Patent Application
20110189383
The invention relates to a device and a method for inert gas cure for leadframe or substrate strips.
Leadframe or substrate strips with polymide (PI) on top of bonded chips are carried and stored in storage cassettes, mostly made of metal. The storage cassettes are placed in a temperature cure oven for a high temperature cure process in order to cure the liquid PI. With the curing process, the liquid PI is transformed into a robust and solid state for protecting the pressure sensitive die surface of high precision parts of the chips. For example, high precision analog parts are protected with this kind of die over coat, which is applied in the assembly sites after mounting and bonding the dies on the leadframes or substrates. Substrates possibly on the base of polymide with copper lines are for example used for ball grid arrays and may also need a die over coat cure. The die over coat needs to be cured at about 300° C. under an inert gas atmosphere, e.g. in a nitrogen purged oven.
Due to the volume of a cure oven, it requires strong efforts to completely remove oxygen from the cure oven in terms of flushing rate and amount of nitrogen. Furthermore, not all cure ovens available are completely gas tight resulting in a remaining oxygen level within the oven atmosphere.
It is an object of the invention to provide a method and a device for improved curing of leadframes or substrates.
In one aspect of the invention, there is provided a cover for use in a cure oven. The cover is configured to enclose an inner volume of a storage cassette air-tightly. The storage cassette is of the kind to store a plurality of leadframe or substrate strips having a die overcoat to be cured. Advantageously, this storage cassette may be the same storage cassette as used in the state of the art in which the bonded chips are carried and stored. The cover is provided with a first opening for supplying an inert gas to the storage cassette and with a second opening for letting the inert gas off.
In one aspect of the invention, the cover encloses the complete storage cassette. No changes are required on the storage cassettes. The cover may be formed by two lids, the two lids being attachable to each other air-tightly.
In another aspect of the invention, the storage cassette is open only on two sides and the cover is formed as two lids being attachable air-tightly to the storage cassette. When the storage cassette itself is already provided with air-tight walls, it is not necessary to have an additional cover at these sides. Thus, two lids attached to the storage cassette are sufficient to enclose the leadframes or substrates and the volume enclosed around the leadframe or substrate strips is even smaller and less material is needed for the cover.
The lids may be adapted to the storage cassette to be covered so as to extend over the storage cassette sufficiently to cover together all openings of the storage cassette.
A smaller volume is easier and faster to purge. Heat transfer to the enclosed volume and thus to the leadframe or substrate strips is less hindered. The material used for the cover and the storage cassette should allow for good heat transfer. Usually, the storage cassettes are made of metal and in an aspect of the invention the cover may also be made of metal.
In an aspect of the invention, the air-tight sealing of the volume enclosed inside the storage cassette between the lid and the storage cassette or between two parts of the cover is achieved by a rubber type fitting or seal.
To the first opening, a pipe of the kind used in the state of the art for coupling to the cure oven may be coupled directly to the cover for supplying the inert gas, for example nitrogen. The second opening allows output of the inert gas. The second opening may be coupled to a pipe leading either to a second storage cassette or to an exhaust pipe system. As the cover encloses the plurality of leadframe or substrate strips air-tightly, the inert gas atmosphere must only be provided inside the storage cassette and not inside the whole cure oven.
The invention further provides a box for use in a cure oven, the box comprising a storage cassette and a cover according to the invention. The cover may be formed as a front and a back cover. With the cover the storage cassette is sealed air-tightly form the inside oven gas environment.
In a further aspect of the invention, the second opening is configured to be coupleable via a second pipe to the first opening of a second box, so as to form a chain. In the case where a plurality of boxes is put into the cure oven to be cured at the same time, this allows a continuous flow of nitrogen through all boxes. Otherwise it is of course also possible to connect a plurality of boxes in parallel to a gas supply.
In an aspect of the invention, the box has a volume which is small compared to the inside volume of the cure oven. The smaller the volume is the better an oxygen-free atmosphere can be realized.
In another aspect of the invention, the first opening for supplying the inert gas is arranged on an end of the box opposite to the end where the second opening is arranged. To provide an oxygen-free atmosphere it must be ascertained that the inert gas fills out completely the volume of the box. Therefore, the two openings must be arranged on the box in a way to make sure that the inert gas will fill up the whole volume.
The invention further provides a method for curing a die overcoat on a leadframe or substrate. The method comprises arranging a plurality of leadframe or substrate strips having a die overcoat to be cured in a storage cassette. The method further comprises enclosing the leadframe or substrate strips in the storage cassette air-tightly in a box which has a first and a second opening. The box is placed in a cure oven and an inert gas is supplied by the first opening into the box for preventing oxidation of the leadframe or substrate strips. The cure oven is heated to cure the die overcoat. Usually, the temperature necessary for curing is about 300° C. The temperature depends of course on the die overcoat used. To make sure that the leadframes or substrates are heated sufficiently, the material used for the cover and the storage cassette must allow for good heat transfer. Usually, the storage cassettes are made of metal and in an aspect of the invention the cover may also be made of metal.
In a further aspect of the invention, the method further comprises preheating the inert gas before being supplied into the box. Thus, the heat is not only transferred by the cover and the walls of the storage cassette, but also by the inert gas itself. Preheating of the inert gas flow may be achieved by a coil design of the pipe coupled to the first opening being located inside the cure oven.
In a further aspect of the invention, the method comprises the step of supplying the inert gas at the beginning of the cure process with a first flush rate and supplying the inert gas during the cure process temperature ramp with a second flush rate, wherein the first flush rate is higher than the second flush rate. Thus, in a first time the oxygen is flushed out and afterwards the pressure is sufficient to maintain the inert gas or nitrogen atmosphere.
Further advantages and characteristics of the invention ensue from the description below of an example embodiment and from the accompanying drawings, in which:
In function, leadframe strips 12 are covered with a die overcoat which is to be cured. Boxes 18 are air-tightly sealed. The inert gas is supplied to boxes 18 through pipe 22 at the beginning of the cure process with a first flush rate. The inert gas is introduced by first openings 20 into boxes 18. Thus, the air inside boxes 18 is pushed out through the second opening provided in each box 18. The gas pushed out of boxes 18 can leave cure oven 14 by exhaust 24.
After a first time period which is judged to be sufficient to provide an oxygen-free atmosphere inside boxes 18, the temperature of cure oven 14 is augmented in a cure process temperature ramp. The gas supply to boxes 18 is reduced to a second flush rate which is lower than the first flush rate. With the gas flowing with the second flush rate the oxygen-free atmosphere in boxes 18 is maintained. Compared to flush rates used when the complete oven volume must be oxygen-free, smaller flush rates are necessary when using boxes 18.
If it is necessary to prevent oxygen to enter by the second opening, which depends on the partial pressure, the second opening may be coupled to an exhaust pipe system. Therefore, it is possible to couple a pipe to the second opening. The heat in the oven is transferred via the walls of boxes 18 and the atmosphere inside the boxes 18 to the leadframe strips 12 and the die overcoat is cured.
In a preferred embodiment which is not shown in
Although the invention has been described hereinabove with reference to a specific embodiment, it is not limited to this embodiment and no doubt further alternatives will occur to the skilled person that lie within the scope of the invention as claimed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 006 393.2 | Feb 2010 | DE | national |
