GB2180959A - Apparatus for the burn-in of integrated circuits - Google Patents

Abstract

Apparatus is disclosed for the burn-in of integrated circuits (12), comprising: a chamber (B) for receiving such circuits; first means (4), for heating such integrated circuits in the chamber (B) with the circuits (12) fed with supply voltages; and control means for controlling the first means (4), including a computer (15) for monitoring power consumed by the circuits, the computer (15) being capable of storing information concerning the number of said circuits and being adapted to provide, from said power, a signal dependent on the mean power dissipation of the circuits: and in which oven the control means controls the heating of the circuits in dependence on said signal. Coolant may also be supplied via a valve (6). <IMAGE>

Description

SPECIFICATION Apparatus for the burn-in of integrated circuits Afterthe manufacture of integrated circuits, it is common for them to undergo a testing process called "burn-in" by being subjected to an elevated temperature in a chamberofan oven with operating supplyand control voltages applied to the circuits.

Two problems associated with known burn-in apparatus and how attempts have been made to overcome them are as follows: 1. When certain integrated circuits are burned-in, if the control and supply voltages are removed before the temperature has reduced to ambient from the high temperature at which the burn-in took place, any induced failures may begin to recover and negate the effect ofthe burn-in process. To preventthis taking place, it is necessary for the supplies to be maintained to a burn-in assembly until ambient temperature is reached, this being known as cooling under bias.

This may be achieved in several ways. Firstly the boards carrying the integrated circuits (the burn-in boards) may be removed from their supplies for a duration of up to 1 minute and plugged into another power supply in a cooling atmosphere. This, however, is now being frowned upon by many users-thetime being considered fartoo long. Secondly, a burn-in board may be ejected into the atmosphere, but this meansthat, with a single common chamberforall the boards, the gapthat is left must be resealed and the supplies must be maintained through eitherflexible cables, which are prone to wear, orthrough a sliding contactsystem which may be inherently unreliable.

2. Most burn-in chambers allow either one temperature zone or at the bestfourzoneswheredifferent temperatures may be setfordifferent kinds of integrated circuits.

To explain why the latter is necessary we may consider two types of integrated circuit in the same package.

Typically, a Dual-in-line (DIL) package has a thermal resistance of 1 00#C/W, that is to say for each watt of dissipation in the circuit, the junction temperature (silicon chip temperature) will rise by 1 000C.

Since burn-in is an accelerated chemical reaction, it follows that this temperature should be precisely controlled.

If we nowconsidertwo devices: (a) CMOS gate with a dissipation of 1 mW; (b) 256K DRAM with a dissipation of 500 mW; applying a figure of 100#C/W,we can seethatthe silicon chip will be at the chamber temperature, plus an additional factor as follows: (a) Silicon temperature Oven temperature (Toven) + (100 x .001) Toven + 0.1"C.

(b) Silicon temperature Toven + (100 x .500) = Toven + 50"C.

This illustrates the problem, showing a 500C temperature difference between types and demonstrates the need for precise temperature control of each burn-in board.

According to the present invention, there is provided apparatus forthe burn-in of integrated circuits, comprising: first means, for heating such integrated circuits with the circuits fed with supply voltages; and second means for controlling the first means, including a computerfor monitoring power consumed bythe devices, the computer being capable of storing information concerning the number of said devices and being adapted to provide,from said power, a signal dependent on the mean power dissipation of the devices, in which apparatus the second means controls the heating of the devices in dependence on said signal.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1, 2 and 3 show end, side and plan views of an individual burn-in assembly; Figure 4shows from thefrontthe shape of an oven incorporating such assemblies; Figure 5 is a view of part of an alternative form of burn-in assembly; and Figure 6shows electronic circuitry associated with each burn-in assembly.

The burn-in apparatus to be described fulfils two important improvements, the first being the ability to cool individual burn-in boards under bias without moving parts, the second being the ability to be able to control individually junction temperatures of integrated circuits being burned-in.

