EP1415519A1

EP1415519A1 - Welded leadframe

Info

Publication number: EP1415519A1
Application number: EP02756051A
Authority: EP
Inventors: Lars Gustafsson; Jan ÖHRN; Per LUNDSTRÖM
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2001-08-07
Filing date: 2002-08-07
Publication date: 2004-05-06
Also published as: CN1270376C; SE0102668D0; TW531462B; SE521528C2; WO2003015485A1; SE0102668L; SE521528C3; JP2004538656A; CN1539256A

Abstract

In joining a lead of a leadframe (3) or component to a contact pad (9) of a substrate (1), the lead and contact pad are locally welded to be electrically connected. For reducing the power used in the welding operation the lead has at least one hole and preferably two holes (11), the material of the lead thereby melting, in the welding step, at the edges of holes to be joined with the heated material of the pad. Thereby a relatively low power can be used in the welding, the local heating used in the welding melting preferably or primarily material in the walls of said at least one hole.

Description

WELDED LEADFRAME

RELATED APPLICATIONS

This application claims priority and benefit from Swedish patent application No. 0102668-1, filed August 7, 2001, the entire teachings of which are incorporated herein by refe- rence.

TECHNICAL FIELD

The invention is related to a method of joining leads to substrates, e.g. in the manufacture of subassemblies mounted on a larger circuit board.

BACKGROUND During reflow soldering of electronic assemblies, there are problems and a potential failure associated with remelting of the solder in some types of subassemblies, a subassembly here taken as comprising a carrier having a conductive pattern, various components mounted thereto and a leadframe. If the subassembly has the carrier, e.g. a ceramic substrate, located under the leadframe, the weight of the carrier and the components may cause the assembly to collapse and the substrate to fall down. This case is illustrated in Fig. 1.

When joining a leadframe to a subassembly there is a potential failure associated with a second reflow or a curing cycle for an adhesive, caused by the temperature stress on the components mounted on the subassembly.

When joining a temperature sensitive component to a substrate or a printed circuit board, common reflow or cycles for curing adhesives can cause component failure.

The conventional solutions to these problems comprise the following:

- Using common solders and accept the remelting of the solder during the reflow process.

- Using a higher temperature solder to avoid remelting of the solder during the reflow process.

- Using a brazing material. - Using an electrically conductive polymer based adhesive.

Solder is herein taken to mean an alloy having a melting point below 450°C, intended for joining two parts at a temperature below the melting points of these parts. A braze is an alloy having a melting point higher than 450 °C and intended for the same use. The drawbacks of the solutions mentioned above include: - Common solders remelt in a second reflow process and temperature sensitive components may be damaged during reflow.

- Known solders in the desired temperature range have disqualifying properties such as brittleness, high cost or that they can contain hazardous substances. Also, many components cannot withstand the elevated temperature needed when using high temperature solders. - The components cannot withstand the high temperature needed for the reflow process of brazing materials.

- For power applications having a requirement of a high efficiency, the electrical properties of known conductive adhesive materials have an insufficient electrical conductivity. Instead of the mentioned methods welding can be used. However, drawbacks of welding processes include the elevated temperatures and the large energy needed for making welds. SUMMARY

It is an object of the invention to provide a method of connecting leads such as in a leadframe or a component to pads of a substrate requiring only local heating with a moderate energy.

It is another object of the invention to provide a method of connecting leads in a leadframe to pads of a substrate so that the joined assembly of leadframe and substrate can stand elevated temperatures, e.g. temperatures used in high temperature soldering.

Generally, in a method for joining a leadframe to pads of a substrate, in e.g. a subassembly, holes are made in the leads of the frame and then the leads are welded to the pads. The holes reduce the energy required for the welding. The holes can preferably cover about 50% of the area heated in the welding process.

Using holes in the leads allows the production of lead-free, practically heat insensitive, mechanical and electrical connections and to connect heat sensitive components to a substrate or a printed circuit board. It is possible to manufacture surface mounted subsystems having lead-free connections and to avoid a global heating, such as when using soldering or curing electrically conductive adhesives. The process allows mounting thick leads using low welding energy input. Furthermore, it allows mounting leads/components on a temperature sensitive device and mounting temperature sensitive components. Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the invention are set forth with particularly in the appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which:

- Fig. 1 is a cross-sectional view of a leadframe joined to a substrate, and

- Fig. 2 is fragmentary plan view of a lead joined to a contact pad.

DETAILED DESCRIPTION In the schematic cross-sectional view of Fig. 1 a subassembly is illustrated comprising an e.g. ceramic substrate 1 joined to a leadframe 3 at joining positions 5. In these positions connection leads or legs 7 of the leadframe are made to be electrically connected to pads 9 of the substrate, see the fragmentary, plan view of Fig. 2. The leads 7 have a thickness considerably larger than then thin contact pads 9. In the joining positions the leads are welded to the pads using e.g. a laser beam, plasma welding, a microflame, as indicated at 10.

In the heating in a welding process the pads will hence be more easily heated than the leads due to the difference in heat capacity resulting from the different thicknesses. However, to reduce the thermal capacity of the leads 7 they are provided with at least one and preferably two holes 11 covering e.g. about 50% of the area heated in the welding. These holes can have any shape, e.g. being circular as illustrated in the figure. In the heating then, a meltdown of the material in the walls of the holes 11 or in the edge regions of the lead at the holes is obtained, the melted material flowing down to be joined with the material of the underlying pad 9. At the same time the leadframe is heated at regions around the holes and also the substrate, i.e. the pads 9 thereof, the latter heating occurring through the holes 11. When making the heating using a powerful laser beam, a rapid formation of a laser light absorbing plasma is formed due to the holes 11 that effectively form cavities when the leadframe is placed over the substrate. The plasma strongly enhances the coupling of the laser light power to the materials to be welded to each other.

Typically the holes 11 can have a diameter in the range of about 0.1 - 0.8 mm made in the leadframe material which can have a thickness in the range 0.1 - 1 mm. The incoming, heating energy indicated by the arrows 10, has a direction that is approximately perpendicular to the large surface of the components to be welded to each other.

In the welding process the leadframe is retained and oriented by a fixture, not shown, in relation to the substrate and pads to make it adopt the wanted position. While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous additional advantages, modifications and changes will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention.

Claims

CLAIMS 1. A method of joining a lead of a leadframe or component to a contact pad of a substrate comprising that the lead and contact pad are locally heated to be joined, characterized in that the local heating is performed as a welding step melting at least partly material of at least one of the lead and the contact pad to make this material or these materials join the lead to the contact and making, before performing the local heating, at least one hole in the lead, thereby allowing a relatively low power to be used in the local heating, the local heating melting preferably or primarily material in the walls of said at least one hole or in the edge regions of the lead at said at least one hole.

2. A method according to claim 1 , characterized in that two holes are made in the lead.

3. A method according to claim 1, characterized in that the at least one hole is made to cover substantially 50% of that area of the lead which is locally heated.