JP2011044661A - Wire bonding structure in semiconductor device and design method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a wire bonding structure which is compact, lightweight and inexpensive, and can sufficiently ensuring durability and reliability even when a semiconductor device is made large in capacity. <P>SOLUTION: With respect to the wire bonding structure in a semiconductor device wherein a semiconductor element S and a circuit component C to be electrically connected to it are installed and fixed apart from each other on a base body B and an electrode part of the semiconductor element S and the circuit component C are connected by a conductive metal wire W extending between them, the wire W includes one end part Wa bonded to an electrode part surface of the semiconductor element S, the other end part Wa' bonded to a surface having a height difference ΔL from the electrode part surface, of the circuit component C, and a loop part Wm connecting the one end part Wa and the other end part Wa' into one body and has at least an intermediate part curved upward convexly. The height difference ΔL is set within a range of 1 to 5 mm, and a loop height h of the loop part Wm is set within a range of 2 to 7 mm. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a wire bonding structure in a semiconductor device such as a power module, in which a semiconductor element and a circuit component to be electrically connected to the semiconductor element are connected by a metal wire extending therebetween.

  The metal wire used for the wire bonding structure in the semiconductor device is made of, for example, a conductive metal such as Al or Cu, and has one end joined to the electrode part surface of the semiconductor element and the other end joined to the surface of the circuit component. And a loop portion in which one end portion and the other end portion are integrally connected and an intermediate portion is bent upward and convex.

  Conventionally, it has been considered that the life of the wire bonding portion as described above of the semiconductor device is dominated by the temperature rise caused by the heat generation of the element with respect to the required life, and is caused by the heat generation of the element. Focusing on the energization current value and energization time and the element temperature obtained under the conditions, an acceleration test by temperature acceleration is performed, and the lifetime is predicted from the endurance diagram obtained from the acceleration test result. However, such a life prediction method performs an acceleration test every time when only one of a part of a semiconductor device, for example, a part of a module component, a wiring position of wire bonding, or the number of wires is changed. Since the endurance life has to be obtained, there is a problem that it takes a very long time to obtain a life chart.

  In addition, as a technique for ensuring a high life tolerance against the power cycle of the bonding wire and improving the reliability, attention has been paid to the shape of the wire joint, and a technique for defining the joint shape has already been proposed ( For example, see Patent Document 1).

Japanese Patent Laid-Open No. 2004-140072

  By the way, the wire bonding part of the semiconductor device, particularly the wire bonding part of the power module, operates even when the temperature change is repeated between the rising temperature that the element can tolerate due to the energization current value in the actual use environment and the temperature that falls when turned off. The durability of the wire has been set by seeking that it can be guaranteed by experiment. In the prior art of Patent Document 1, the shape of the wire joint is defined in consideration of the stress generated in the wire joint due to the temperature change.

  However, the durability of the wire is not only due to the shape of the wire bonding part, but also to the height of the loop, especially the height of the bonding surface of the semiconductor element / circuit component to which both ends of the wire are bonded (so-called hit point height difference). A large dependence has been determined by the inventors' research.

  An object of the present invention is to provide a wire bonding structure in a semiconductor device and a design method thereof that are small, light, inexpensive, and sufficiently reliable, based on the research results.

  In order to achieve the above object, according to the present invention, a semiconductor element and a circuit component to be electrically connected to the base body are installed and fixed at intervals on the base body. In a wire bonding structure in a semiconductor device in which a wire is connected by a conductive metal wire extending therebetween, the wire is joined to the surface of the electrode portion of the semiconductor element, and the circuit component The other end joined to the surface of the electrode portion and the surface having a height difference, and a loop portion integrally connected between the one end and the other end, and at least an intermediate portion bent upwardly, The first feature is that the height difference is set within a setting range of 1 to 5 mm, and the loop height of the loop portion is set within a setting range of 2 to 7 mm.

  In addition to the first feature, the present invention has a second feature that the wire is made of aluminum.

  Furthermore, the present invention is a method for designing a wire bonding structure in the semiconductor device having the first or second feature, wherein the bonding portion between one end of the wire and the surface of the electrode portion of the semiconductor element, and the Estimate the distortion generated at the joint between the other end of the wire and the circuit component, and based on the correlation value between the distortion and the reliability, the height difference and the loop height are within the set range. Setting is a third feature.

