GB2370421A

GB2370421A - Printed circuit board with recessed component

Info

Publication number: GB2370421A
Application number: GB0031493A
Authority: GB
Inventors: Thomas Albert Gaskell; Richard Townsend; Gary Hawkes
Original assignee: Ubinetics Ltd
Current assignee: Aeroflex Cambridge Ltd
Priority date: 2000-12-22
Filing date: 2000-12-22
Publication date: 2002-06-26
Also published as: GB0031493D0

Abstract

A PCB (1) is constituted by a plurality of insulating layers (3), a plurality of conductive track layers (2), and at least one component (4) in electrical and mechanical engagement with the PCB. The insulating layers (3) alternate with the track layers (2). The or each component (4) is mounted within a hole (5) formed in a surface of the PCB (1) and extending through at least one but not all of the insulating layers (3). The PCB construction provides a low profile board enabling components on either side of the board to be recessed.

Description

Printed Circuit Board

This invention relates to a printed circuit board (PCB).

Components are normally fitted to a PCB on the or each side thereof. There are two standard mounting methods, namely surface mounting and through-hole mounting. Surface mounting requires a component to be fixed with its base flush with a surface of the PCB, and through-hole mounting requires a component to be fixed to the PCB via a lead that goes through the PCB to be soldered on the other side thereof.

Both of these mounting methods have the disadvantage that finished PCBs have components projecting from one or both sides thereof. This is particularly disadvantageous where the PCB is to be used in applications for which extremely low profiles are required, as for example with PC cards, compact flash cards, mobile telephones, personal digital assistance (PDAs), and other products with similar electronic packaging formats. In an attempt to reduce the profile of a PCB, it is known to mount one or more components so as to be wholly or partially fitted into a hole cut through the PCB. Unfortunately, although this does reduce the profile of the PCB, it is inefficient in use of the area of the PCB, as it prevents copper tracks from passing through the area in question, and prevents components being fitted to one side of the board. It can also be an expensive or mechanically less reliable way of mounting components.

The aim of the invention is to provide a low profile PCB which does not suffer, to the same extent, from the disadvantages of the prior art low profile PCBs.

The present invention provides a PCB constituted by a plurality of insulating layers, a plurality of conductive track layers, and at least one component in electrical and mechanical engagement with the PCB, the insulating layers alternating with the track layers, wherein the or each component is mounted within a hole formed in a surface of the PCB and extending through at least one but not all of the insulating layers

Advantageously, the or each hole extends through at least one insulating layer, and the or each component associated with that hole is electrically connectable to the track layer overlying the insulating layer exposed at the base of that hole.

In a preferred embodiment, each of the insulating layers is formed with a conductive track layer on either side thereof. In this case, at least one through-via may be formed extending between opposite surfaces of the PCB. Alternatively, the PCB is constituted by two two-layer PCB members, each of which is constituted by an insulating layer sandwiched between two conductive track layers, the two PCB members being bonded in face to face engagement. In the latter case, the two PCB members are bonded together by means of pre-preg or other insulating adhesive material, and each of the PCB members is formed with at least one through-via, whereby, when the two PCB members are bonded together, blind vias are formed in the opposite surfaces of the PCB.

Two forms of PCB, each of which is constructed in accordance with the invention, will now be described, by way of example, with reference to the drawings, in-which :- Figure 1 is a schematic sectional view of the first form of PCB; and Figure 2 is a schematic sectional view of the second form of PCB.

Referring to the drawings, Figure 1 shows a PCB 1 of six-layer construction. The PCB 1 has copper tracks 2 formed on opposite sides of cores 3. The cores 3 are made of glass fibre, and the copper tracks are formed from copper foil (or other conductive material). An electronic component 4 is fixed within a hole 5 formed in the upper surface of the PCB 1. A through-via 6 leads from the base of the component 4 to the opposite side of the PCB 1. A similar through-via 7 is provided to extend between the opposite surfaces of the PCB 1. The through-vias 6 and 7 provide for interconnects between the layers of the PCB 1, and also to assist the introduction of components having leads.

