This application claims priority to German Patent Application 10 2005 002 862.4, which was filed Jan. 20, 2005, and is incorporated herein by reference.
The invention relates to the field of semiconductor production. In particular, the invention relates to the enveloping of semiconductor chips after their mounting on a carrier substrate.
During the production of semiconductor components, semiconductor chips are mounted on substrates. In this case, it is also possible for a plurality of semiconductor chips to be mounted one above another. A substrate has a conductor structure serving for connecting the semiconductor chip to an external circuitry. The conductor structure comprises conductive tracks, contact pads and ball pads.
FBGA housings (FBGA=Fine Ball Grid Array) are known for connecting the conductor structures to an external circuitry. In this case, the substrate has a chip side, on which the chip or the chip stack is mounted. The substrate has a solder ball side opposite to the chip side. The conductor structures, or at the very least parts of the conductor structures, are arranged on the solder ball side.
In one embodiment of FBGA housings, the semiconductor chip (the bottom semiconductor chip in the case of a chip stack) is mounted onto the chip side of the substrate with its bonding pads directed toward the chip side of the substrate. In this case, a bonding channel is provided in the substrate. The bonding channel comprises an opening in the substrate, preferably in the form of an elongated slot or a quadrangular hole. Through the bonding channel, wire bridges are drawn from the bonding pads on the semiconductor chip to corresponding contact islands in the conductor structure on the solder ball side for the purpose of electrically connecting the semiconductor chip to the conductive tracks of the conductor structure.
The ball pads of the conductor structure are arranged on the solder ball side. The ball pads are provided with solder balls via which, in a later process, the semiconductor component can then be mounted and electrically conductively connected to the external circuitry, for example on a printed circuit board. For this purpose, the solder is heated until it melts. In order to prevent the solder from then flowing away from the solder balls in an uncontrolled manner, the solder ball side is coated with a soldering resist, which is patterned so as to avoid such an uncontrolled flowing away of solder during the later mounting process.
In order to stabilize the entire semiconductor component, the latter is provided with a housing made of a mold compound. The housing comprises an upper housing part on the chip side and—in the case of FBGA housings with a bonding channel—of a lower housing part on the solder ball side. The lower housing part in particular also protects the bonding wires led through the bonding channel by virtue of the bonding channel being filled with mold compound with molding of the lower housing part and the bonding wires thus being enclosed by mold compound.
Encapsulation molds having the negative mold of the housing parts are used for the molding of these housing parts. Around the negative mold of a housing part, the encapsulation mold is provided with a sealing web. During the production of the housing parts, the encapsulation mold is placed onto the respective substrate side and pressed with a pressure force onto the respective substrate side. In this case, the negative molds enclose the parts of the semiconductor component to be encapsulated. Mold compound is then forced into the negative molds. In this case, the sealing web and the pressure force are intended to prevent mold compound from flowing out of the negative mold. It has been shown in practice, however, that this is accomplished only inadequately.
This is because the mold compound is heated to a temperature of approximately 180° C. during encapsulation, as a result of which it has very low viscosity and as a result emerges between the substrate side and sealing web. Even when there is good sealing, so-called “bleeding” still takes place, during which components of the mold compound, such as resin components, pass through the sealing.
This is disadvantageous particularly when molding the lower housing part on the solder ball side, since this escaping mold compound may also pass onto the solder balls or in the vicinity thereof and thus adversely influence later mounting processes, such as soldering-on. This problem is exacerbated by the fact that in FBGA housings having a lower housing part, the conductor structure is quite generally arranged on the solder ball side. The conductor structure comprises metal conductive tracks elevated over the rest of the substrate surface. As a result, leakages occur between the encapsulation mold and the substrate surface on the solder ball side since the surface is not even.
Various measures are possible for avoiding this problem. Firstly, it is possible to increase the pressure force. This may be done in particular by increasing the transfer pressure, that is to say by pressing the encapsulation molds for the upper and lower housing parts against one another.
However, increasing the pressure force has the disadvantage that the substrate is exposed to a very high mechanical stress, which may result in the substrate breaking.
A further possibility for improving the seal between encapsulation mold and substrate surface consists in the sealing webs of the encapsulation mold for the lower housing parts being kept very narrow, to the point where they have the form of a cutting edge. As a result, the pressure per unit area between sealing web and substrate surface is increased and the sealing webs can penetrate into the substrate surface of the solder ball side, that is to say into the soldering resist situated thereon.