  In the present invention, the “loop height” refers to the bonding surface of the semiconductor element or circuit component to which the higher end of the one end and the other end of the wire is bonded to the highest portion of the loop portion. Say height.

  According to the first aspect of the present invention, the structure of the bonding wire in the semiconductor device, in particular, the loop height of the loop portion, and the height difference of the bonded surface of the semiconductor element / circuit component to which both ends of the wire are bonded (dots) By defining the height difference within a specified range, it becomes possible to effectively increase the durability and reliability of the wire bonding part without specially increasing the number of wires or the number of wires used. Therefore, even if the capacity of the semiconductor device is increased, a wire bonding structure capable of sufficiently ensuring durability and reliability can be obtained which is small and light, inexpensive, and contributes to reduction in size and weight of the semiconductor device and cost reduction. it can.

  According to the second feature of the present invention, the use of aluminum as the wire material can contribute to further weight reduction and cost saving of the semiconductor device.

  According to the third aspect of the present invention, the strain generated in the wire bonding portion is estimated, and the optimum structure of the bonding wire (especially the loop height and both ends of the wire is determined based on the correlation value between the strain and the reliability. The above-mentioned difference in the hit point height of the part) is set so that the life of the product specification for thermal stress relaxation can be easily estimated from the test results of only the standard sample, and the wire joint part of the actual product structure In addition, the thermal stress generated at the wire joint can be easily estimated from the loop shape of the wire. Further, it is possible to easily set a wire bonding wire shape that can sufficiently enhance the durability and reliability of the wire bonding portion without particularly increasing the thickness of the wire or increasing the number of wires used.

Sectional drawing of the principal part of the power module which concerns on one Example of this invention Graph showing the relationship between strain at wire joints and reliability correlation value (number of durability cycles) Schematic diagram showing a simplified model of wire joints and wires with varying loop heights Graph showing the relationship between the wire loop height and the stress generated at the wire joint Schematic diagram showing a simplified model of wire joints and wires with the loop top position changed Graph showing the relationship between the loop top position of the wire and the stress generated at the wire joint Explanatory drawing which shows simply the mode of action of displacement and force accompanying thermal deformation of the bonding wire A graph showing the relationship between the difference in height of the wire joint during wire heating (the difference in the height of the hitting point) and the generated strain A graph showing the relationship between the height difference of the wire joint (the difference in the hitting point height) and the durability increase rate / strain decrease rate

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  First, in FIG. 1, a power module PM as a semiconductor device includes a heat-dissipating metal base plate B as a base body, an insulating circuit substrate P installed and fixed on the base plate B, and the insulating circuit substrate P. A plurality of semiconductor elements S and S ′ (only two are shown in the drawing) joined to each other via solder H, and a circuit that is also installed and fixed on the base plate B and electrically connected to the semiconductor element S And at least a part C.

  A predetermined gap is provided between the semiconductor element S and the circuit component C, and at least one thin wire W made of a conductive metal called a bonding wire is connected between the semiconductor element S and the circuit component C. In the illustrated example, W is made of aluminum. In addition, as a constituent material of the wire W, another metal having excellent conductivity such as copper or gold may be used instead of aluminum.

  As the semiconductor elements S and S ′, conventionally known power semiconductor elements, for example, various elements such as IGBT, MOS-FET, or FWD are used. As the insulating circuit board P, for example, a conventionally known DCB board is used.

  The wire W has one end Wa joined to the flat electrode part surface of the semiconductor element S, one end rising part Wb that continues to the one end Wa and rises obliquely upward toward the circuit component C, The other end Wa ′ joined to the flat surface of the circuit component C, the other end rising portion Wb ′ connected to the other end Wa ′ and obliquely rising upward toward the semiconductor element S, and both rising portions The loop portion Wm is bent upward and convex so as to smoothly connect the upper ends of Wb and Wb ′.

  There is a height difference ΔL between the bonded surface (surface of the electrode portion) of the semiconductor element S to which both ends Wa and Wa ′ of the wire W are bonded and the bonded surface of the upper surface of the circuit component C, and this height difference ΔL. Is set within a setting range of 1 to 5 mm for the reason described later. Further, another semiconductor element S ′ is interposed between the semiconductor element S and the circuit component C. However, the loop portion Wm of the wire W can easily straddle the other semiconductor element S ′. The roof top portion is formed in a loop shape that is offset to the semiconductor element S side.

