DE112020007317T5 - METHOD OF CONNECTING SURFACE MOUNT ELECTRONIC COMPONENTS TO A CIRCUIT BOARD - Google Patents

Abstract

A method of connecting an electronic component to a circuit board is disclosed. First, a substrate and an electronic component with solder sandwiched between them are placed under a flash lamp. Multiple pulses of light from the flash lamp are applied to the electronic component, substrate and solder until the solder reflows. During the exposure to the light pulses, the power of one of the light pulses from the flashlamp and the temperature of the electronic component are measured, the measured power is converted into radiation exposure, and in response to the measured temperature of the electronic component, the duty cycle of a next light pulse is dependent is adaptively adjusted by the radiation exposure of the one light pulse.

Description

TECHNICAL AREA

The present application relates to manufacturing methods in general, and more particularly to a method of connecting surface mount electronic components to a printed circuit board.

BACKGROUND

In general, electronic components can be connected to a circuit board by soldering. With the advent of surface mount technology and wave soldering, the need to manually solder electronic components onto a circuit board has largely disappeared from a production perspective.

The surface mount technique requires the use of solder paste with solder particles dispersed in an organic flux. Solder paste is selectively applied to a printed circuit board, electronic components to be soldered are placed thereon, and then the printed circuit board is placed in a reflow oven equipped with a conveyor. When heated, the reflow oven reflows the solder paste to activate the flux in the solder paste, clean the surfaces, and melt the solder material. The molten solder wets the surfaces and solidifies, creating a good electrical/mechanical connection between the electronic components and the circuit board.

However, the reflow process has some limitations. Because the entire assembly must go through the reflow oven, the entire PCB and electronic components must be able to withstand the high temperature required to reflow the solder paste. With the standard SAC-305 solder paste, the temperature is around 217 °C. Another limitation of reflow is the long time it takes to process the solder paste to allow the solder to reflow. Typically it takes about 10 minutes as the thermal profile of the reflow oven is carefully controlled and can involve several predetermined steps of ramping up, soaking and ramping down the temperature zones.

One solution to the high temperature requirements of reflow is to use solder paste with a lower reflow temperature. Currently, such solder pastes tend to have poor thermal and mechanical properties compared to standard SAC-compliant solder pastes. Another solution to the high temperature requirements of reflow is to use electrically conductive adhesives instead of solder paste. This enables processing at almost room temperature (25°C). As with low-temperature solder pastes, the mechanical and electrical robustness of the connections is sacrificed.

In the reflow process, energy is transferred to the electronic components, the circuit board and the solder paste by convection, which leads to another problem. The shear of the air in the reflow oven can cause electronic components to be blown off the board or misaligned before they are soldered to the board. This becomes a bigger problem when the electronic components are very small, since their aerodynamic cross-section is larger in relation to the weight.

An alternative approach to reflow is to use a laser. With this approach, a solid-state laser can be aimed at solder joints or electronic components to heat them and reflow or reflow the solder without heating the entire circuit board. The laser process is able to deliver a precise amount of power and energy to the solder joints to reflow the solder paste in about a second. However, the laser process is serial in nature, so the total time required for a board with many solder joints increases with the number of solder joints. In addition, the laser process requires alignment with solder joints. In other words, it requires either a vision system and/or careful alignment and programming of the position of each spot to be soldered. In addition, the laser process can be affected by uneven beams, which can lead to uneven heating of the electronic components. This leads to poor solder connections, damage to other parts of the circuit board, or "tombstoning" of electronic components.

Another alternative approach to reflow is to use a continuous light source such as a tungsten filament lamp or a tungsten-based halogen lamp. A problem with this approach is that turning the light source on and off takes a finite amount of time, about a second. Unlike the laser process, a continuous light source cannot be modulated on the time scale of the thermal equilibrium time of solder paste deposition, which is usually less than 100 milliseconds. This limitation makes it difficult to precisely control the power and energy profile applied to the material being processed. This is especially true for an exposure where the processed target material is stationary during the exposure. In the case where the target material is being conveyed, a similar problem exists since the ideal exposure is generally coupled to the conveying speed. Adjusting either the conveyor speed or the exposure level cannot be done quickly enough to control the machine's power output to ensure even heat treatment of the material being processed.

