Patent Application
20190371989
The invention describes a method of manufacturing a lead frame.
In the manufacture of light-emitting diode (LED) lighting devices, one approach is to mount an LED die or wafer-level package onto a lead frame or metal carrier. The lead frame comprises contact regions that make electrical contact with the anode and cathode of an LED die, and a voltage source can be connected across the LED by means of the electrode contact regions. The lead frame also acts as a primary heat spreader, since it is directly connected to the LED package. Advances in LED chip technology have led to very small die size with favourably high light output. The small die size requires a small gap between the anode and cathode contact regions of the lead frame or carrier. During operation of an LED, the anode and cathode become hot so that any contact areas to which the die is mounted are subject to thermal deformation. U.S. Pat. No. 4,797,726 describes a method of protecting a semiconductor chip from such thermal stress.
A lead frame is effectively a frame that holds a plurality of leads in place. The leads are the contact regions to which electrical components will later be soldered. The “frame” itself may be entirely or partially discarded by cutting away any unnecessary areas after the electrical components have been mounted to the leads. A lead frame is generally stamped or punched from a piece of sheet metal or a sheet metal strip, using a stamping press with an appropriately shaped stamping tool and corresponding die. It is a challenge to manufacture lead frames for very small LED die packages, since the gap between anode and cathode may comprise only a fraction of a millimetre. For example, LED die packages with an anode-cathode clearance of only 150 μm-250 μm are currently being manufactured, and this clearance may be expected to decrease further as the technology advances. However, there is a limit to the smallest clearance that can be achieved by a conventional stamping tool, particularly since the lead frame can have a thickness in millimetre range in order to be able to effectively transport the heat dissipated by an LED. For example, a practicable sheet metal thickness may lie in the range 0.5-3.0 mm in order for the stamped lead frame to function satisfactorily as a heat spreader. Generally, a conventional stamping tool is designed to stamp through sheet metal so that the space between adjacent regions is no less than the thickness of the sheet metal. In other words, the ratio of sheet thickness to the smallest achievable gap is 1:1, so that the gap width achievable by a conventional stamping press is too wide for the LED die packages mentioned above. This constraint arises from the need to ensure that the stamping procedure can be performed without interruption for an essentially unlimited number of times.
This problem can be addressed by developing a stamping press that can achieve the desired narrow clearance between electrode contact regions. However, re-design of a stamping press is associated with significant expense, particular since the gap between electrode contact regions must be very precise and uniform. Special tooling modifications to achieve small gap widths are therefore generally limited to the production of lead frame prototypes. Another way of dealing with the problem may be to stamp a lead frame with a large region that will later be sub-divided by cutting or milling into two, four or more individual electrode contact regions. A cutting or milling step can use finer tools that are capable of cutting or forming the desired narrow slit or gap over which the small LED dies can later be mounted. However, owing to the level of precision required in forming the gap or slit between contact regions, such additional tools and the additional tooling step add significantly to the overall cost of the LED devices.
Therefore, it is an object of the invention to provide a more economical way of manufacturing a lead frame.
The object of the invention is achieved by the method of claim 1 of manufacturing a lead frame and by the lead frame of claim 12.
According to the invention, the lead frame manufacturing method comprises the steps of preparing a stamping tool to stamp a lead frame from sheet metal, so that the lead frame includes a plurality of LED electrode contact regions or tiles, wherein complementary anode/cathode contact regions are separated by an initial gap width; and also a number of narrow connecting bars, bridges or fillets, wherein a connecting bar extends between two regions of the lead frame. The method further comprises a step of using the stamping tool to stamp the lead frame; and subsequently deforming at least one connecting bar of the stamped lead frame to reduce the gap width between complementary contact regions to a final gap width.
Of course, the tool preparation step need only be carried out once. After preparing the tool of a conventional stamping press to stamp the desired lead frame shape, the tool can be used repeatedly to stamp lead frames from sheet metal, for example from a continuous band of sheet metal fed from a roll into the stamping press at an appropriate rate. Such a stamping press can be prepared to stamp or punch one or more lead frames from a sheet. Equally, a stamping press can stamp a series of lead frames in a continuous strip of sheet metal.
During stamping, portions of the sheet metal are punched out, leaving a desired arrangement of interconnected regions, most of which serve a specific purpose in the end device. Specifically, the stamped lead frame will comprise several electrode contact regions or “tiles” and any number of connecting bars. Initially, the electrode contact regions are separated by gap widths that are easily achievable by the conventional stamping press. As indicated above, such gap widths are too large for use with very small LED die packages that require a clearance of only a few hundred μm or less. The deformation step of the inventive method is carried out on the stamped lead frame to deform at least one connecting bar of the stamped lead frame, with the result that the gap width between complementary contact regions or tiles is reduced to the desired clearance width. An advantage of the inventive method is that a final gap width or clearance of only 250 μm or less can easily be achieved, even if the initial stamped gap width was the same as the sheet metal thickness, even for relatively thick sheet metal. Another advantage of the inventive method is that the lead frame can be produced with relatively little effort and at low cost.
According to the invention, the lead frame achieved by the inventive manufacturing method comprises a plurality of LED electrode contact regions, wherein complementary contact regions are separated by a gap width of at most 250 μm after the deformation step. The inventive lead frame is cheap to manufacture and can be used as a carrier for very small LED die packages.