Referring to Figures 1 to 4 of the accompanying drawings, apparatus for the burn-in of integrated circuits comprises an oven defining a plurality of individual chambers 1 lagged bythermal insulation 2, in each of which chambers is received a respective burn-in board heater assembly as shown by Figures 1,2 and 3. Each burn-in board heater assembly comprises: an upper sealed chamber; a lower sealed chamber B (the chambers A and B being separated by an interface surface 3); a series of highlythermally conductive, e.g.

aluminium or copper, electrically energised heater plates 4 (only one being shown on the surface 3; a cooling fluid inlet 5 communicating with the chamberavia a control valve 6, a cooling fluid exhaustoutlet7fromthe chamberA; a temperature sensor8 in the chamberA; afan 9 inthechamber B driven by a motor 10; and cooling fluid baffles 13 in the chamber A. In use, the chamber B receives a burn-in board 11 carrying integrated circuits 12 undertest.

The chambers A and B are sealed from each because a nitrogen atmosphere may be admitted to chamber B to preventtarnishing ofthe contacts during burn-in, whilst the coolant fluid which may be air or, under certain circumstances, a liquid is admitted into chamberA and the two must not mix. The interface surface 3 controls the heat flow between chamber A and chamber B, the heaters 4 being positioned to provide an isothermal plane whose temperature may be controlled by a separate system (see below), the temperature being monitored by the temperature sensor 8.

The still airthatwill be generated in chamber B at the control temperature is moved by fan 9 and normally this temperature will be maintained by the temperature sensor and associated system. Alternatively, a heater 4 may be placed in the position shown in Figure5, in which reference numeral 14 denotes a direct radiation shield. If, however, the dissipation of the integrated circuits being burned-in exceeds that of the natural losses through the lagging,then coolant control valve 6 may be opened by the control system to admit cooling fluid which will flow over the surface 3 by means ofthe cooling fluid baffles 13. This fluid will then exitthroughthe exhaust outlet7 and the product of the specific heat and mass of cooling fluid will provide the necessary cooling function.This cooling function, however, may also be employed atthe end of the burn-in processto enable the particular burn-in assembly to be taken to approximately ambient temperature. In the first mode, that is maintaining a burn-in temperature where excess heat must be removed, the heaters 4 would normally be off and a very short burst of cooling fluid would be admitted, the integrating function ofthetemperature being taken care of by the mass of the assembly walls and interface surface 3. For the cooling under bias mode, however, the operation is different. The system controlled (see below)would command a continuous flow of cooling fluid, in which case the temperature would be taken to approximately ambient.

Aburn-in assembly as described above relies upon the fact that it generates a control signal from the temperature sensorS, receives power into the heaters 4 and has a control signal to operate coolant control valve 6. Electronic circuitry to perform these functions will now be described with reference to Figure 6. The system allows automatic allowance forvariations in integrated circuit dissipation, though the system may also be adjusted manually to achieve the main benefits.

The system shown in Figure 6 comprises a computer 15 which accesses two comparators 16 and 17 via a digital to analogue (D-A) converter 18; a heater relay switch 19 controlled by the output of comparator 16; a control valve relay switch 20 controlled by the output of comparator 17, the comparators 16 and 17 receiving inputs from the temperature sensor 8; power supplies 21, which supply D.C. to the burn-in board monitored voltage and current values being output from supplies 21 via lines denoted by reference numeral 22 to an analogue to digital (A-D) converter 23 whose output is fed to the computer 15.

The D-A converter 18 provides an analogue signal to drive comparators 16 and 17. The function ofthese comparators is such that a temperature sensitive signal derived from the temperature sensor8 is compared with a known aiming point generated within the computer and outputted on to a digital bus 24 into the D-A converter 18. Comparator 16 controls heater switch 19, which may be either a conventional relay or more commonly a solid state relay with zero voltage switching. When the level defined by comparator 16 is reached, the heaters 4 are turned. When the temperature exceeds this level by a difference of voltage V (an internally generated offsetvoltage), switch 20, which again may be either a conventional relay or a solid state relay, opens control valve 6, admitting the cooling fluid, which may be eitherairora liquid.The resultant coolantflowsweeps overthesurface of chamberAthus removing heatfromthe chamber B containing the burn-in board.