Accordingly, it would be desirable to provide an improved method of electrically and mechanically connecting surface mount electronic components to a circuit board.

BRIEF DESCRIPTION OF THE INVENTION

According to one embodiment, a substrate and an electronic component with solder material sandwiched between them are placed under a flashlamp. Multiple pulses of light from the flash lamp are applied to the electronic component, substrate and solder until the solder reflows. During the application of the light pulses, the power of one of the light pulses from the flashlamp and the temperature of the electronic component are measured, the measured power is converted into radiation exposure and in response to the measured temperature of the electronic component the duty cycle or " duty cycle” for a next light pulse is set adaptively depending on the radiation exposure of the one light pulse.

All of the features and advantages of the present invention will become apparent in the detailed description that follows.

character list

• 1 eine isometrische Ansicht eines Systems zur thermischen Bearbeitung von Lötpaste zum Verbinden von oberflächenmontierbaren elektronischen Bauteilen mit einer Leiterplatte gemäß einer Ausführungsform;
• 2 ein detailliertes Blockdiagramm des Systems aus 1 gemäß einer Ausführungsform;
• 3 ein Flussdiagramm eines Verfahrens zum Verbinden von oberflächenmontierbaren elektronischen Bauteilen mit einer Leiterplatte; und
• 4 eine grafische Darstellung der Intensität der Blitzlampen-Emission gegenüber der Wellenlänge bei zwei unterschiedlichen Zündspannungen der Kondensatorbank.

The invention proper, as well as a preferred mode of use, other objects and advantages of the invention, are best understood by reference to the following detailed description of an exemplary embodiment when read in conjunction with the accompanying drawings; show:

• 1 14 is an isometric view of a solder paste thermal processing system for connecting surface mount electronic components to a circuit board according to an embodiment;
• 2 a detailed block diagram of the system 1 according to one embodiment;
• 3 a flowchart of a method for connecting surface-mountable electronic components to a printed circuit board; and
• 4 Figure 12 shows a plot of flashlamp emission intensity versus wavelength at two different capacitor bank ignition voltages.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to the drawings and in particular to 1 1 is an isometric view of a system for thermally processing solder paste for connecting electronic components to a circuit board, according to one embodiment. As shown, a system 100 includes a housing 110 that rests on an equipment mount 160 . The housing 110 has a flap 120 which can be opened using a handle 140 . A semi-transparent window 130 is provided in the flap 120 through which a user can observe all processes within the housing 110 . At least one flash lamp (not shown) is positioned within housing 110 to facilitate thermal processing. Users may enter and receive information from the system 100 via a touch screen 150 .

The housing 110 may contain a controlled environment chamber (not shown), which may be filled with an inert gas, such as nitrogen, or a reactive gas, such as formic acid, or with a vacuum, in which thermal processing of electronic components is performed can be executed. In addition, the housing 110 may include a conveyor (not shown) for transporting electronic components during thermal processing.

Various mechanical devices and electrical components designed to operate the system 100 may be housed in the device rack 160 .

With reference to 2 Illustrated is a block diagram of system 100 according to one embodiment. As illustrated, the system 100 includes a capacitor bank charging power supply 210, a capacitor bank 220, an IGBT (Insulated Gate Bipolar Transistor)-based switching device 230, a frequency controller 240, a flash lamp 250, a photodiode 260, a Integrator 265, a bolometer 270, an infrared camera (IR camera) 280 and a computer 290. Computer 290 includes a processor and various memory devices well known to those skilled in the art. The capacitors in the capacitor bank 220 are, for example, electrolytic capacitors. Some of the components mentioned above can be installed in the equipment carrier 160 (from 1 ) be housed.

The capacitor bank 220 can be charged by the capacitor bank charging power supply 210 . The charges from the capacitor bank 220 are then discharged into the flashlamp 250 via the IGBT-based switching device 230, the IGBT-based switching device 230 being repeatedly turned on and off by the frequency controller 240 during the discharge. The frequency controller 240 regulates the drive of the IGBT-based switching device 230, which in turn controls the switching frequency of the discharge. The repeated turning on and off of the IGBT-based switching device 230 serves to modulate the flow of current from the capacitor bank 220 to the flashlamp 250, which in turn turns the flashlamp 250 on and off. In other words, this means that the frequency or pulse length of the light pulses emitted by the flash lamp 250 is predetermined by the frequency controller 240 .

