JPS6349594B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Ceramic chip capacitors are widely used in hybrid modules and printed circuit modules due to their durability, volumetric efficiency, wide application range, and low cost. Generally, these chips are made by alternating rectangular electrode plates and dielectric layers, which are alternately attached to two terminal bands on opposite ends of the chip. The majority of ceramic chip capacitors generally have 1 as their dielectric material.
Manufactured using mill-thick barium titanate ceramic. Palladium silver is often used as the electrode plate, generally with a thickness of about 0.1 mil. Soldering is typically used to connect these terminal bands to the module's circuitry.

Generally, the primary means for soldering chip capacitors and chip components is with a soldering iron, which can damage not only the component but also the module. For example, when heat is transferred to an element, internal pressure is generated because it cannot escape in proportion to the expansion of gas molecules trapped within the element. Also, if adjacent layers of the element have different coefficients of thermal expansion, shear stress will occur within the element. Furthermore, even if materials are selected so that they have similar coefficients of thermal expansion, it is usually relatively difficult to make them have similar thermal conductivities, so a temperature gradient occurs between the layers, which causes shear stress. . Furthermore, stress is substantially generated at the soldering point due to two conditions. First, during the heating process, the chip is free to expand, but the module region is unable to expand due to mechanical restraint provided by the surrounding cooler material, resulting in compressive stress in localized regions of the module rather than expansion. Then, upon cooling to room temperature, the chip and the solder joints become under tensile stress as the chip is partially restricted from shrinking to its original length by the solidified solder. Second, if the solder spots are made one at a time, even if contact with the chip is minimized during the formation of the second solder spot, significant stress will be placed on the spots and the chip.

The internal component stresses described above can damage the components and thereby significantly reduce the reliability of the module. Examples of structural defects caused by stress are delamination of layers, twisting of the material, bubbles, and cracks. These effects include, for example, shorting or opening the circuit, or changing the capacitance characteristics. Furthermore, stress at the soldered points can lead to poor connections. Conventionally, various methods have been used to solder chip capacitors, and the most common method is to use a soldering iron. Other methods include the use of flame, hot air, or focused radiant energy. However, there remains a need for an apparatus and method for soldering chip capacitors that minimizes internal component stresses and solder point stresses.

It is an object of the present invention to provide the above-mentioned soldering device and method.

According to the invention, electromagnetic radiation having a wavelength in the range from 1 millimeter to 1 nanometer is separated into a plurality of beams, which beams have a spacing on a support device for supporting the material to be deposited. is directed to the collision area. In particular, this radiation preferably consists of pulsating coherent laser energy with a pulse width of less than 10 milliseconds, a pulse repetition frequency of less than 10 pulses/second, and a pulse train width of less than 1 second. Preferably, the parameters of these pulses are also selectable to suit various applications.

This device is used for soldering. In particular, the device is preferably used as a device for soldering chip elements onto circuit modules. In that case, the radiation preferably has a wavelength of 1.06 micrometers and a pulse width of 4 to 6 milliseconds.
The module may also be supported on an X-Y table controlled by a microprocessor and used to accurately position the module during soldering of chip components.

FIG. 1 shows the operation of an embodiment of the present invention. Two laser beams 2 and 4 are formed by directing a coherent laser beam 6 onto a double slot aperture disk plate 8. Preferably, the disc plate is selected from a plurality of disc plates so that the lengths and widths of the slots can be interchanged for various applications. Any pulsed laser, such as the Raytheon model SS-380, can be used as a laser radiation source in the present invention. Preferably, laser radiation having a wavelength of about 1.06 micrometers is used, for reasons explained below. Further, preferred pulse widths, pulse repetition frequencies, and total energy requirements will be discussed later. The combined energy of the two shaped beams is monitored by directing the beams to a conventional beam splitter mirror 10 which reflects 1% of the incident energy to a conventional energy monitor 12 as shown in FIG. . Remaining
99% passes through this mirror 10 to the reflecting prism 14
to go into. The function of this prism is to provide a sufficiently large spacing between the beams so that they simultaneously impinge on both ends of element 16 as shown in FIG. Each beam is then connected to a conventional cylindrical lens 18 and 2
Through 0, the energy is directed or focused into the dimensions and shapes described below. Each focused beam passes through a beam energy splitter mirror 22.
and enters 24. This mirror has the property of reflecting 99% of the beam's energy at that frequency towards the soldering surface 32 where the solder is to be melted. mirror 22
and 24 pass the remaining 1% of the energy to energy monitors 26 and 28. The position of mirror 24 is adjusted as shown in FIG. 1 so that the beam enters the soldering surface at different spacings to facilitate simultaneous soldering of opposite ends of differently sized elements. Furthermore, mirror 2
Similar to the adjustment in step 4, the adjustment is performed by giving a stepwise change to the position of the mirror 22. Also, the first
As shown, 100% reflective mirrors 34 and 36 direct images of visible light 38 and 40 from the soldering surface to television cameras 42, 44 to direct the two laser beams to the line of sight of element 16. Perform position alignment. Visible light may be transmitted to the soldering surface using optical fibers (not shown).
The configuration described above is well known.

