GB2239416A

GB2239416A - Metal bonding method

Info

Publication number: GB2239416A
Application number: GB8929304A
Authority: GB
Inventors: John Covell Collier
Original assignee: ITT Industries Ltd
Current assignee: ITT Industries Ltd
Priority date: 1989-12-29
Filing date: 1989-12-29
Publication date: 1991-07-03
Also published as: DE4039787A1; GB8929304D0

Abstract

In a metal bonding method such as for bonding electrical contacts 26 the metal contact-attachment pads 3 by a solder reflow step, self-alignment between the contacts and their respective pads during solder reflow is improved by providing the contacts with a convexly-shaped underside. <IMAGE>

Description

- 'I 1 1, IMPROVED METHOD OF METALLIC BONDING This invention relates to

methods of metallically bonding objects together in precise positional relationship.

The present invention is especially. but not exclusively applicable to the metallic bonding of electrical contacts (e. g. dome-shaped) to precisely positioned metal contact-attachment pads on a printed circuit or wiring board for insertion into an electrical edge connector so that the electrical contacts are engaged by spring contacts of the edge connector. Printed circuit or wiring boards of this form are described, for example, in our co-pending Patent Applications Nos. 8919167 (Collier 18-4) r 8923548.5 (Collier 21) and 8923547.7 (Collier 22) to which attention is hereby directed.

In the above-mentioned co-pending Patent Application No. 8919167 (Collier 18-4) the electrical contacts (e.g. dome- shaped) are metallically bonded to metal contact-attachment pads provided on the board. These pads may comprise copper foil pads of the same shape and dimensions as the bases of the contacts (e.g. circular or rectangular) and for the purpose of metallically bonding the electrical contacts to the foil pads solder paste may be applied thereto and after placing the electrical contacts on the contact-attachment pads so that they are generally in their proper positions, the solder in the solder paste is then melted whereby the contacts are displaced by surface tension into precise positional vertical alignment with the f 1 2 pads after which the molten solder is allowed to harden for the bonding of the precisely positioned contacts to' the printed circuit board contact-attachment pads.

it has been found with this solder reflow method of bonding electrical contacts to precisely positioned contactattachment pads on the printed circuit or wiring board that in some cases, such as due to the 6ases of some contacts being defective, there is a tendency for the solder bond to be defective and/or for the surface tension alignment action of the molten solder on the contacts to be deficient.

The present invention is based on the realisation that the configuration or profile of the metal surfaces to be bonded together by a solder reflow self-alignment technique is important and accordingly the present invention provides in its broadest aspect a method of metallically bonding together metal surfaces of at least substantially the same shape and dimensions by a solder reflow selfalignment technique, in which at least one of the opposed metal surfaces is curved convexly or protrudes towards the other opposed metal surface.

It is believed that the convex curvature of the metal surface prevents gas produced from the flux of the solder when the solder melts from being trapped under the contacts and causing distortion of the surface tension effect leadina to deficient centralisation or alignment between the opposed metal surfaces and/or defective bonding therebetween.

In carrying out the present invention to effect solder bonding of printed circuit board contacts to metal (e.g. copper foil) contact-attachment pads on the printed circuit board the contacts may be dome-shaped having upper and lower surfaces of convex configuration. The board padst however, may be flat but the convexity of the underside of the dome-shaped contacts avoids the problem of gas from the flux of the solder paste becoming trapped as previously referred to. This problem of gas being trapped seems to be particularly troublesome when the underside of board 1 1 3 contacts have bitherto been of concave configuration as a result of the contacts being produced by punching or stamping from a metal sheet. - By way of example the present invention will now be 5 described with reference to the accompanying drawing which shows in Figures 1 and 2 enlarged diagrams of a part of a printed circuit board having alternatively shaped contacts metallically bonded to a contact-attachment pad on the printed circuit board.

Referring to Figure 1 of the drawing, the printed circuit board, a fragment of which is indicated at 1, is provided near an edge thereof with a multiplicity of domeshaped button contacts one only of which is indicated at 2. As can be seen the button contact is symmetrical about the horizontal centre line. Printed circuit boards having raised (e.g. dome-shaped) button contacts are fully described in our co-pending Patent Applications No. 8919167.

As can be seen from Figure 1, the dome-shaped contact 2 is bonded to a copper foil contact-attachment pad 3 which way be formed as a small integral part of an overall conductive pattern provided on the board surface by an etching process. The pad 3 is of circular shape in the present example and of the same diameter as the contact 2. The contact 2 is metallically bonded to the metal foil pad 3 by means of solder 4.

In order to effect the metallic bonding between contact 2 and the contactattachment pad 3 solder paste is introduced between the contact 2 and the pad 3. Solder paste applied to the contact attachment pad 3 may be used to hold the contact 2 in place on the pad once the contact has been positioned on the pad by a pick and place mechanism for example prior to the solder reflow bonding step.