Photodiode 260 must be calibrated before system 100 can be used for thermal processing. The photodiode 260 can be calibrated using the bolometer 270, which is preferably traceable to the National Institute of Standards and Technology (NIST). During calibration, both the photodiode 260 and the bolometer 270 are exposed to a single pulse of light emitted by the flashlamp 250, respectively. The bolometer 270 measures the radiation exposure or energy per area (in units of J/cm ² ) of the single light pulse, and the photodiode 260 measures the instantaneous power (in units of W) of the same light pulse. The instantaneous power signals from the photodiode 260 are then integrated by the integrator 265 to obtain a radiation exposure value of the same single light pulse, and the radiation exposure measurement from the bolometer 270 is divided by this radiation exposure value by the integrator 265 to generate a calibration factor as follows: $$Kalib\mathrm{right}ie\mathrm{right}\mathrm{and}nGsfaktO\mathrm{right}=\frac{St\mathrm{right}aHl\mathrm{and}nGsexpOsitiOnsmess\mathrm{and}nG}{St\mathrm{right}aHl\mathrm{and}nGsexpOsitiOnswe\mathrm{right}t}$$

After calibration, the photodiode 260 and integrator 265 combination can be used to obtain information about the radiation exposure of each light pulse emitted by the flashlamp 250. Basically, the radiation exposure information of a light pulse emitted by the flashlamp 250 can be obtained by multiplying the calibration factor obtained during calibration by the output value of the integrator 265 (which is the radiation exposure value of the light pulse emitted by the flashlamp 250 obtained by integrating the light pulses emitted by the photodiode 260 measured instantaneous power signals of the light pulse emitted by the flash lamp 250) are calculated.

Although photodiodes are technically designed to measure instantaneous power and not radiation exposure, the rationale for using photodiodes, such as photodiode 260, to obtain radiation exposure information of the light pulses from flashlamp 250 in system 100 is that photodiodes are reliable and small are enough to be placed near the flashlamp 250 or sample light from a small optical fiber that does not obscure the light pulses. Because photodiodes have a relatively high sampling rate of over 1 MHz, they are also inexpensive.

However, as mentioned above, in order to use a photodiode to obtain the radiation exposure information of light pulses from a flashlamp, the photodiode must be calibrated. A reason for the calibration can be found in 4 , which plots the emission intensities of two flashlamp discharges with identical pulse length as a function of wavelength for two different capacitor bank voltages, namely 300 V and 400 V. The light pulses emitted by the flashlamp are broadband - from -250 nm to ~1,700 nm. The integral of the intensity over the emission range is proportional to the emitted power in W/cm ² . With increasing voltage, emission increases in all bands, but emission in the shorter wavelengths increases faster than that in the longer wavelengths. In other words, when the discharge voltage of the capacitor bank changes, the spectral emission distribution also changes. Unfortunately, a photodiode has a sensitivity that varies with wavelength. Therefore, the direct output of the photodiode cannot tell anything about the quantitative power output due to the emission of the flashlamp, since the magnitude of the signal is a convolution of the photodiode's sensitivity and emission intensity, both of which vary with wavelength.

The calibration method mentioned above is designed to overcome the deficiencies of a photodiode. By using a bolometer, such as bolometer 270, and an integrator, such as integrator 265, the photodiode can be generated by integrating the power signals from a single output from a flashlamp Light pulses are calibrated. A bolometer, unlike a photodiode, can measure the radiation exposure (total energy) emanating from a light pulse emitted by the flashlamp. This is essentially due to the fact that the radiation detection element has a flat or even sensitivity over the entire emission spectrum. Thus, a shift in the emission spectrum of the flashlamp does not affect the bolometer's ability to measure all light striking it. But also the radiation exposure of light pulses from the flashlamp cannot easily be measured using a bolometer. This is because the time response of a bolometer is very poor. A typical time resolution of a bolometer can be around 1 Hz, that of a photodiode in excess of 1 MHz. Also, a bolometer is usually quite large and will therefore block some of the light from a flashbulb.