To explain the operation with reference to FIG. 2, the protection plate 46
is opened by the operator to expose a conventional X-Y table (not shown). Generally, a printed circuit module 70, as shown in FIG.
6 is placed over a conductor pad 74 which is part of the printed circuit module and to which elements are to be soldered. After the protective plate is closed, an X-Y table control panel 48 having coarse and fine adjustment devices for both axes, respectively, is used to position the module, and when the laser 50 is activated, the two axes described in connection with FIG. so that the beams strike the bases at both ends of the chip element simultaneously. Proper positioning of the X-Y table is achieved by using a television monitor 5, which is connected to the television camera in FIG.
2.54 may be used. The laser is activated after the laser parameters described below are selected using the laser control panel 58. Laser radiation energy absorbed by the solder on the pad and the solder on the chip elements is converted to thermal energy and melts the solder. Once this energy is removed and the solder cools, solder joints are formed on each end of the chip element to connect it to the module's circuitry. The energy in the overall energy monitor and the individual beam monitors is indicated in units on the laser control panel.

In other embodiments, a microprocessor (not shown)
is used to control the position of the X-Y table and the excitation of the laser so that multiple step elements can be sequentially soldered onto a single module without operator intervention. Whether in automatic or manual mode, the X and Y coordinates of the X-Y table are displayed on a digital reader 59 on the X-Y control panel.

The soldering apparatus described thus far provides a soldering technique that is a significant improvement over prior art. These improvements result substantially from simultaneous melting of the solder at both ends of the chip and from the operator of the machine having very precise control over the soldering parameters.

Referring to FIG. 3 and the prior art described above, a chip capacitor is fabricated by alternating square electrode plates 60 and dielectric layers 62, each alternating with termination bands 64 at opposite ends of the chip. For example, the electrode plates are made of palladium silver and the dielectric is ceramic. The chips may be immersed in solder 66 during manufacture. When different strips of two materials with different coefficients of thermal expansion are joined along an interface and heated isothermally, the composite will be It curves like a concave. Neutral regions occur within each material creating a locus of unstressed points that define the boundaries of compressive and tensile stresses. And the maximum compressive and tensile stresses are generally at the surface of these materials.
In the case of chip capacitors in which bending moments are balanced by successive layers of ceramic and electrode plates such that the interfaces remain flat, these stresses are minimum at the center of each layer and maximum at the boundaries. Also, in the case of chip capacitors, even if adjacent layers have similar coefficients of thermal expansion, these materials generally have differences in thermal diffusivity such that these stresses are caused by differences in expansion caused by temperature differences, rather than differences in coefficients of thermal expansion. . Thermal diffusion of a material is defined as its thermal conductivity divided by the product of its specific heat and specific gravity. Therefore, to prevent damage to chip capacitors or chip components, it is generally necessary to minimize heat transfer to the chip during soldering.

The laser soldering apparatus described above allows very precise control over the amount and shape of energy transferred to the solder, thereby providing a means to minimize heat to the chip. For example, the basic formula for the energy required to melt a substance is E _M /V=δ(C△T+H _S ), where E _M is the desired melting energy (joules),
V is the volume to be melted (cm ³ ), δ is the density of the substance (g/cm ³ ), and C is the specific heat of the material (calories/g/°C) △
T is the difference between the melting temperature and the initial material temperature (°C), and H _S is the thermal diffusion (calories/g). Generally, the solder should be about 30-50 degrees above its melting temperature to obtain good fusion. Therefore, the required total energy is approximately E〓/V=δ(C△T ₁ +H _S )+δ(C△T ₂ ). However, ΔT ₂ is the difference obtained by subtracting the melting temperature from the required fusion temperature.

