There are known methods for coating a lead section of a lead frame, on which a semi-conductor device is mounted and packaged within a synthetic resin, with a soft material such as solder material. One is an electro-solder plating method, and the other is a dipping or immersion method. Many variations of these processes have been proposed. Roughly speaking, in one variation the packages are cut off with the leads from the lead frame, and after making a single device, each side of the device is soaked or immersed in a liquid solder bath. In a second variation the lead section of the lead frame is soaked or immersed so as to be coated without cutting off the single device from the lead frame.
The Assignee of the present invention discloses in Laid-down Japanese Patent Application SHO 63-80963 and U.S. Pat. No. 4,836,131 a solder coating apparatus which coats the lead section of the lead frame, before cutting off each packaged device from the lead frame, by a pair of two coating rolls arranged in a vertical plane. However, in this apparatus, the number of pairs of coating rolls required corresponds to the number of packages on the lead frame. This can be undesirable.
The Assignee of this invention has also proposed a new solder coating method as shown in Laid-down Japanese Patent Application HEI 1-18077 and U.S. Pat. Nos. 4,858,554 and 5,057,337, in which two liquid solder baths cooperate with a pair of opposed coating rolls so that a lead frame can pass through between the opposing rolls, whereby the liquid solder material is coated on both sides of the lead frame passing through between the two rolls. A lower part of each roll is immersed in the liquid solder material and the rolls rotate in opposite directions to coat the lead frame as it is moved between the two rolls. However, in this method, movement of the lead frame must be vertical when said lead frame is coated with solder material. After it is coated, the lead frame must be moved horizontally. This requires an intricate transfer mechanism, and makes the overall apparatus more complex than desired.
Another coating method is to use the capillary phenomenon of liquid solder material. In this method, however, only a lower part of the lead frame can be coated with the solder material. It is difficult to simultaneously coat both sides (upper side and lower side) of the lead frame. As to such method, the Assignee has proposed a coating device in Japanese Patent Application Serial No. 1-251 989 and U.S. application Ser. No. 07/764 730 (now U.S. Pat. No. 5,200,368) in which a lead frame is pushed in and passes through the coating breech of a solder coating block so that a section of the lead frame contacts liquid solder which comes up through a slit from the bath and fills the passage of the lead frame.
The solder coating apparatus mentioned in the foregoing paragraph has desirable features in that the lead frame can be coated with solder film while the workpiece is transferred continuously in horizontal lay. The apparatus does not have moving parts except for the feed-in and take-out mechanisms of the workpieces, and the coating process itself is simple as is its structure.
But in this above-described method and structure, while the soldering operation proceeds continuously, nevertheless metal oxides, deteriorated flux and gross grow and gradually attach around the coating block and its passage. They grow to such extent that these contaminates cause defects in the coated film as the contaminates become mixed with the solder material flow and cause partial passage blockage and uneven film thickness. Thus, the solder bath and the solder coating block must be frequently cleaned to make soldering operation satisfactory.
In the present invention, a new solder coating apparatus is proposed in which liquid solder material at a bottom of the solder bath is forcibly supplied to each slit of the coating block in a constant amount to avoid adherence of the unusable materials.
In an attempt to attain the above, this invention provides a solder coating apparatus in such way that the coating block has a passage for the lead frame and a second passage for the package, and a liquid solder supply slit is cut along the lower two sides of the lead frame passage. A pump, the main body of which is immersed in the liquid solder bath, supplies liquid solder through a connecting pipe to the liquid solder supply slit.
Moreover, the solder coating apparatus is furnished with an intermediate solder reservoir which is installed under the solder supply slit, and is connected with the connecting pipe.
In performance of the invention, clear, melted solder material is always supplied by the pump to the lead frame passage through the solder material supply slit, and abundantly fills said passage; the lead section of the lead frame moves through the passage filled with liquid solder material, whereby both sides of the lead section are coated with solder. Molten solder material is returned and reserved in the solder reservoir, and is again sent to the lead frame passage through the solder material supply slit which is connected with the solder reservoir.