When the solder paste is melted the surface tension of the molten solder acts to centralise or vertically align the contact 2 with the contact-attachment pad 3. To enhance this self- alignment action during solder reflow the underside of the contact 2 is of a convex configuration, as 1 4 illustrated, and this contact convexity which should have a high decree of symmetry serves to avoid the problem of gas from the flux of the solder paste being transferred below the contact and thereby tending to distort the surface tension effect of the molten solder paste leading to contact alignment deficiencies and/or bonding defects.

It will be appreciated that since the copper foil contact attachment pad 3 on the printed circuit board 1 is located at a precise position on the board it is necessary for the contact 2 to be precisely located relative to the pad 3 if the contact is to be properly engaged by a spring contact of an edge connector with which the board is to be used.

Referring now to Figure 2 of the drawing, this shows a printed circuit board contact configuration which is considered to be an improvement on the dome-shaped contact configuration depicted in Figure 1.

As can be seen in the f igure, a button contact 5 is metallically bonded at 6 to a circular contact attachment pad 7 on printed circuit board 8 by means of a solder reflow technique utilising solder paste as already described with reference to Figure 1. However, in the present embodiment the contact 5 which is symmetrical about its centre line has upper and lower central flat surfaces 9 and 10 but radiused edges 11 and 12 so that a partially flat and partially convex contact profile is presented towards the contact pad 7 on the board 8. This contact profile serves to prevent gas released from the solder flux of the solder paste as the solder is melted from being trapped between the lower surface of the contact 5 and the contact pad 7 and thereby causing contact alignment deficiencies and/or bonding defects.

Apart from providing a better solder joint the contact configuration of Figure 2 also has the advantage that when the contacts are being fed along the feed tracks of an assembly machine which may include a pick and place mechanism for positioning the contacts on the contact pads 11 of the printed circuit board, contacts are less inclined to ride up over each other as they travel along the feed tracks.

It may here be mentioned that the contact pad size is such that the diameter x of the pad 7 should ideally be greater than the diameter dl of the flats 9 and 10 but it should not be greater than the diaineter d2 which is the overall diameter of the contact 5.

Although the method of the invention has been specifically described in connection with metallic bonding of button contacts to a printed circuit or wiring board it will be understood that the invention could have many different applications. For example, the invention could be used to metallically bond button contacts to the wafer of a rotary wafer switch.

in another application which is envisaged it may be required to bond an electro-optic device to a board or substrate so that a light generative or responsive element of the device is located in precise positional relationship with another device element co-operating therewith and for this purpose the electro-optical device and the board or substrate may each be provided with a number of correspondingly spaced and dimensioned metal pads which are to be solder bonded together using solder paste and the reflow self-alignment technique. In this case the metal pads on the electro-optic device and/or the board or substrate could have outwardly convex or protruding surface profiles to enhance the self-alignment bonding of the pads together during the solder reflow step.

Although in both of the specific examples described the underside of the board contact 2 or 5 is curved convexly at least partially towards the contact attachment pad it is contemplated that at least some advantage could be obtained by having the underside of the contact with some configuration other than convex which protrudes towards the contact attachment pad and which serves to eliminate or at least reduce the trapping of gas referred to.

6 Although 'in the' method described solder paste is utilised to provide the metallic bond between the button contacts and the contact attachment pads the alternative use of solder with flux is also contemplated.

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7

Claims

CLAIMS:

1. A method of metallically bonding together opposed metal surfaces of the same size and shape by a solder reflow technique, in which at least one of the opposed metal surfaces is curved convexly or protrudes towards the other opposed surface in order to enhance self-alignment of the opposed surfaces during the solder reflow step.

2. A method as claimed in claim 1, in which the opposed metal surfaces are surfaces of a contact-attachment pad of a printed circuit or wiring board and an electrical contact. respectively, and in which the underside of the contact has a convex profile.

3. A method as claimed in claim lt in which the opposed metal surfaces are surfaces of a contact attachment pad of a printed circuit or wiring board and an electrical contact. respectively, and in which the underside of the contact has a central flat portion and a radiused peripheral portion providing a partial convex profile presented towards the contact attachment pad.

4. A method as claimed in claim 2 or 3, in which the upper and lower parts of the electrical contact have the same profile.

5. A method as claimed in any of claims 2 to 4. in which the contactattachment pad is coated with solder and flux is introduced before or during the solder reflow step.

6. A method as claimed in any of claims 2 to 4, in which solder paste is applied to the contact attachment pad or the contact before the positioning thereon of the contact after which the solder reflow step is carried out.

7. A method as claimed in any preceding claim, in which a plurality of said opposed metal surfaces are metallically bonded together simultaneously.

8. A method substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawing.

9. A printed circuit or wiring board having near 4.. 1 8 an edge thereof a multiplicity of raised button contacts for engagement by spring contacts of an electrical edge connector, in which the raised button contacts are bonded to contact attachment pads by the method according to any one of the preceding claims.

I Published 1991 atThe Patent Office. State House. 66171 High Holhom. London WC I R 41P. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point, Cwmfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray. Kent.