0.2 mil silver 1.5 mil copper followed by 1.5
Consider a chip capacitor of dimensions A (0.035 x 0.055 x 0.65 inches) with terminations made of Mil's 63Sn/37Pb solder, and these three materials are combined at a temperature of 32°C higher than the melting point of 63/37 solder, or 215°C. In terms of temperature, the total energy required to make each solder joint is 0.434 Joules. 50 more in solder
% reflection factor and 90% optical transfer factor, approximately 0.964 Joules should be provided to each beam for these conditions. Similarly, for an F size (0.235 x 0.075 x 0.210 inch) chip capacitor, each beam should provide 3.7 joules. A series of tests were conducted with the device and energy monitoring data was taken for various dual slot disk plate and laser parameter combinations. Accurate amounts of energy, theoretically calculated as described above taking into account errors that occur in practice, can be supplied from the device with high reproducibility and consistency.

The laser model SS-380 is a neodymium yttrium laser that operates at a wavelength of 1.06 micrometers.
It is an aluminum garnet (Nd-YAG) type. The pulse width can be selectively adjusted by changing the pulse repetition rate to 6 pulses/second, 4 pulses/second, 2 pulses/second, or 1 pulse/second.
Variable from 2 to 10 milliseconds in pulses per second and manual single pulses. The maximum output is 40 watts. In the preferred embodiment, this SS-380 laser was modified to provide even greater flexibility. Pulse widths up to 30 ms can be obtained in 0.1 ms increments. Also, 50 watts is 30 joules/
Pulse energy can be produced. Furthermore, burst mode is applied at 10 pulses/second or more.

Using a wavelength of 1.06 micrometers minimizes light reflection from the solder compared to a 10.6 micrometer carbon dioxide laser. As the energy absorption by the solder increases, the reflectance to the chip decreases and stress within the chip increases. Also, since this energy is coherent, the cylindrical lens can be designed to precisely focus nearly all of the energy so that virtually no incident energy goes directly into the chip. Additionally, the preferred laser energy shape is a series of pulses with a relatively narrow pulse width. For example, it has been found that for soldering a size A chip capacitor, it is effective to apply six pulses each having a pulse width of 4 to 6 milliseconds at a repetition rate of 6 pulses/second. The double slotted aperture disc plate used provides a beam with a depth approximately equal to the depth dimension (0.035 inches) as shown in FIG. Also, the laser amplitude is set to a maximum of 20%. When changing from one element size to another, it is better to change the double slot aperture rather than the amplitude to change the pulse repetition rate, pulse width, and duration to suit the purpose.

The narrow pulse train energy profile described above provides great protection against element damage during and immediately after soldering. First, the degree of heat release of liquid phase solder is considerably greater than that of solid phase solder. Therefore, a relatively high proportion of the heat applied to the solder after it becomes a liquid is transferred to the element. The energy applied to the solder is therefore precisely controlled so that it becomes liquid only for a very short period of time, on the order of milliseconds, minimizing the amount of heat that enters the element. Then, for any given point within the chip capacitor, the stress is greatest when the temperature difference between the pole and dielectric layers is greatest. Furthermore, the maximum difference is approximately 3
It was found that this phenomenon occurs with two time constants. That is,
One time constant is defined as the time for one point to rise to 63% of the total temperature rise caused by heating the material. This is logical because if the temperature rises very slowly over a relatively long period of time, the difference in thermal diffusivity of these layers will have little effect and the temperature difference between the two layers will be very small. Assuming that these expansion coefficients are very close, the stress to the limit becomes smaller as the number of time constants increases. Further, if the number of time constants is smaller than 3, the material does not have enough time to react. In many industrial applications, production rate is important and it is undesirable to take the time to slowly raise the temperature of the element. Therefore, it is important to transfer heat to the solder quickly to minimize stress. A 4-6 millisecond pulse introduces heat into the solder in much less than one time constant.

Simultaneous soldering of both ends of a chip capacitor using two laser beams provides a significant improvement in stress relief compared to conventional methods. Specifically, after cooling, the stress in the soldered portion becomes smaller, and therefore the stress applied to the chip also becomes smaller. Melting the solder on both ends of the element simultaneously without external contacts allows the element to float on top of the liquid solder. This provides a sufficient thickness of solder between the component and the printed circuit board. Specifically, to control stress within the chip and plastic fatigue of the solder, the thickness of the solder solidifying under the chip should be less than 0.064 mm (0.0025 inch).