The hole 5 formed in the upper surface of the PCB can be formed by any suitable manner, for example by routing away the upper layers of the PCB. Alternatively, the hole 5 could be formed by cutting away the material from a formed glass fibre core, and then forming one or more upper layers on the core. Subsequently, this sub-assembly could be fixed to a number of complete lower layers in the normal way, using for example epoxy pre-preg material.. Flow of this material, or of other forms of adhesive material, into the hole 5 would be prevented by the use of masking or blanking materials. Pre-preg is a pre-impregnated fibre glass mat which is initially non-activated. In order to bond the sub-assembly to the lower layers, the impregnated resin in the mat is activated.

Figure 2 shows a PCB 11 constituted by two two-sided PCBs 1 la and lib. Each of the PCBs 1 la and lib has copper tracks 12 formed on opposite sides of a core 13. The cores 13 are made of glass fibre, and the copper tracks 12 are formed from copper foil (or other conductive material). The two adjacent surfaces of the PCBs 1 la and 1 lb are

fixed together using pre-preg 1 Ic or other insulating adhesive material.

A hole 15 is formed in the PCB 11 a prior to it being fixed to the PCB lib. Plated through-vias 16 are formed in each of the PCBs 1 la and lib prior to their being fixed together. In this way, blind vias are provided in the finished PCB 11 at a lower additional cost, bringing the benefits of increased packing density.. An electronic component (not shown) is fixed within the hole 15 formed in the PCB 1 la.

The exposed inner-layer conductive track may be plated with a suitable material such as gold or nickel to give protection against oxidation or corrosion. In either of the embodiments described above, the electronic component can be fixed within its recess 5 or 15 using industry-standard techniques (with the exception of wave soldering).

Obviously, the PCB I or 11 can be formed with a plurality of holes 5 or 15, and these can be positioned on one or both sides of that PCB.

It will be apparent that each of the PCBs I and 11 described above has the advantage that surface-mounted electronic components can be fixed to such a PCB at a lower level than if they were fixed to the upper surface of the PCB. This leads to a consequent reduction in volume and/or lowering of the profile of the PCB. Moreover, compared with cutting a hole all the way through a PCB, and subsequently fitting a through-hole or surface-mount component, the PCBs described above enable lower inner layers and the bottom surface of the PCB to remain intact, allowing components to be placed on the other side of the PCB (under the recessed components), with a subsequent increase in packaging density.

Another advantage of mounting components in this way, is that these could be standard parts available at a lower cost than modifying through-hole or surface-mount components to fit through a hole in the PCB, if such modifications were required to meet a specific height requirement. Moreover, components mounted in this way could be more physically robust than modifying through-hole or surface-mount components to fit through a hole in a PCB, if such modifications were required to meet a specific height requirement.

Finally, very low profile components mounted in this way, that is to say largely below the upper surface of the PCB, could be straddled by components mounted on the top surface of the PCB, with a consequent increase in packaging density.

Claims

1. A PCB constituted by a plurality of insulating layers, a plurality of conductive track layers, and at least one component in electrical engagement with the PCB, the insulating layers alternating with the track layers, wherein the or each component is mounted within a hole formed in a surface of the PCB and extending through at least one but not all of the insulating layers.

2. A PCB as claimed in claim 1, wherein the or each hole extends through at least one insulating layer, and the or each component associated with that hole is mechanically and electrically connectable to the track layer overlying the insulating layer exposed at the base of that hole.

3. A PCB as claimed in claim 1 or claim 2, wherein each of the insulating layers is formed with the conductive track layer on either side thereof.

4. A PCB as claimed in claim I or claim 2, wherein the PCB is constituted by two two-layer PCB members, each of which is constituted by an insulating layer sandwiched between two conductive track layers, the PCB members being bonded in face-to-face engagement.

5. A PCB as claimed in claim 4, wherein the two PCB members are bonded together by means of pre-preg or other insulating adhesive material.

6. A PCB as claimed in any one of claims I to 5, wherein at least one through-via is formed extending between opposite surfaces of the PCB.

7. A PCB as claimed in claim 4 or claim 5, wherein each of the PCB members is formed with at least one through-via, whereby, when the PCB members are bonded together, blind vias are formed in the opposite surfaces of the PCB.