A test head is a very important part of a semiconductor IC tester (hereinafter tester) which is connected with the tester by means of thousands of cables to test a semiconductor device (hereinafter DUT) supplied thereto.
The test head usually contains several tens of circuit boards (hereinafter board(s)) consisting of ICs and other electronic parts. When the boards loading the ICs and electronic parts are in operation, they generate a great amount of heat. For instance, when the room temperature is 20.degree. C., the temperature inside the housing of the test head rises up to 40.degree. C. even if a built-in fan of the test head operated to cool off inside the housing.
In general, the electronic parts such as ICs loaded on the boards are very sensitive to temperature changes in that some functions such as delay time of ICs are easily affected by such changes. Thus, it is desirable that the changes of the room temperature should be minimized.
Moreover, when the inner temperature of the housing of the test head becomes too high, it causes the high defect rate of the boards, requiring replacement of the boards, adversely affects the reliability of the tester.
In addition, the test head is relatively heavy, for instance, it weighs 200 Kg. Thus, it is a very difficult and tiresome task to replace the boards.
An example of conventional technology is explained with reference to FIGS. 6 and 7. FIG. 6 is a partially cut-away plan view showing the inside of the test head housing. FIG. 7 is a partially cut-away side view showing the inside of the test head housing.
Inside of the test head housing is arranged to be cooled by a forced air cooling system by a room temperature of a room where the tester is located. A bundle of cables 90 is connected to sockets of boards 80 and 81 in a housing 9 of the test head. The boards 80 and 81 are inserted to board racks 70 and 71. The board rack 70 is positioned between fans 38 and 39. Similarly, the board rack 71 is positioned between fans 40 and 41.
In the conventional example, first, the fan 38 generates an air flow by drawing air at the room temperature in the room where the tester is located. Then, the air generated by the fan 38 cools the boards 80 mounted in the board rack 70, wherein the fan 39 blows out the air warmed by the boards 80. In a similar manner, the fan 40 generates an air flow by drawing air. Then, the air generated by the fan 40 cools the boards 81 mounted in the board rack 71, wherein the fan 41 blows out the air.
In order to improve the reliability of the test head, the junction temperature of the IC devices mounted on the boards needed to be low.
In addition, it is necessary to maintain the inner temperature of the test head and the junction temperature of the IC devices constant and not be affected by the temperature change of the outside atmosphere.
Particularly, if the temperature inside the test head greatly changes, for instance, more than 3.degree. C., a calibration operation for adjusting delay times throughout the system is required. As a result, a test through-put of the tester is lowered, causing a low test efficiency.
Thus, there is a need in the industry to freely control the temperature in the housing of the test head in a range of, for instance, .+-.1.degree. C.
The recent advancement of technology has realized a high density structure wherein a large number of boards are mounted in the test head. However, the advanced technology has also revealed a problem that the temperature inside the test head cannot effectively be controlled by the conventional technology.