GB2520570A

GB2520570A - LED Holder

Info

Publication number: GB2520570A
Application number: GB1320853.3A
Authority: GB
Inventors: John Philip Lincoln Binns
Original assignee: LUMOTRIX Ltd
Current assignee: LUMOTRIX Ltd
Priority date: 2013-11-26
Filing date: 2013-11-26
Publication date: 2015-05-27
Also published as: GB201320853D0

Abstract

A LED holder 1 is used as its own soldering jig. Wires from the LED, 7 and 9, are assembled into the holder prior to soldering. The holder has tunnels that feed the bare ends of the wires down onto the solder pads 8 and the holder is made of plastic that will not melt at the solder temperature. Soldering is achieved by re-flow of solder paste or manually running solder wire onto the solder pads. The LED sits within a pocket or recess on the back surface of the holder.

Description

LED Holder This invention relates to innovations in a moulded plastic holder which houses a LED.

LED5 made by the COB (chip on board) method comprise a thin flat rectangle of ceramic with a (usually) circular arrangement of small LED chips bonded on top and covered with a phosphor coating which blends and re-radiates their light to give a single light emitting surface. Electrical contact pads are brought out from the LED to clear areas towards the edge of the ceramic. LED holders are used to provide fixing means to pull a LED hard onto a heat-sink, to provide a suitable interface surface for connection of a reflector mounted in front of the LED and to provide wire guiding means to allow electrical connection to the contact pads, either by spring-loaded contacts onto the pads, or soldered connections.

Our previous known design of LED holder is flat, is 2mm thick and has a 1mm deep pocket on the back to accept the LED ceramic. The wires are soldered to pads on the ceramic and lie in channels on top of the plastic. The channels are sufficiently deep so that the top of the wires lie below the surface, and hence the surface is substantially flat to allow the easy fitting of most reflectors. However, the existing design has a disadvantage. During assembly the wires, which are quite stiff, need to be held exactly in place with a jig prior to soldering onto the LED pads. Next the LED with attached wires has to be removed from the jig. Then, in order to fit the LED into the pocket on the back of the holder, the wires have to be bent upward and passed through holes in the holder at the same time as the LED is being inserted into the pocket. This allows them to be brought onto the front of the holder and laid in the channels. This process is slow.

According to the present invention the holder itself is used as a jig. The wires and the LED are assembled into the holder prior to soldering. The holder has tunnels that feed the bare ends of the wires down onto the solder pads and the holder is made of plastic that will not melt at the solder temperature. A preferred method of soldering is to re-flow solder paste on the solder pads, but the holder perfectly facilitates manual soldering by the method of heating the LED from underneath and running solder wire onto the solder pads. Thus the usefulness of this holder is not restricted to a particular method of soldering.

The invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows the application of the LED holder attached to a heat-sink with a reflector on the front.

Figure 2 shows a type of LED.

Figure 3 shows the front of the LED holder with the LED in place.

Figure 4 shows the rear of the holder with wires in place but without the LED.

Figure 5 shows a magnified portion of figure 4 to demonstrate the tunnel features.

Looking at figure 1 we can see the LED holder 1 in application on a heat-sink 2. Two mounting screws 3 (only one visible) are used to locate the LED holder on the heat-sink and the LED inside the holder will be pulled on to the heat-sink which will cool the LED during operation. A reflector 4 is attached to the front of the LED holder with its aperture centred on the light emitting surface 5 of the LED.

Figure 2 shows the LED itself that fits inside the holder. In this case the size of the ceramic substrate 6 is 24mm x 20mm x 1mm. The light emitting surface 7 is 0.8mm thick and will fit inside the circular aperture provided in the LED holder. In this case there are two solder pads 8 to provide electrical connection to the LED chips under the light emitting surface. Less commonly there can be four solder pads. Sometimes spring loaded contacts might be used for making electrical connection to the pads, but the reliability of this contact over long periods of time is unknown, so soldered wires are preferred.

Figure 3 shows the front of the holder 1. It can hold a LED with two solder pads or one with four solder pads. In this case it is holding a LED with two solder pads 8 with mounds of solder on top. The light emitting surface of the LED 7 is visible in the centre of the holder. There are two wires 9 in channels.