With reference to 3 Illustrated is a flowchart of a method of connecting an electronic component to a circuit board according to an embodiment. Beginning with a step 300, a photodiode, such as photodiode 260, is turned off 2 , first calibrated using a bolometer, such as the 270 bolometer 2 , as shown in block 310. Calibration is accomplished using a single pulse of light from a flashlamp, such as the 250 flashlamp 2 to obtain a calibration factor as mentioned above. After calibration, the photodiode is ready to be used to measure the radiant power of each light pulse emitted by the flashlamp.

Next, a circuit board or substrate with some solder paste is placed between an electronic component and the circuit board under the flashlamp, as shown in block 320 . The solder paste can be any solder paste, including SAC compliant solder paste or thick film material greater than 50 microns thick. The substrate can be a printed circuit board using standard printed circuit board material such as FR4, a thermoset or a thermoplastic with a maximum working temperature of less than 200°C. Multiple pulses of light from the flashlamp are then applied to the electronic component, solder paste, and circuit board, as shown in block 330 .

During the application of light pulses (ie, thermal processing), the radiation exposure value of each light pulse from the flashlamp is determined by the calibrated photodiode and an integrator such as integrator 265 2 , is detected and the temperature of the electronic component being soldered onto the circuit board is viewed through an IR camera, such as IR camera 280 2 , determined as shown in block 340.

In conjunction therewith, the IR camera monitors the circuit board to measure the temperature of the electronic component and/or the temperature distribution across the circuit board during the application of light pulses. Because the emission from the flashbulb is in the range of 250nm to ~1700nm and the IR camera mainly "sees" in the 10 micron region of the electromagnetic spectrum, there is no interference of the flashbulb with the infrared emission through the PCB . This allows the IR camera to "see" the heating across the PCB during flashlamp processing.

The power information measured by the photodiode is converted to a radiation exposure value using the calibration factor determined during the calibration process (i.e., at block 310). Based on the radiation exposure value of a light pulse and the measured temperature information of the electronic component, the frequency and/or duty cycle of a subsequent light pulse may be adaptively adjusted prior to application of the subsequent light pulse, as illustrated in block 350 .

For example, a computer, such as the computer 290 from 2 , a frequency controller such as frequency controller 240 2 , to adjust the duty cycle for an upcoming light pulse to be emitted by the flashlamp based on the radiation exposure value and the measured temperature information to track a desired temperature profile of the electronic component on the circuit board. A temperature profile of an electronic component on a printed circuit board can look like this: time unit/s Temp/°C 0 25 1 240 5 240 6 180

Next, a determination is made as to whether or not the solder paste has reflowed, as shown in block 360 . If the solder paste has not reflowed, the process returns to block 330; however, once the solder paste has reflowed, the application of light pulses may be terminated, as shown in block 370. Those in blocks 330, 340, 350 and 360 steps shown are carried out iteratively. After reflowing the solder paste, it then cools and solidifies. At this time, the electronic component is connected to the circuit board via the solidified solder.

For a given ignition voltage of a capacitor bank, such as capacitor bank 220, off 2 , the integrated photodiode signal is proportional to the radiation exposure of each pulse over a wide range of pulse length and pulse frequency. There are two requirements for this technique. The first requirement is that, in order to correlate the energy emitted by the flashlamp on a pulse-to-pulse basis using the integrated photodiode signal over a range of firing frequencies, a capacitor bank charging power supply, such as the 210 out of 2 , must be large enough to accommodate the capacitor bank, such as the 220 capacitor bank 2 , fully recharge before the respective light pulse is emitted. The second requirement is that in order to correlate the integrated power signals from the photodiode to the bolometer over a range of pulse lengths, the voltage across the capacitor bank must drop minimally during discharge. This ensures that the spectral peak output intensity remains relatively constant during the discharge. Therefore, a large capacitance is preferable, and the size of the capacitor bank depends on the accuracy required for the power measurement. Preferably, less than 5% of the stored energy of the capacitor bank is discharged with each pulse.