  The bonding between one end Wa of the wire W and the surface of the electrode portion of the semiconductor element S and the bonding between the other end Wa ′ of the wire W and the circuit component C are performed by using this kind of wire as a semiconductor. This is performed by a conventionally known method of bonding and bonding to the electrode portion of the element, for example, ultrasonic bonding.

  The outer diameter of the wire W is set to the same size as that of a conventional ordinary bonding wire, for example, 400 μm. Moreover, the wire W is set so that the loop height h of the loop portion Wm is in the range of 2 mm or more and 7 mm or less for the reason described later.

  The horizontal distance between both ends Wa and Wa ′ of the bonding wire W is limited in the layout of the semiconductor elements S and S ′ and the circuit component C in the standard size power module PM, and the rigidity of the wire W itself is ensured. From this point of view, it is set to a predetermined value or less (preferably 30 mm or less).

  Thus, the wire bonding portion of the power module PM, that is, the wire bonding portions b and b ′, is between the rising temperature that can be allowed by the semiconductor elements S and S ′ depending on the value of the energized current in the actual use environment and the temperature that decreases when the power is turned off. When temperature changes are repeated, cracks are generated due to the thermal stress generated at the wire joint, and this crack gradually develops and eventually breaks and peels off the joint, causing a disconnection failure. It has been known. The inventor of the present invention is not limited to the shape of the wire joints b and b ', but the durability of the wire joint is not limited to the loop shape of the wire, particularly the loop height h or the height difference between the wire joints b and b'. The thermal analysis and cross-sectional structure of the wire joint greatly depends on (that is, the height difference ΔL of the surface to be joined of the semiconductor element S and the circuit component C to which the wire both ends Wa and Wa ′ are joined). The analysis was conducted sequentially, and the analysis method will be specifically described below.

  In the analysis, an analysis model of the semiconductor element S, the insulating circuit board P, the wire W, etc. is created, and then a temperature condition corresponding to each switching operation test of the power module PM is set, and then a thermal stress analysis is performed. Thus, distribution data of stress generated in the wire joint is obtained.

  According to such an analysis technique, the joint b between the one end Wa of the wire W and the electrode part surface of the semiconductor element S, and the joint b ′ between the other end Wa ′ of the wire W and the circuit component C are used. It is possible to estimate the strain generated in the wire joints b and b ′ from each cross-sectional shape and the shape of the wire W, and based on the correlation value between the strain and the reliability, that is, the optimum structure of the bonding wire (ie, The loop shape, in particular, the loop height h and the height difference ΔL between the wire joints b and b ′ can be set as described later. Therefore, it is possible to easily set a wire bonding wire shape that can sufficiently enhance the durability and reliability of the wire bonding portion without particularly increasing the thickness of the wire or increasing the number of wires used.

By the way, when the present inventors examined the correlation between the distortion of the junction between the electrode part of the semiconductor element and the wire and the reliability of the IGBT element and the FWD element, the experimental result of FIG. 2 was obtained. In this graph, the horizontal axis is the strain ε (%) generated at the wire joint, and the vertical axis is the cycle number L _C (the number of endurance cycles) of the temperature cycle test until the wire joint breaks. L _C = C · ε ⁻ⁿ was obtained as an experimental approximate expression. In this equation, C is a constant.

According to this graph (approximate expression), it can be determined that the durability (reliability) is higher as the number of cycles L _C is larger, so that the number of cycles can be used as a correlation value between durability and reliability.

Thus, also from FIG. 2, it is clear that the greater the strain ε of the wire joints b and b ′, the lower the cycle number L _C , that is, the durability and reliability of the wire joint.

  Based on the above analysis technique, the wire loop shape of the wire module b, b ′ (and hence the durability / reliability) is examined by variously changing the wire loop shape. The durability and reliability of b 'is not only the shape of the wire joint itself, but also the loop shape of the wire W, particularly the height h of the loop part Wm, and the height difference ΔL of the wire joints b and b' ).

