GB2261620A

GB2261620A - Soldering

Info

Publication number: GB2261620A
Application number: GB9124573A
Authority: GB
Inventors: Peter M Beckett; Andrew Victor Polijanczuk; Donald Gill Whitehead
Original assignee: University of Hull
Current assignee: University of Hull
Priority date: 1991-11-20
Filing date: 1991-11-20
Publication date: 1993-05-26
Also published as: GB9124573D0

Abstract

Parts to be soldered are subjected to at least two different levels of energy input during heating. High power semiconductor laser diodes 10 can be used and by varying the laser diode power during the process more effective use can be made of the available laser energy. The same laser device can be used to provide the preheat energy and also to illuminate the workpiece for initial positioning under the observation of a camera. As shown a computer 13 controls the input 12 to the laser diodes 10. The radiation is conducted by fibre optics 14. <IMAGE>

Description

SOLDERING The invention relates to soldering.

It is well known that there are several distinct phases through which a solder passes as it is heated during the formation of a joint. During these phases, the ability of the solder to absorb heat energy depends on many factors associated with the physical state of the solder. For example, if solder paste is used, too much heat energy directed on to the solder base before it reaches a molten state will cause the paste to blow apart. Too little energy, and the flux will melt and disperse without melting the solder.

According to a first aspect of the invention, there is provided a method of soldering comprising assembling parts to be soldered with the solder between them and then heating the solder in a heating cycle that includes the application of heat to the solder at two different energy levels.

In this way, once the solder has reached its melting point, more heat energy can be applied to create rapidly the soldered joint. In addition, pre-heating of the joint can be performed at a lower heat energy, prior to melting, when solder paste is used.

Although the application of lasers to soldering is a fairly recent technology, as early as 1974 C.F. Bohman ('The Laser and Microsoldering', Bohman C.F., Society of Manufacturing Engineers, Tech.Paper AD74-810 Mich.48128, 1974,pup19) discussed the use of lasers in soldering. The more recent impact of microelectronics and surface mount technology coupled with the availability of reliable laser units has, however, now made the technology economically viable ('Laser Soldering of Surface Mounted Devices', Meyer F.G. Brazing and Soldering, Spring 1987, Vol.12.

pp40-41) Both Nd:YAG and CO2 lasers have been successfully applied to the production of soldered joints for the electronics industry ('Application of Laser Microsoldering to SMD's' Lish.E, Martin Marietta Aerospace, Orlando,Fl. International Symposium on Microelectronics, Anaheim 11-14 Nov.1985). The advantage lies in the ability to direct the energy beam accurately and precisely on to the target area to be soldered without heating the surrounding parts. The minimum rise in substrate temperature reduces mechanical stress and the rapid melting and cooling of the solder prevents the formation of inter-metallic compounds which, when present, can cause brittleness and possible joint failure.

However, rapid precise and controlled variation in output power levels during the process of soldering a joint, where the total energy pulse may be required to be present for typically 100 milliseconds, is not possible when Nd:YAG or CO2 lasers are used as the heat energy source.

According to a second aspect of the invention, there is provided a method of forming a soldered joint comprising assembling parts to be soldered and a solder between the parts, directing at the joint a beam from a solid state laser diode and adjusting a forward current supply to the diode to so vary the radiant power of the beam as to produce at least two different radiant power levels.

High-power solid state laser diodes have an extremely fast response to variations in their forward current level. By adjusting the current through the diode, and hence the radiant power output, several desirable characteristics can be achieved.

Preferably, the solder is first subjected to a lower radiant power level to heat the parts without melting the solder and is then subjected to a second higher power level to melt the solder.

Additionally or alternatively, as the solder is melted by the beam, the radiant power of the beam may be altered in accordance with the absorption properties of the solder.

The diode may be controlled to emit a first lower radiant power beam until the solder is melted and then to emit a second higher radiant power beam to create the joint.

The diode may be controlled to emit initially a beam at lower radiant power for use in positioning the joint relative to the diode and then be controlled to emit a beam at least one higher radiant power level to melt the solder.

The following is a more detailed description of an embodiment of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a schematic view of a soldering system including a laser diode, Figure 2 is a graph showing the variation in output power with forward current of a typical laser diode of the system of Figure 1, and Figure 3 is a graph showing the variation of power with time during the soldering method.

Referring first to Figure 1, the system comprises a laser diode assembly 10 provided with a heat sink 11. The diode assembly 10 is connected to a controllable power supply 12 which supplies a forward current to the diode and which is in turn connected to a computer 13. The output of the diode assembly 10 is connected to an optical fibre 14 which terminates at its end in a focussing device 15 for the laser energy.

The power supply 12 adjusts the forward current of the laser diode assembly 10. As seen in Figure 2, the forward current may, in one particular embodiment, be varied between about 0 and 15 amps to produce output powers of between 0 and 4.5 watts. Between about 6 and 15 amps, the correlation is linear.

In use, the system produces a joint between, for example, the parts 16,17 shown in Figure 1. These are assembled with solder or a solder paste 18 between them and can be held in place by a positioning tool 19. In practice, the parts 16,17 are located within a chamber and are viewed through a camera (not shown).

In order to aim the focussing device 15 correctly relative to the part 16,17 the power supply 12 is adjusted so that the output power of the diode 10 is very low. This provides a low radiant energy beam at a wavelength of 800-900 nanometers which is within the waveband to which the camera is sensitive. The beam can thus be viewed and the device 15 suitably aimed.

The power supply 12 is then adjusted by the computer 13 in accordance with the desired sequence of energy levels most suited to the particular soldering application. One such sequence is shown in Figure 3. In this sequence, the diode assembly 10 has its lower power output in the period from tl to t2 with the radiant power level being sufficient to preheat the parts 16,17 without melting the solder 18. In the period from t3 to t4, the radiant power output is raised to the level at which the solder or solder paste 18 melts without "blowing apart". Once it is melted, the radiant power output level is raised in the time period t5 to t6 to the correct temperature to create the soldered joint. The power is then switched off.

In this way, fast and more reliable laser soldered connections can be made.

It will be appreciated, of course, that the cycle shown in Figure 3 can be varied in a number of ways. For example, the diode assembly 10 can be controlled only to provide solder melting and joint forming temperatures.

Alternatively, it could be controlled to provide only preheating and joint forming, or any other combination of these operations. For example, using a 63Sn37Pb solder paste, a TSOP lead can be soldered by applying laser diode energy at 0.25 Watt for 4 seconds for preheat and then increasing the power to 3 Watts for 200 milliseconds to form the joint.

Claims

1. A method of soldering comprising assembling parts to be soldered to form a joint with solder between the parts and then heating the solder in a heating cycle that includes the application of heat to the solder at at least two different energy levels, the heating cycle melting the solder to form said joint.

2. A method as claimed in claim 1 wherein the solder is heated by directing at the joint parts a beam from a solid state laser diode having a radiant power and adjusting a forward current supply to the diode to so vary the radiant power of the beam as to produce said heating cycle.

3. A method as claimed in claim 2 wherein the heating cycle includes a first lower radiant power level to heat the parts without melting the solder and includes a second higher power level to melt the solder.

4. A method as claimed in claims 2 or 3 wherein, as the solder is melted by the beam in the heating cycle, the radiant power of the beam is varied in accordance with absorption properties of the solder.

5. A method as claimed in any one of claims 2,3 or 4 wherein the heating cycle includes controlling the diode to emit initially a beam at lower radiant power for use in positioning the joint relative to the diode.

6. A method substantially as hereinbefore described with reference to the drawings.