SU1248747A1 - Method of forming solder lug on printed circuit board backing - Google Patents

Description

The image relates to soldering, in particular to methods for producing solder protrusions on printed circuits, and can be used in electronic industry.

The purpose of the invention is to increase the accuracy of forming the protrusions,

The drawing shows a device for carrying out the proposed method.

A stencil 2 with holes 3, made of a material that is not wettable by solder, such as titanium, is fixed on the printed circuit board strip 1. Holes 3 in the stencil 2 coincide with the contact pads of the substrate 1 (not shown). A gap 4 is formed between the stencil and the substrate with a size less than the diameter of the smallest. Aperture 3 in the stencil 2. The stencil 2 is sealed to the nozzle 5, and the gap 4 enters the inside of the nozzle 5. The stencil 2 and the nozzle 5 are immersed in a bath with pa. fused solder 6. In the same bath, the shutter 7 is immersed, made in the form of a plate of non-wettable solder material, such as titanium. The surface of the solder is covered with a layer of protective liquid 8, for example glycerol.

The process of forming solder protrusions is carried out as follows.

In the bath with the molten solder b, the shutter 7 is immersed to a predetermined depth. Then, the stencil 2 with the nozzle 5 and the substrate is immersed in the bath. After immersion between the sides of the stencil 2, a pressure differential is established equal to the hydrostatic pressure of the liquid npi column. As the stencil 2 and the nozzle 5 are immersed, the hydrostatic pressure increases and becomes sufficient to fill the holes with solder 3, but still not enough to fill the gap 4, since the gap 4 is much smaller than the diameter of the hole 3 in the stencil and, consequently, - T (it is more likely to distort the surface of the solder, which requires a proportionally greater pressure. All this is true only if the solder does not wet the stencil material and the substrate. When filling the holes, 3 melts solder displaces the gap 4 inside the nozzle 5 that has fallen into the holes when immersed

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 shield fluid 8 and wets the surface of the contact pads. Groundwater is freely squeezed out through the gap, as it has low viscosity and perfectly wets the stencil material and the substrate. Upon further immersion of the surface of the stencil 2 and the shutter 7 are in contact and the apertures 3 are intersected. The shutter 7 is immersed to a depth at which the hydrostatic pressure exceeds a value sufficient to fill the holes with solder in the stencil, but does not reach the value at which the solder fills the gap 4.

The order of the operations depends on the task. To form solder protrusions due to surface tension forces, the underlay and stencil are separated before the solder crystallizes, after the stencil, the shutter and the substrate are raised into the protective liquid layer 8. , the shutter and the substrate are removed from the bath with solder and cooled. In any form of molding, the stencil material and the shutter should not be wetted with solder, since otherwise solder will remain in the holes of the stencil.

Other options are possible to create a pressure drop on the sides of the stencil — by evacuating the volume inside the nozzle 5, or by sealing the bath with solder 6 and applying pressure to the liquid solder,

The separation of the substrate and the stencil is performed both before and after the crystallization of the solder.

Separation before crystallization allows the protrusions to be molded due to the surface tension of the solder, at which the protrusions are self-centered due to the wetting forces and do not require precise alignment of the holes in the stencil with the contact pads of the substrate. It is advisable to use this molding option for large ceramic substrates having low dimensional stability due to the uncontrolled shrinkage of ceramics during heat treatment.

Disconnection after cristoplization allows the formation of protrusions with enhanced crack resistance.

The method of forming solder protrusions is tested in laboratory conditions on microcorps with a matrix distance; the position of the contact pads having 25 to 100 contact pads dia- meter: 1.5 mm, located with a step of 2.5 mm. Stencils made of titanium and aluminum are used.

Solder protrusions with a height of 0.5-2 mm are formed with an accuracy of 0.005 mm, which ensures a high-quality installation of micro-corpuses. The gap between the substrate and the stencil is approximately 0.05 mm. Stable filling of the holes in the stencil with a diameter of 0.8 - 1.5 mm does not depend on the stencil thickness when immersed in liquid solder to a depth of 50 mm.,

Editor O. Bugir Order 4167/11

Compiled by F. Konopelke

Tehred N. Bonkalo - Corrector M.Sharopsh

Circulation 1001 Subscription

VNIIPI USSR State Committee

for inventions and discoveries 113035, Moscow, Zh-35, Raushsk nab., d. 4/5

Production and printing company, Uzhgorod, st. Project, 4

The proposed method makes it possible to exclude the operations of predosing the solder blanks, laying them out on the stencil holes, combining the operations of filling the holes in the stencil and melting the solder on the surface of the contact pads, which

significantly increases productivity.

Cases of partial filling with solder of holes in the stencil and paste are excluded, which increases the molding accuracy.

In addition, it is possible to simultaneously apply various amounts of solders to contact pads, which expands the functional possibilities of the method and allows, for example, on the same substrate to simultaneously form solder protrusions for mounting microcaps of various designs.

Claims (4)

1. METHOD FOR THE FORMATION OF SOLDER OUTBARS ON A PRINTED SUBSTRATE • SCHEME, in which a stencil is fixed on the substrate, the holes in the stencil are filled with molten solder, and the substrate with the stencil is disconnected, characterized in that, in order to increase the forming accuracy, a gap is formed between the stencil and the substrate. less than a diameter _{: a} meter of the smallest hole in the stencil, filling the holes with solder is carried out under pressure, providing contact of the solder with the substrate, then the holes in the stencil are closed with a shutter. 2. The method of pop. 1, characterized in that the dose of solder is set by the thickness of the stencil and the area of the hole in the stencil. 3. The method of pop 1 and 2, characterized in that the separation of the substrate and the stencil is carried out before crystallization of the solder. 4. The method of pop 1 and 2, characterized in that the separation of the substrate and the stencil is carried out after crystallization of the solder. SU,. „1248747