  That is, one end Wa of the wire W is joined to the electrode portion of the semiconductor element S, and the other end Wa ′ is joined to the circuit component C to be a fixed end, and the loop top portion of the loop portion Wm is a free end. Therefore, due to heat generation, both ends of the wire joints b and b ′ are subjected to stress caused by a difference in thermal expansion between the upper and lower materials sandwiching the wire joints b and b ′ and thermal deformation (expansion / contraction) of the loop part Wm. It is considered that the stress generated by the deformation load exerted on the portions Wa and Wa ′ is considered to act in a composite manner. In the present invention, a method for reducing the latter stress in particular has been investigated as described in detail below. Is.

  First, the present inventor assumes a simplified model of a wire joint and a wire as shown in FIG. 3, and the wire W having both ends fixed by walls is deformed by thermal deformation of the loop portion accompanying a temperature rise. The vertical pushing force F was evaluated. Since the lateral force F generates a moment in the direction of peeling the wire joints b and b ', the greater the force F (and hence the moment), the greater the stress on the wire joints b and b'. Occurring and the durability and reliability of the joints b and b 'are lowered.

  FIG. 4 shows the loop height h obtained when the wire diameter is 300, 400, 500 μm, the distance (support span) between the wire joints b, b ′ is 8 mm, and the rising change temperature ΔT is 100 ° C. The relationship between (horizontal axis) and force F (vertical axis) is shown. According to this, regardless of the wire diameter, the higher the loop height h is, the smaller the force F is, and particularly, the force F rapidly decreases when h = 2 mm or more. Therefore, in the present invention, the lower limit of the loop height h is set to 2 mm, while the upper limit is set to 7 mm from the viewpoint of securing the rigidity of the wire W and reducing the size and weight of the semiconductor device. The reason why the force F becomes smaller as the loop height h is higher is that the higher the loop height h, the lower the rigidity of the middle portion of the loop, and the lower the force that pulls the wire between both walls. It is believed that there is.

  6 shows the loop top position t (distance from one loop rising portion) and the force F when the position t of the loop top of the wire W is changed as shown in FIG. The relationship is shown. According to FIG. 6, it is clear that even if the loop top position t is changed, there is almost no influence on the force F. Therefore, in the present invention, the loop top position is not particularly limited.

  Next, the present inventor makes a difference in height between the wire bonding portions b and b ′ of the wire W (that is, a difference in height ΔL between the bonded surfaces of the semiconductor element S and the circuit component C to which the wire both ends Wa and Wa ′ are bonded) The effect of the height difference)) on the durability of the wire joints b and b ′ was examined.

  That is, when the wire W generates heat due to the operation of the power module PM, the loop portion Wm tends to be displaced upward due to thermal expansion, and at the same time, both ends Wa and Wa ′ of the wire W are Acting on the wire joints b and b ′, which are fixed ends, to generate bending stress on the wire rising parts Wb and Wb ′ and shear stress on the wire joints b and b ′ (ie, Force to try to peel off the wire joints b and b '. In this case, as the height difference ΔL is increased, the tensioning action of the wire W is reduced, and accordingly, the shear stress is also reduced. Therefore, the strain generated in the wire joints b and b ′ is also reduced, so that the wire joint It is considered that the durability reliability of the parts b and b ′ is increased.

  Accordingly, when the relationship between the height difference ΔL of the wire joints b and b ′ and the generated strain when the wire W is heated to a predetermined temperature is examined, as the height difference ΔL increases, as shown in FIG. 8, the wire joint b , B 'is confirmed to be small.

Therefore, when the relationship between the height difference ΔL of the wire joints b and b ′ and the strain reduction rate η / durability increase rate α is further examined, the result shown in FIG. 9 is obtained. Here, the strain reduction rate η means the ratio of the strain ε _a when ΔL is a to the strain ε ₀ when the hit point height difference ΔL is 0 (that is, η = ε _a / ε ₀ ), On the other hand, the durability increase rate α is the difference in hit point height ΔL for the endurance cycle number L _C of the temperature cycle test expressed using the graph shown in FIG. 2 (experimental approximate formula L _C = C · ε ⁻ⁿ ). It means the ratio between α and a (α = L _C a / L _C o = C · ε _a ⁻ⁿ / C · ε ₀ ⁻ⁿ = (η) ⁻ⁿ ). In this case, if the durability increasing rate α has a cycle increasing effect of 20% or more (that is, α = 1.2) or more, it is a clear and objective effect that is not a factor of variation due to manufacturing tolerance, material strength, and the like. It can be judged.