The control temperature, however, may be adjusted to conform to the burn-in requirements of the circuits undertestand the reason for this variation may be explained with reference to thefollowing two simple calculations (see also above). Since burn-in is primarily a chemical reaction, the temperature of the silicon chip itself must be controlled. If we consider a typical package of the dual in line type which will have a thermal resistance of approximately 1 00'C per watt, it can be seen that a CMOS chip, whose dissipation is effectively zero during a burn-in period, will have a junction temperature almost exactly equal to that ofthe oven temperature. A second case, however, we must consider is a complex chip that may have a dissipation of, for example, 500 mW.In this case, the dissipation at the chip will raise the silicon junction area approximately 50 C. above that of the ambient air. We can thus see that if the chambertemperature is 1 500C, then a CMOS device would be burned-in at a junction temperature of 1 50"C. The more complex chip, of which we have previously given the example, would be burned-in at a junction temperature of approximately 200"C.

It can, therefore, be seen that the vital component that must be defined accurately is (in a standard burn-in chamberwhere all integrated circuits are subject to one temperature) not at all precisely controlled.

The function of the control system of Figure 6 overcomes this problem by the following method.The computer 15 monitors the power supply to determine the power being fed into the burn-in board, and an operatorwill have already informed the computer how many devices are located on this board. The mean dissipation of each device may be therefore calculated by the computerwith a knowledge of the device package thermal resistance. It can be seen that, quoting the example of the second device type, the burn-in temperature could be then set to 1 00'C as opposed to the 1 500C required forthe CMOS device. This simple case therefore illustrates that, with a knowledge of power in and the number of devices, the junction temperature (the critical temperature in a burn-in procedure) may be controlled from one individual burn-in assembly to another, thus overcoming one of the majorvariables in the burn-in equation.

The comparators 16 and 17 are required to enable a small "deadband" to be achieved between the heaters i turning off and the coolant being applied. This is standard in any closed loop controlled system. The coolant demand signal is therefore derived from changing the output potential of the D-Aconverter 18 being fed to comparator 17. In this manner, by setting the comparison voltage to approximately ambient, an uninterrupted flow of coolantfluid will willbe admitted, thusfulfilling the cooling under bias requirement as described above.

Claims (7)

1. Apparatus for the burn-in of integrated circuits, comprising: a chamberfor receiving such circuits; first means, for heating such integrated circuits in the chamberwith the circuits fed with supply voltages; and control means for controlling the first means, including a computer for monitoring power consumed by the circuits, the computer being capable of storing information concerning the number of said circuits and being adapted to provide, from said power, a signal dependent on the mean powerdissipation of the circuits: and in which oven the control means controls the heating ofthe devices in dependence on said signal.

2. Apparatus according to claim 1 comprising second meansforcooling said chamberandwherein said control means also controls said second means to effect a reduction in thetemperature of said chamber.

3. Apparatus according to claim 2 wherein said second means includes a second chamber sealed offfrom said adjacent the or each said chambers with a thermally conductive division or divisions there between such that cooling fluid in said second chamber effects cooling ofthe or each chamber.

4. Apparatus according to claim 3 wherei n said first means is mounted in the second chamber and said chamber is heated by thermal conduction through said division.

5. Apparatus according to claim 1 wherein said first means is mounted in the or each said chamberand with a baffle to prevent direct radiation of heat onto aid circuits and meansfo providing circulation of heat from said first means to said circuits.

6. Apparatus according to claim 2 wherein said signal provides a reference signal for comparator means which comparator means also receive a temperature signal indicative of the temperature in said chamberthe comparator means providing control signals for said first or second means to produce heating or cooling respectively to maintain a circuit semiconductor junction temperature substantially at a specified desired temperature by taking into account the warming effect of said junction by the respective calculated power dissipation.

7. Apparatus for the burn-in of integrated circuits substantially as hereinbefore described with reference to the accompanying drawings.