injected into the second mould cavity. Said hard thermoplastic cools and forms the hard thermoplastic 2. The hard thermoplastic 8 attaches to the soft thermoplastic 2 preferably by chemical bonding, but also a mechanical bonding or combination of chemical and mechanical bonding is possible.

Figure 2 is a schematic representation of another embodiment of the arrangement of the invention as a cutaway diagram.

A first major difference to the embodiment shown in Figure 1 is that the soft thermoplastic 2 has enveloped the edges of the PWB 1. The soft thermoplastic 2 comprises enclosure parts 12 which are located under the second side 6 of the PWB 1. Said enclosure parts 12 establish the first interface 5 between the soft and hard thermoplastics 2, 10 beneath the PWB 1. Here again, the first interface can 5 encase all the edges 4 of the PWB, or only some se- lected sections of the edges 4.

A second major difference to the embodiment shown in Figure 1 is that in the hard thermoplastic 8 there is an offset 13 on the side facing the PWB 1. The offset 13 is located around the edge of the PWB 1 , and the enclosure parts 12 as well as the first interface 5 is arranged at least partly in said offset. There is a gap or clearance 14 between the soft thermoplastic 2 and the outer edge of the offset 13. This gap 14 allows the soft thermoplastic 2 and thus the PWB 1 move more freely and it is recommended in cases where the PWB 1 outside of the offset 13 steps up over the plane 7. Width of the gap 14 is in range of 0 to 1 mm, preferably 0.2 to 0.6 mm, depending on the materials used and the size of the PWB 1. The bottom of the offset needs not to be even or arranged parallel with the plane 7.

The embodiment shown in Figure 2 guarantees a highly reliable bonding between the soft thermoplastic 2 and the hard thermoplastic 8. Additionally, it gives more space for the edges of the PWB 1 to move in respect with the hard thermoplastic 8.

The embodiment shown in Figure 2 is manufactured as the embodiment in Figure 1 , but the first mould cavity is formed so that at least some locations of edges of the PWB 1 the soft plastic reaches the second side 6 of the PWB 1 at least close to said edges 4 in order to form the enclosure parts 12.

Figure 3 is a schematic representation of a third embodiment of the arrangement of the invention as a cutaway diagram. The third embodiment differs from the embodiment shown in Figure 2 in that there are apertures or cuts 15 extending between the first side 3 and second side 6 of the PWB 1. These apertures 15 can be arranged on the edges 4 of the PWB 1 and/or at a distance form said edges 4. The apertures shown in Figure 3 are perpendicular to the PWB 1 , but this is not necessary. Number, placing and size of the apertures 15 can vary.

The apertures 15 are filled with the soft thermoplastic 2 in the extent that the soft thermoplastic 2 adhered to the hard thermoplastic 8 at the level of second side 6 of the PWB 1 or underneath the second side 6.

The apertures or cuts 15 are formed prior to the injection of soft thermoplastic, preferably prior to the insertion of the PWB 1 into a mould. The apertures or cuts 15 further improve the attachment between the PWB 1 and the soft thermoplastic 2. It is clear that the apertures or cuts 15 can also be adapted to the embodiment shown in Figure 1.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. A method of integrating a printed wiring board (PWB) into a plastic item, characterized by accommodating the PWB (1 ) into an injection moulding mould, injecting a soft thermoplastic (2) capable to attach to the PWB (1) into a first mould cavity formed by the mould and the PWB (1), on a first side (3) of the PWB (1) and on at least some locations of edges (3) of the PWB by said PWB (1) such that soft plastic (2) reaches the plane (7) of a second side (6) of the PWB (1 ), the soft plastic (2) forming a first interface (5), providing a second mould cavity formed by a mould, the PWB (1) and the soft plastic (2) attached to said PWB (1), injecting hard thermoplastic (8) incapable to adhere to the PWB (1) and capable to adhere to the soft thermoplastic (2) into the second mould cavity such that it adheres to the soft thermoplastic (2) at the a first interface (5), the hard thermoplastic (8) forming the plastic item, so that the second side (6) of the PWB (1 ) and the hard thermoplastic (8) form a second interface (9), wherein the first interface (5) is at the same level with or underneath as the second interface (9).