The technique described in the present disclosure allows for a traceable measurement of the radiant energy emitted by the flashlamp. For example, at a firing rate of 10 Hz, the average power output (or cumulative energy) at a data rate of 10 Hz can be known. With a firing rate of 50 Hz, the average power output can be known at a data rate of 50 Hz. In addition, during the processing interval greater than 1 second, the firing rate and/or duty cycle of the flashlamp can be electronically adjusted by frequency control to produce a tailored performance profile. By varying the pulse rate and/or the duty cycle, the amount of energy delivered over time is varied.

The heating profile for an electronic component can be preprogrammed or modified during processing, as detected by a circuit board (or material) trained temperature sensor during processing. For the latter, knowing the emission energy from the flashlamp is only half the problem. The entire board is illuminated, including the circuit board, metal traces, electronic components, and solder paste. Because each element has a different absorptivity for flashlamp radiation, thermal mass, and thermal connection to surrounding components, each element is heated to a different temperature by a uniform emission source. It is therefore important not only to know the emission of the flashbulb, but also the heating behavior of the various electronic components on a circuit board due to the emission.

During the processing time, which is 1 to 10 seconds, several thermal processes can be performed by the system 100, such as activating the flux, evaporating the flux, heating the solder particles to their melting point, melting the solder particles, and reflowing the solder Lots. Because all of the above processing steps are performed in a very short period of time, it is necessary to be able to deliver a known amount of power to a circuit board and to be able to vary that power during processing.

The system 100 is also capable of delivering the same average power over a period of time with a varying spectral output power. This allows for a better distinction between what is heated by the beam and what is not heated on a circuit board, since different parts or electronic components on a circuit board have different absorptivity across the emission spectrum of the flashlamp 250 . In particular, when the ignition voltage of the capacitor bank 220 is higher, the total power across the spectrum is higher than at a lower voltage, but the proportion of light with shorter wavelengths is higher (see Fig 4 ). In other words, with a lower ignition voltage of the capacitor bank 220, the spectral emission is lower in all bands, but the proportion of light with longer wavelengths is higher. However, the same amount of radiant power delivered over many pulses can reach a given level regardless of the ignition voltage because the flashlamp duty cycle can be adjusted. This can be achieved by varying the pulse length and/or the pulse frequency. More specifically, if an area of the circuit board to be heated is more absorptive at longer wavelengths of the emission spectrum, a longer pulse length or high frequency pulsing of the flashlamp 250 can be used and the same che emission power profile as applied at a higher voltage, shorter pulse length and shorter frequency. In addition, an optical filter can be used for the emission to allow even better discrimination. For the system 100, the control parameters include the capacitor bank voltage, the pulse length, the pulse rate, and the total duration of the pulse train.

System 100 may include one or more lamp housings, each containing at least one flashlamp, such as flashlamp 250 . This enables thermal processing of solder material on a printed circuit board as well as a thick film on a substrate by irradiation from above (e.g. irradiation from the thick film side), irradiation from below (e.g. irradiation from the substrate side) or both. The top and bottom flashbulbs can be independently synchronized in their firing, or they can receive the same firing trigger signal.

The conveyor within the system 100 has two modes of operation, namely a stationary mode and a synchronized mode. In the stationary mode, a circuit board is not transported during processing and the exposure profile is controlled electronically by the frequency controller 240. In this case, the pulse frequency and thus the power emitted by the flash lamp 250 can be changed over the course of the processing. Alternatively, the pulse length of each pulse can be altered during processing to adjust the energy delivered by the flashlamp 250 to the circuit board per pulse. In the synchronized mode, the pulse rate of the light pulses from the flash lamp 250 can be synchronized with the conveyor speed to obtain the same radiation exposure, ie total energy, expressed in J/cm ² , applied to the board being transported. In this case, the emitted radiation exposure can be independent of the conveyor speed.

During the synchronized mode, the flashlamp housing that holds the flashlamp 250 and is equipped with a reflector to direct the light from the flashlamp 250 towards the circuit board can be tilted upwards, e.g. rotated in the conveying direction or against the conveying direction, while the circuit board is moved past the flashlamp housing. When the flashlamp housing is placed parallel to the transported circuit board, the exposure is very uniform, with a variation of about 2 to 3% over the exposure area. When the flashlamp housing is farther from the circuit board, the light pulses are less intense. By tilting the head of the flashlamp 250, the intensity of the exposure can be increased or decreased as the circuit board is moved past the head of the flashlamp 250. This feature allows better control of the gas generated during processing of the solder paste, so that the electronic components can be attached successfully and without errors.