Figures 4A and 4B illustrate the creep distance greater than the printed circuit module at each end of the element required to compensate for expansion of the element during soldering for a given δ, where the creep angle θ is the solder thickness. becomes smaller as increases. Specifically, since t ₂ is greater than t ₁ , θ ₂ is smaller than θ ₁ . FIG. 5 provides empirical data to show that the tensile stress applied to the element after cooling decreases as the thickness increases from 0.076 mm (0.003 inch) to 0.025 mm (0.010 inch). Alternatively, this simultaneous melting substantially eliminates the normal stresses on the chip when soldering one end of the element and then the other end.

Although soldering of chip capacitors has been described in detail above, it goes without saying that this technique can be easily applied to soldering of other items.

Some embodiments of carrying out the present invention are listed below.

(1) providing electromagnetic radiation having a wavelength in the range of 1 millimeter to 1 nanometer; separating the radiation into multiple beams of substantially equal energy; A method of bonding an element to a substrate comprising the steps of: directing the beams in a direction; and removing the plurality of beams to substantially simultaneously form a bond bonding the element to the substrate.

(2) a source of electromagnetic radiation with a wavelength in the range from 1 millimeter to 1 nanometer, a device for spatially separating this radiation into a plurality of beams, and a support device for supporting the object to be soldered; A laser soldering device comprising: a device for directing collision areas with a spacing of .

(3) The laser soldering device according to aspect (2), wherein the wavelength is 1.06 micrometers.

(4) The laser soldering apparatus according to aspect (2), wherein the electromagnetic radiation comprises a plurality of pulses having a pulse width in a range of 4 to 6 milliseconds.

(5) The laser soldering apparatus according to aspect (2), wherein the support device comprises an X-Y table controlled by a microprocessor.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the operation of an embodiment of the present invention;
Fig. 2 is a schematic external view of a laser soldering device, Fig. 3 is a partial sectional view showing a chip capacitor soldered to a module circuit, and Figs. 4A and 4B are two FIG. 5 shows the creep angle relationship for different solder thicknesses t ₁ and t ₂ . FIG. 5 shows the tensile stress in the chip as a function of temperature for two different solder thicknesses. 8...Double slot opening disk plate, 10...
Beam splitter mirror, 12... Energy monitor, 14... Reflection prism, 16... Element (chip condenser), 18, 20... Cylindrical lens,
22, 24...beam splitter mirror, 26,
28... Energy monitor, 38, 40... Visible light, 42, 44... Television camera, 46...
...Protection plate, 50...Laser, 70...Printed circuit module, 74...Conductor pad, 48...X-
Y table control panel, 52, 54...television monitor, 58...laser control panel, 60...
... Electrode plate, 62 ... Dielectric layer, 64 ... Termination band, 66 ... Solder.

Claims (1)

[Claims] 1. A method of soldering an electronic chip element having solder at both ends to two support conductor pads connected to a substrate, comprising a pulse width of 10 milliseconds or less and a pulse of 10 pulses/second or less. providing pulsed coherent laser radiation with a repetition rate, pulse train duration of less than 1 second, dividing the radiation into two simultaneous spatially separated beams of substantially equal energy; A method of soldering comprising the steps of: directing a beam inwardly to remelt the solder; substantially simultaneously forming a solder joint between the element and the pad and removing the beam. 2. The method of claim 1, wherein the pulse width is in the range of 4 to 6 milliseconds. 3. The method of claim 2, wherein the pulse train consists of 6 pulses. 4. The method of claim 1, wherein the wavelength of the radiation is 1.06 micrometers. 5. An apparatus for soldering an electronic chip element having solder at both ends to two supporting conductor pads connected to a substrate, the apparatus comprising: a radiation source providing laser radiation having a wavelength in the range from 1 millimeter to 1 nanometer; Apparatus for simultaneously forming two spatially separated beams from said laser radiation, said beams having a pulse width of 10 milliseconds or less, a pulse repetition rate of 10 pulses/second or less, and a pulse train connection of 1 second or less. a beam forming device comprising time-pulsed coherent laser radiation; and a device for directing the beam toward opposite ends of the electronic chip element to substantially simultaneously form a solder joint between the element and a pad; A soldering device consisting of. 6. The soldering apparatus according to claim 5, wherein the energy of the pulse train is selectable.