PREPARING THE LEADS OF A SURFACE-MOUNTABLE COMPONENT

Abstract: The leads (4) of a surface-mountable component are embedded in a web (10) of deformable material, such as a solder foil, prior to their separation from a matrix of lead material. The embedding can be done by either cold or hot pressure forming the web (10) onto the leads (4). The unwanted portions (6,8) of the matrix of lead material are then removed, for example, by die shearing. The leads can be bent into predetermined configurations either before or after they are embedded in the web of deformable material.

Description: This invention relates to a method of preparing the leads of a surface-mountable component prior to assembling the component on to a substrate. Advances in electronic device chip technology provide higher density circuit packages. Higher density packaging creates an increased demand for higher I/O at the module component packing level and/or card package levels. The use of surface mount technology allows for higher density than pin in hole component leads. Smaller leads with more stringent dimensional tolerances necessitate special handling considerations so as not to damage the leads or destroy their planarity during processing. Conventional methods for assembling surface mount components onto printed circuit substrates include applying solder paste to the circuit board in a predetermined pattern corresponding to the lead pattern of the components. Frequently, paste is applied through a stencil. Components are then placed on their respective solder pads and the whole is then subjected to heat to reflow the solder. Good registration between component leads and the solder pads on the substrate is essential for acceptable, reliable soldered joints. With high density very fine component leads, the current standard assembly methods have shortcomings. The delicacy of the leads, enhances the opportunity for easy damage or distortion of their planarity. Since leads must be in alignment in the X, Y and Z directions to ensure satisfactory solder joints, it becomes increasingly important to maintain that alignment for high yields. U.S. -A- 4,505,035 relates to methods of aligning and mounting a plurality of electrical leads to a plurality of terminals and discloses aligning leads by urging them into spaced grooves of a workholder and subsequently forming a deformable material, such as a solder wire, between and across each of the leads to retain established spaces therebetween. It is known and practised in the industry to leave lead frame material connected to the leads of edge connectors until after the leads are attached to the substrate. The excess lead frame material is then removed. The present invention seeks to overcome the shortcomings noted above. The invention provides a method of preparing the leads of a surface-mountable component comprising the steps of (a) embedding the leads in a web of deformable material prior to their separation from a matrix of lead material and (b) removing the unwanted portions of the matrix of lead material. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawing, in which: Fig. 1 represents a component prior to lead forming and kerf removal; Fig. 2 represents the component of Fig. 1 after lead forming and kerf removal; Fig. 3 represents a component prior to lead forming after solder foil is positioned for pressure forming on the leads; Fig. 4 represents a component with formed leads after solder pressure forming; Fig. 5 is a schematic detail of the leads after solder foil is pressure formed thereon; Fig. 6 represents the component of Fig. 3 after the leads, still attached to the kerf, are formed; Fig. 7 is an enlarged schematic diagram illustrating the kerf removal process; and Fig. 8 represents a component ready for attachment to a printed circuit board. Figs. 1 and 2 illustrate a known component packaging method. Fig. 1 shows a component with its leads incompletely formed, that is the excess lead material matrix, kerf, from which the leads are stamped or otherwise formed is not removed. In Fig. 1 the component is indicated at 2. Individual leads 4 extend from each side of the component 2. Leads 4 are connected to each other by kerf 8. The corners of the kerf are shown at 6. Fig. 2 shows the same component 2 after leads 4 are bent and the excess kerf section 8 has been removed. When this known technique is used with very fine leads, their fragility dictates extreme caution in handling during manufacture, shipment and subsequent process steps for attaching, that is pick, place and attach to the printed circuit board. Thus some of the leads 4' are shown already misaligned and no longer planar. Fig. 3 is similar to Fig. 1. However, a solder foil 10 has been positioned for pressure