These wires are soldered to solder pads 8. There are also two channels 10 without wires. It can be seen that the wires 9 lie in channels on the surface of the holder but as they approach the solder pads 8 they pass through tunnels down to the solder pads. In this case, the holder has four holes with the same shape as the solder pads, to allow visual inspection of the pads. Two of the holes are empty because the LED has only two solder pads. Looking into empty hole 11 it can be seen how the tunnel exits into the hole close to where a solder pad would be. This arrangement is similar for all four holes. Once soldered the LED and wires cannot be removed from the holder. In this case, the holder also features various screw holes 12 for heat-sink mounting and reflector location holes 13. It will be noted in this case that a LED with two solder pads has been fitted but in principle a holder could be designed to accept a LED with any number of solder pads.

Figure 4 shows the rear of the holder without the LED. It has a pocket 14 to take the LED ceramic and circular aperture 15 to accommodate the light emitting surface. Tunnels are aligned so that, when the LED is in place, wires can poke through them and touch the solder pads. Two tunnels 16 are empty and two have wires 9 in them.

To explain the invention fully it is necessary to understand the purpose of the tunnels. Experiments were made based on our previous known design of LED holder where a wire was brought out of the channel and across the ceramic of the LED such that the bare end of the wire lay above the solder pad. Since the insulation had been stripped at the end and the insulated part of the wire was lying on the ceramic surface of the LED there was a gap (corresponding to the thickness of the insulation) between the bare wire and the pad. When solder paste was applied and heated to re-flow it, the molten solder was unable to bridge the gap between the pad and the wire and a joint was not made. The molten solder is drawn into the joint via surface tension, so it is necessary for the wire to touch the pad before soldering. Attempts to crush the wire down onto the pad were not successful because the wire is relatively stiff, and it bounced back up. Thus it is necessary to have tunnels formed in the holder, preferably with sloping roots, such that as the wires are fed into the tunnel the bare ends are forced down onto the solder pads. In a preferred version of the holder the tunnels are like square funnels with the sides coming in as well, to bring the strands of the bare wire together for better soldering.

Referring to figure 5 we can see two tunnels 16 which are symmetrical, one is empty and the other contains a wire. The plane where the top of the ceramic and the solder pads would lie is the bottom of the pocket 14. The ceramic itself is not in the pocket in this view. The sloping roof 17 can be seen in the empty tunnel, and its effect can be seen on the wire bare end 18 in the other tunnel as follows: the wire bare end comes out of the insulation 19 separated from the top of the ceramic and is brought towards the surface ot the ceramic where it can touch a solder pad because it has to follow the profile of the sloping roof. The total movement of the wire towards the ceramic is very small, just enough to bridge the gap caused by the insulation. It should be noted that the wire bare end is shown as a single strand for ease of drawing but will usually be multiple strands. Looking at either tunnel we have a wire entrance with a wide aperture where the insulated wire will fit, and a narrower wire exit where only the bare wire will fit. This is caused by funnel sides, and looking at the empty tunnel we can see one complete funnel side 20 and just the top edge 21 of the other. Acting together their effect is to reduce the aperture and bunch the bare wires together for consistent soldering. There is a second benefit that the aperture is too small for the insulation to pass through, so there is no chance that, during assembly, the wire will be pushed too far. In fact pushing the wire in until the insulation wedges in the funnel gives a consistent method of locating the right length of bare wire over the solder pads. It should be noted that the roof and sides of the tunnel do not have to be flat or discrete surfaces -they could be blended together and still achieve the same result.

Claims

Claims 1. A LED holder to facilitate soldered wire connections to the LED such that the wires and LED are inserted into the holder prior to soldering and the holder acts as a jig for the soldering.
2. A LED holder according to claim 1, where the wires lie in channels in the holder.
3. A LED holder according to claim 2, where the channels are on the top surface of the holder and a LED pocket is on the back surface of the holder.
4. A LED holder according to claim 2 or 3 where one end of the channels incorporate sloped wire guiding means to bring the bare ends of the wires to touch the solder pads of the LED.
5. A LED holder according to claim 4 where the sloped wire guiding means are tunnels with roofs that slope down towards the solder pad.
6. A LED holder according to claim 5 where the sides of the tunnels slope inwards to create a funnel.
7. A LED holder according to any of the previous claims where the plastic is high temperature plastic to resist the soldering temperature.
8. A LED holder according to any of the previous claims where soldering is achieved by re-flow of solder paste.
9. A LED holder according to claims 1 to 7 where soldering is achieved by running solder wire onto a solder pad whilst the holder is heated from below.