  9 is examined. As the height difference ΔL of the wire joints b and b ′ increases, both the strain reduction rate η and the durability increase rate α tend to increase. When the value exceeds 5 mm, the upper limit of ΔL is determined to be 5 mm from the viewpoint of securing the rigidity of the wire W and reducing the size and weight of the semiconductor device.

  In particular, regarding the durability increase rate α, it can be determined that there is a clear and objective cycle increase (durability improvement) effect at 20% increase (that is, 1.2) or more as described above. The lower limit of the height difference ΔL of b ′ is set to 1.0 mm at which the durability increase rate α is increased by 20%.

  As described above, the strain generated in the wire joints b and b ′ is obtained by the analysis method described above, and the optimum structure of the bonding wire (especially the loop height) based on the correlation value between the obtained strain and reliability. h, the height difference ΔL) of the wire joints b and b ′ is set. At the time of setting, the loop height h is set within a setting range of 2 to 7 mm and the wire joint portion b is within a range that satisfies various design conditions and requirements required for the power module PM and can be actually manufactured. If the height difference ΔL of b and b ′ is appropriately set within a setting range of 1 to 5 mm, the durability / reliability of the wire bonding part is increased, or the number of wires W is increased specially. It can be sufficiently secured without any problems.

  As a result, even if the capacity of the power module PM is increased, it is possible to easily obtain a wire bonding structure that is small, lightweight, inexpensive, and sufficiently durable and reliable, and as a result, the power module PM can be reduced in size, weight, and cost. Can be achieved. In addition, the life of product specifications for thermal stress relaxation can be easily estimated from the test results of only the standard sample, and the wire joint and wire loop shape of the actual product structure can be used for the wire joint. The generated thermal stress can easily estimate the strain.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various design changes can be made without departing from the present invention described in the claims. Is possible.

  For example, in the above-described embodiment, as the circuit component to be electrically connected to the semiconductor element S via the wire W, the semiconductor element S and another circuit element C that is spaced from the insulating circuit substrate P to which the semiconductor element S is bonded are shown. However, the concept of the circuit component of the present invention includes various components regardless of form and function as long as the circuit component is to be electrically connected to at least the electrode portion of the semiconductor element via a wire. Also included are metal circuit layers and lead frames exposed on the surface of the circuit board.

B ··· Base plate C for heat dissipation as base body ··· Circuit component PM · · Power module S as semiconductor device · · · Semiconductor element W · · · Wire Wa · One end Wa ′ ·· other end Wm ··· loop portion b, b '·· joining portion h ··· loop height ΔL · ·· height difference

Claims (3)

On the base body (B), the semiconductor element (S) and the circuit component (C) to be electrically connected to the semiconductor element (S) are installed and fixed at intervals, and the electrode part of the semiconductor element (S) and the circuit component ( C), a wire bonding structure in a semiconductor device in which a conductive metal wire (W) extending therebetween is connected,
The wire (W) is bonded to the surface of the electrode part of the semiconductor element (S) and the surface of the circuit component (C) having a height difference (ΔL) from the surface of the electrode part. The other end portion (Wa ') to be joined, and a loop portion (Wm) in which one end portion (Wa) and the other end portion (Wa') are integrally connected and at least an intermediate portion is bent upwardly are provided. ,
The semiconductor is characterized in that the height difference (ΔL) is set within a setting range of 1 to 5 mm, and the loop height (h) of the loop portion (Wm) is set within a setting range of 2 to 7 mm. Wire bonding structure in equipment.   The wire bonding structure in the semiconductor device according to claim 1, wherein the wire (W) is made of aluminum. A method for designing a wire bonding structure in a semiconductor device according to claim 1 or 2,
The joint (b) between one end (Wa) of the wire (W) and the electrode part surface of the semiconductor element (S), and the other end (Wa ') of the wire (W) and the circuit component (C) Estimate the strain generated at the joint (b ') with (C),
A design of a wire bonding structure in a semiconductor device, wherein the height difference (ΔL) and the loop height (h) are set within each setting range based on a correlation value between the distortion and reliability. Method.

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