forming to leads 4 while kerf sections 6 and 8 remain attached thereto. It has been found that cold compression moulding is a preferred technique although other means, such as hot pressure forming, for applying the solder foil to the leads can be used. The critical process parameters relate to the pressure employed and a settling time for achieving adequate flow of the solder material around the component leads. In one exemplary implementation a solder foil composition of 63/37 eutectic tin lead was used. The composition of the solder may vary in both constituents and alloy percentages as long as the foil has suitable malleability to compression form about the leads 4. Solder thickness was 0.007 to 0.02 cm (0.003 to 0.008 inches). Thickness may vary, but is a function of the lead and corresponding printed circuit pad dimensions as well as the actual amount of solder required for satisfactory bonding. Using a typical component lead width of .015 inches (approximately .35 millimeters) and a pitch, that is lead centre to lead centre distance, of .031 inches (approximately 0.8 millimeters) and solder foil width of .04 inches (approximately 1 millimeter) the following critical parameters apply. pressure of about 6.9 × 10³ to 3.5 × 10² Pa (1 to 5 lbs per square inch) per lead and a pressure application rate of 2.5 to 12.5 mm per second for a dwell time of approximately 1 to 5 seconds is appropriate. The dwell time depends on alloy and solder hardness. The above data pertains to a process run at ambient temperature. The use of higher temperatures would, of course, reduce the dwell time and pressure required as well as increase the rate of applying pressure in most cases. Fig. 4 is a partial cross sectional view of component 2 with formed leads 4 from one side in between elements of pressure forming apparatus. Top element 12 and base 13 move vertically relative to each other to compress solder foil 10 about leads 4. In Fig. 4, compression has occurred and component 2 is ready for removal from the pressure forming apparatus. As a result of the pressure forming solder foil 10 is deformed and at 14 extends up between the leads 4. Alternatively, the solder pressure forming step may occur before lead forming. The two steps may also be performed simultaneously. Fig. 5 is an exploded view of some leads 4 after solder 10 has been cold pressure formed and shows those portions 14 of solder foil 10 more clearly. Once the leads are fixed in the solder, the device is rigid enough to withstand subsequent handling. Lead formation, before, after or during solder pressure forming is readily accomplished using conventional lead forming apparatus and techniques. Known methods are satisfactory because the concern of damaging the fragile leads has been reduced since kerf sections 6 and 8 remain attached. Fig. 6 is a perspective view of component 2 after removal from the solder pressure forming apparatus. Kerf sections 8 are removed by suitable shearing apparatus as illustratively shown in Fig. 7. A shearing tool having a cutting element 18 and base 20 is schematically shown with kerf section 8 positioned therebetween. After shearing, component 2, as shown in Fig. 8 is ready for attachment using conventional techniques without danger of disturbing lead planarity. The leads 4 can be bent into predetermined configurations either before or after they are embedded in the solder foil 10. Conventional placement techniques are used to position the devices over land pads on printed circuit boards where applied flux assists in maintaining position until the solder is reflowed.

Claims: A method of preparing the leads (4) of a surface-mountable component comprising the steps of (a) embedding the leads (4) in a web (10) of deformable material prior to their separation from a matrix of lead material and (b) removing the unwanted portions (6,8) of the matrix of lead material. A method as claimed in claim 1, in which the deformable material is a web of solder foil. A method as claimed in claim 1 or claim 2, in which the web is cold pressure formed onto the leads. A method as claimed in claim 1 or claim 2, in which the web is hot pressure formed onto the leads. A method as claimed in any preceding claim, in which the unwanted portions of the lead material matrix are removed by die shearing. A method as claimed in any preceding claim, comprising the further step of bending the leads into predetermined configurations after step (a) and before step (b). A method as claimed in any of claims 1 to 5, comprising the further step of bending the leads into predetermined configurations before step (a).

Inventors: FRY, BOYD MICHAEL, STANKUS, JOHN JOSEPH
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: H05K13
IPC codes: H01L23, H05K13, H05K3