As described above, the present invention provides an improved method of electrically and mechanically connecting surface mount electronic components to a printed circuit board.

Although the system 100 is designed to process solder paste, it is also suitable for processing any thick film over 50 microns due to the same problems, such as the need to apply a precise and large amount of energy (>30 J/cm ² ). apply a precise power profile over a period of 1 to 10 seconds with multiple light pulses. In addition, the disclosed method enables the mounting of thermally sensitive electronic components on a printed circuit board by reducing the overall thermal budget over the prior art. The reduced thermal budget also enables the use of processing thick films on thermally sensitive substrates with maximum operating temperatures of less than 200°C. Examples of this are thermoplastics such as PVC, TPU, polyester, PET, PEN, paper, etc.

While the invention has been shown and described in detail with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention.

Claims (20)

A method of connecting electronic components to a substrate, the method comprising the steps of: placing a substrate having solder material between an electronic component and the substrate under a flashlamp; applying a plurality of light pulses from the flashlamp to the electronic component, the solder and the substrate until the solder reflows; during the application of light pulses, measuring the power of one of the light pulses from the flashlamp and converting the measured power to a radiation exposure value; measuring the temperature of the electronic component being soldered onto the substrate; and in response to the measured temperature of the electronic component, adaptively adjusting a duty cycle of a next light pulse dependent on the radiation exposure value of the one light pulse. procedure after claim 1 , wherein power measurement is performed using a photodiode. procedure after claim 2 , further comprising calibrating the photodiode to generate a calibration factor. procedure after claim 3 , wherein converting further comprises converting the measured power to the radiation exposure value using the calibration factor. procedure after claim 1 wherein measuring a temperature is performed using an infrared (IR) camera. procedure after claim 1 wherein adjusting a duty ratio is performed by changing a frequency of the light pulses. procedure after claim 1 wherein adjusting a duty ratio is performed by changing a pulse length of the light pulses. procedure after claim 1 , wherein the substrate is a printed circuit board. procedure after claim 1 , wherein the substrate is a thermoplastic with a maximum working temperature of less than 200°C. procedure after claim 1 wherein the solder material is a thick film material having a thickness greater than 50 microns. procedure after claim 1 , further comprising transporting the electronic component during the application of the light pulses. procedure after claim 11 , further comprising synchronizing the speed of transporting the electronic component with a frequency of the light pulses. procedure after claim 1 , further comprising placing the electronic component in a controlled environment chamber prior to applying the light pulses. System for connecting electronic components to a substrate, the system comprising: a capacitor bank; a capacitor bank charging power supply for charging the capacitor bank; an IGBT-based switching device; a frequency controller; a flashlamp for emitting light pulses, the flashlamp receiving charges from the capacitor bank via the IGBT-based switching device, while the IGBT-based switching device is repeatedly turned on and off by the frequency controller to increase the flow of charge from the capacitor bank to the flashlamp modulate, which in turn turns the flashbulb on and off; a photodiode for measuring the power of a light pulse from the flashlamp; an infrared camera for measuring the temperature of an electronic component to be soldered on a substrate; and a processor responsive to the sensed temperature of the electronic component to adaptively adjust a duty cycle for a next pulse of light via the frequency control dependent on a radiation exposure derived from the sensed power of the pulse of light from the flashlamp. system after Claim 14 , further comprising a bolometer for calibrating the photodiode. system after Claim 14 , wherein the capacitor bank comprises a plurality of electrolytic capacitors. system after Claim 14 , wherein the processor adjusts the duty cycle of the light pulses by changing a frequency of the light pulses via the frequency controller. system after Claim 14 , wherein the processor adjusts the duty cycle for the light pulses by changing a pulse length of the light pulses via the frequency control. system after Claim 14 , further comprising a conveyor for transporting an electronic component and a substrate to be processed. system after Claim 14 further comprising a controlled environment chamber for containing the electronic component during the application of the light pulses.

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