An LED lighting device comprising a lead frame according to the invention can therefore comprise one or more LED die packages mounted onto complementary anode and cathode contact regions of the lead frame, and the LED die packages can have an electrode clearance of only 250 μm or less. The LED lighting device which can be achieved using the inventive lead frame can therefore be favourably economical to manufacture, since there is no need for expensive tooling solutions in the preparation of the lead frames for the required narrow electrode clearance.
The dependent claims and the following description disclose particularly advantageous embodiments and features of the invention. Features of the embodiments may be combined as appropriate. Features described in the context of one claim category can apply equally to another claim category.
A stamped lead frame generally includes various areas that will later be discarded. The reason for this is to simplify the stamping process and also more importantly to simplify the component mounting process. For example, LED die packages can be mounted to the electrode regions of a continuous series of lead frames in a soldering step. In this way, hundreds of devices can be prepared in an efficient and cost-effective manner. After a soldering step, the individual devices can be cut from the lead frame strip or sheet. For this reason, the stamping tool is generally prepared to include such “superfluous” or “waste” regions in the stamped lead frame, and the useful regions (such as the electrode contact regions) are initially held in place by connecting bars, which may also be later cut from the lead frame and discarded or recycled. The connecting bars may form an outer frame around the tiles, or can extend between an outer frame and the tiles.
In the inventive method, the electrode contact regions or tiles are initially stamped so that they are separated by a relatively large gap, and are held in place within the frame by the connecting bars. In one preferred embodiment of the invention, the stamping tool is prepared to stamp one or more connecting bars extending from a tile to a side bar of the lead frame. Preferably, such a connecting bar extends diagonally from a tile to a side bar of the lead frame. A connecting bar is preferably narrow so that it can be deformed with relatively little effort. To this end, the stamping tool is preferably prepared to stamp a connecting bar that has a width corresponding essentially to the thickness of the sheet metal. A connecting bar width in the millimetre range is easily achievable by most conventional stamping press arrangements that handle that thickness of sheet metal.
In one preferred embodiment of the invention, the step of deforming a connecting bar comprises bending or displacing the connecting bar within the plane of the lead frame, which has the effect of adjusting the angle of the connecting bar. In this way, the tile at the end of this connecting bar will be displaced towards the other tile(s), thereby effectively reducing the gap or clearance between those tiles.
In another preferred embodiment of the invention, the step of deforming a connecting bar comprises forcing a short length of the connecting bar out of the plane of the lead frame. This results in a small “arch” formed from a section of that connecting bar. The deformation only involves a geometrical alteration but does not involve stretching, so that an arch, extending out of the plane of the lead frame, effectively shortens the length of the connecting bar. In this way, any electrode contact regions linked by this connecting bar will be displaced towards each other, effectively reducing the gap or clearance between them.
The inventive method provides a simple and straightforward way of achieving a very narrow clearance between electrode contact regions. However, care must be taken to ensure that a gap between electrode contact regions is uniform over its entire length. Furthermore, the final gap width or clearance to be achieved also depends on the basis of thermal requirements of the type of LED die to be mounted over complementary contact regions of the lead frame. The final gap width should therefore not be excessively narrow. A minimum gap width of about 50 μm may be sufficient to avoid solder from “bridging” the gap and causing a short circuit. To this end, in a preferred embodiment of the invention, the method includes a step of inserting a gauge in the space between complementary contact regions to limit the extent of deformation of a connecting bar. The gauge preferably has the same width as the desired clearance, and can be realised as a shim or similar device. The gauge can be held in place while the deformation step is being carried out. These techniques ensure that the gap is narrow and that the vertical faces of the opposing tiles are parallel over the entire length of the gap.
The deformation step is performed after stamping and prior to soldering. Between these stages, the tiles should preferably not depart from their new positions.
Therefore, in a further preferred embodiment of the invention, the deformation step is followed by a stabilizing step in which the stresses arising during the deformation step are reduced and/or in which the positions of the electrode contact regions are spatially fixed. For example, an annealing procedure can be carried out, in which the lead frame is heated and cooled. Annealing can be carried out during the deformation step and/or after the deformation step. Alternatively or in addition, a vertical deformation of the connecting bars (out of the plane of the lead frame) can follow their horizontal deformation (within the plane of the lead frame) to fix the positions of the electrode contact regions. Another way of fixing the electrode contact regions in place might be by applying an over-moulding technique in which, after the deformation step, the lead frame is aligned in a moulding tool and spatially fixed by plastic over-moulding. For example, plastic over-moulding applied on either side of the lead frame can hold the electrode contact regions in place. A moulding tool is preferably shaped to hold at least the tiles in place while applying the over-mould plastic material. The plastic material should be electrically isolating and should have favourable thermal characteristics since a soldering step will follow. Stabilizing the electrode contact regions by one or more of the above steps will reduce stresses in the connecting bars or bridges and ensure that the lead regions are spatially fixed in place and will not change position.
In a later manufacturing step, one or more LED dies for example surface mount LED dies—can be attached in a reflow soldering process across the narrow slit between the electrode contact regions. This step can be followed by cutting away any waste or superfluous material from the lead frame to remove short circuit connections. The device can then be packaged as required.
Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.
In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.
The result of such a deformation step is shown in
Although not shown in this diagram, the deformation step can involve forming “arches” as illustrated in
Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For example, the inventive method can be used to provide a lead frame for carrying any electrical device that requires a very narrow clearance between leads.
For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/CN2016/105450 | Nov 2016 | CN | national |
| 16203504.2 | Dec 2016 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/078293 | 11/6/2017 | WO | 00 |
