Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
1997-07-03
2001-01-16
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S282000, C427S372200, C427S468000, C427S510000
Reexamination Certificate
active
06174562
ABSTRACT:
THE FIELD OF THE INVENTION
The present invention relates to a method and a device for manufacturing of solder stops in plated through-holes in printed boards and a solder stop for improving the solder quality in soldering connected to through-holes.
STATE OF THE ART
A product which consists of an electrically insulated material with a pattern of an electrically conducting material, e.g. a printed board, often comprises a protective layer of lacquer on predetermined surfaces of the product. In the case that only certain parts of the pattern are to be coated with solder, then the other parts are delimited or covered with a so-called solder resist lacquer. The lacquer prevents on the one hand that bridges of solder occur between adjacent electrically conducting surfaces and on the other hand that solder flows out along the pattern. The solder resist lacquer also function as an insulating protection against moisture, particles, components and corrosion which can damage the electrically conducting material. Certain types of solder resist lacquer comprises i.a. a type of curing agent which reacts to UV-light and to heat.
There are often so-called plated through-holes in printed boards. These holes go straight through the printed board whereby the holes are coated with a conducting material (plating). A through-hole is used, for example, for connecting an electrically conducting surface on one side of the printed board to an earth plane on the other side of the printed board.
The solder resist lacquer can be applied on the printed board by a number of known methods. Many of these methods imply that the solder resist lacquer on certain occasions can flow down into the through-holes on the printed board. This is considered to be a big disadvantage and an example of bad process control, whereby the printed board becomes the subject of a complaint. In European Patent Application EP-A2-0 433 720 a method is described in which solder resist lacquer is removed from a through-hole in a printed board assembly. The method comprises that the solder resist lacquer is coated with a layer of photo resist. U.S. Pat. No. 4,230,793 also relates to a method for coating a printed board with solder resist lacquer. The through-holes on the printed board are carefully cleaned of solder resist lacquer.
In order to save space on the printed board or form shorter connections, a through-hole can be placed under or close to a pad. A pad is the surface of the electrically conducting material to which a component is connected.
During so-called re-flow soldering of a printed board assembly it can occur that the solder to which the components are attached flows down into the through-hole. This can occur without hindrance in the case that the metal surface of the through-hole has been laid bare, for example, with the above mentioned method. The soldered joints are thereby impaired.
In order to prevent solder from being able to flow down into an adjacent through-hole during re-flow soldering it is also earlier known to seal the through-hole with an epoxy paste.
DISCLOSURE OF THE INVENTION
The present invention tackles a problem which occurs during re-flow soldering of a printed board assembly, i.e. a printed board with soldered components which are connected to surfaces with plated through-holes. More precisely, the problem occurs in the case where the solder on a printed board assembly during re-flow soldering becomes liquid and flows down in the through-holes. This means that the quality of the soldering deteriorates.
A similar problem occurs during wave soldering and drag soldering, where the solder can unintentionally climb up into the through-holes.
The object of the present invention is consequently to prevent solder from passing through a through-hole, e.g. a plated through-hole, during different types of soldering.
The invention gives a method, a device which is used during the method and a solder stop for solving said problems. The inventive idea is that by using the earlier mentioned disadvantage where solder resist lacquer in certain situations flows down into the through-holes, through influencing the viscosity of the solder resist lacquer so that a certain quantity of lacquer intentionally flows down into the through-holes. The lacquer in the hole cures and forms a solder stop in the through-holes. Said solder stop thereby prevents the liquid solder from passing through the through-hole by the solder stop made of solder resist lacquer preventing the solder from flowing down and wetting the walls of the through-hole.
More concretely, the invention relates to a method where the viscosity of the solder resist lacquer is chosen so that the solder resist lacquer always flows down in one or more through-holes. Thereafter parts of the product are delimited so that non-transparent and transparent surfaces cover the lacquer. The non-transparent surfaces which cover said through-holes completely or partially surround transparent center surfaces. A subsequent illumination of the solder resist lacquer then takes place from both sides of the product. This means that the solder resist lacquer in the holes is illuminated partly through the earlier mentioned transparent center surfaces on one side of the product, and partly through the openings of the through-holes on the opposite side of the product. In this way, said solder stop is formed in the through-hole when the solder resist lacquer is polymerized, i.e. cured.
A device for producing said delimitation is formed according to the invention by a mask which comprises the said non-transparent and transparent surfaces, where the non-transparent surfaces completely or partially surround certain predetermined transparent center surfaces.
The advantages of the present invention is that it is simple and very cheap to perform. The invention can be performed with existing equipment and as a part of already known production steps, which means that it does not take any extra time in the manufacturing process. The invention is suitable both for small series production and for mass production. No extra material is required to be inserted in the through-hole in order to prevent the solder from passing through the through-holes.
The invention can be performed during different types of soldering, e.g. wave soldering and re-flow soldering.
REFERENCES:
patent: 4230793 (1980-10-01), Losert et al.
patent: 5134056 (1992-07-01), Schmidt et al.
patent: 5373985 (1994-12-01), Chiba et al.
patent: 5402314 (1995-03-01), Amago et al.
patent: 0 433720 A2 (1990-11-01), None
patent: 2 200 253 (1987-06-01), None
patent: 6880 (1991-01-01), None
patent: 97588 (1992-03-01), None
PCT Search Report, SE 96/00799, Feb. 19, 1997.
Burns Doane Swecker & Mathis L.L.P.
Talbot Brian K.
Telefonaktiebolaget LM Ericsson (publ)
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