2. The method as claimed in claim 1, characterized by attaching the soft thermoplastic (2) to the PWB (1) by chemical bonding.

3. The method as claimed in claim 1 or 2, characterized by attaching the soft thermoplastic (2) to the PWB (1) by mechanical bonding.

4. The method as claimed in any one of the preceding claims, characterized by injecting the soft thermoplastic (2) into a first mould cavity formed by the mould and the PWB (1) and on the first side (3) of the PWB (1) and on at least some locations of edges (4) of the PWB (1) by and behind said PWB (1) such that soft thermoplastic (2) reaches the second side (6) of the PWB (1) close to the edges (4) and forms the first interface (5) under the second side (6) close to the edges (4) of the PWB (1 ).

5. The method as claimed in any one of the preceding claims, characterized by forming apertures or cuts (15) through the PWB (1 ), injecting the soft thermoplastic (2) through said apertures or cuts (15) such that the soft thermoplastic (2) reaches the plane (7) of a second side (6) of the PWB (1), and injecting the hard thermoplastic (8) into contact with the soft thermoplastic (2) located in said apertures or cuts (15).

6. The method as claimed in any one of the preceding claims, characterized by forming an aperture (10) through the PWB (1 ), and injecting hard plastic (8) through said aperture (10) for establishing a mechani- cal lock between the PWB (1 ) and the hard plastic (8).

7. The method as claimed in any one of the preceding claims, characterized by providing the second mould cavity by replacing a mould-half of the mould by another half of mould.

8. The method as claimed in any one of claims 1-6, character- i z e d by providing the second mould cavity by transferring a movable core of the mould to another position.

9. The method as claimed in any one of claims 1-6, characterize d by providing the second mould cavity by transferring the PWB (1) and the soft thermoplastic (2) to another mould.

10. The method as claimed in any one of the preceding claims, characterized by forming the hard thermoplastic item to a cover.

11. The method as claimed in claim 10, characterized by the cover is a cover of an electronic device, e.g. of a portable electronic device.

12. An arrangement comprising a printed wiring board (PWB), characterized in that the arrangement further comprises: a soft thermoplastic (2) attached to the PWB (1 ) on a first side (3) of the PWB (1 ) and on at least some locations of edges (4) of the PWB (1 ), the soft thermoplastic (2) establishing a first interface (5) at or underneath the plane (7) of a second side (6) of the PWB (1 ), a hard thermoplastic (8) forming a second interface (9) with the second side (6) of the PWB (1 ), the hard thermoplastic (8) being not adhesive to the PWB (1) but adhesive to the soft thermoplastic (2) at the first interface (5).

13. The arrangement as claimed in claim 12, characterized in that the soft thermoplastic (2) is partly arranged to the second side (6) close to the edges (4) of the PWB (1), and the first interface (5) is located under the second side (6).

14. The arrangement as claimed in claim 12 or 13, characterize d in that the hard thermoplastic (8) comprises at least one offset (13) on its side facing the PWB (1), the offset (13) being located around the edge (4) of the PWB (1), and that the first interface (5) is arranged at least partly in said offset (13).

15. The arrangement as claimed in any one of claims 12-14, characterized in that the PWB (1) comprises apertures or cuts (15) extending between the first and second sides (3, 6) of the PWB (1), said aper- tures or cuts (15) being filled with the soft thermoplastic (2), and that the soft thermoplastic (2) filling said apertures or cuts (15) is adhered to the hard thermoplastic (8) at the same level with or underneath the second interface (9).

16. The arrangement as claimed in any one of claims 12-15, characterized in that the PWB (1 ) comprises an aperture (10) through it, and that the hard thermoplastic (8) is arranged in the aperture (10) so as to establishing a mechanical lock between the PWB (1) and the hard thermoplastic (8).

17. The arrangement as claimed in any one of claims 12-16, characterized in that the hard plastic item is a cover.

18. The arrangement as claimed in claim 17, characterized in that the cover is a cover of an electronic device, e.g. of a portable electronic device.