Such a configuration of encapsulation molds requires very small manufacturing tolerances, which leads to costly manufacture. On the other hand, the service lives of the encapsulation molds are short since the slightest damage to the sealing web results in the encapsulation mold being unusable. Finally, the problem of the substrate breaking is increased even further when the narrow webs are combined with increasing the pressure force.
Fixing the encapsulation mold by means of vacuum has furthermore been provided. This leads to a pressure force distribution that is as uniform as possible, but does not avoid the risk of the substrate breaking.
Finally, one possibility for reducing bleeding consists in using mold compound having a lower viscosity. A flowability reduced in this way has the effect that it is more difficult for mold compound to emerge through leakages between the encapsulation mold and the surface on the solder ball side. However, devising a mold compound of this type is associated with considerable costs. Moreover, the lower flowability may give rise to problems during the molding process itself.
Embodiments of the invention relate to a method in which a semiconductor chip is mounted on a chip side of a substrate. The semiconductor chip is electrically conductively connected to a conductor structure on the substrate. Solder balls are applied to ball pads of the conductor structure on a solder ball side of the substrate. The solder ball side is provided with a mask made of a soldering resist. An encapsulation mold for producing a lower housing part is pressed onto the solder ball side and the encapsulation mold is filled with encapsulation composition, further called mold compound. Other embodiments of the invention relate to a substrate for carrying out the method.
In one aspect the invention improves a sealing of the encapsulation mold for the lower housing part on the solder ball side with respect to the substrate surface thereof and in so doing reducing the risk of the substrate breaking and the production outlay on encapsulation molds and increasing the service lives thereof.
According to embodiments of the invention, a method is provided wherein the substrate surface on the solder ball side, at least in the region in which the encapsulation mold is placed onto the substrate surface of the solder ball side, is formed as a sealing region in such a way that the substrate surface is provided with seal elements. The encapsulation mold enters into a sealing connection with the seal elements. Thus, the seal between the encapsulation mold and the substrate surface, in contrast to the prior art, is not realized by the encapsulation mold but by the substrate. As a result, the requirements made of the encapsulation mold are reduced and possible damage to the encapsulation mold thus has a considerably small influence on the seal than in the case of the prior art. The seal between the substrate and the encapsulation mold is realized by the respective substrate. If possible leakages occur, then these faults will only occur at one substrate and will not be reproduced in a plurality of substrates.
In one variant of the method according to embodiments of the invention, it is provided that the conductor structure is patterned in such a way that there is a minimum lateral distance between the individual conductive tracks in the sealing region. The conductive tracks are thus formed in a particular manner at least in the sealing region. This may be done, for example, by virtue of the conductive tracks having area extensions in this region, so that the individual conductive tracks come as close as a minimum distance to one another. What is thus achieved is that the conductive tracks form almost a closed surface in the sealing region, whereby the sealing effect is considerably improved.
In a further variant of the method according to embodiments of the invention, it is provided that the surface of the soldering resist is patterned in uneven fashion at least in the sealing region. When the encapsulation tool is brought close to the surface of the soldering resist, this will then be placed first on the most elevated parts of the surface structure. The pressure force thus acts with a particularly large pressure per unit area in these regions, as a result of which the soldering resist is pressed into the regions of the adjacent depressions. Consequently, the soldering resist itself creates a sealing area between the encapsulation mold and the substrate surface, in this case the soldering resist surface.
In this case, it is possible for the unevennesses in the surface of the soldering resist to be patterned by means of a photolithographic process or by means of a mechanical process, in particular by means of a pressing operation.
Both variants bring about the same effect, namely that the most elevated parts are pressed into the less elevated “valleys” by the encapsulation mold, since the soldering resist has a certain flexibility.
With regard to the structure of the surface, a groove structure may be produced in which longitudinally extended depressions and elevations alongside one another run in one direction. This groove or wave structure brings about the effect of pressing “wave crests” into “wave troughs” if the encapsulation tool is emplaced in some way transversely with respect to the direction. For this reason, it is particularly expedient that in regions of the sealing region, which have a longitudinal extent, the depressions and elevations are introduced transversely with respect to the direction of the longitudinal extent.
With regard to the form of the grooves, an undulatory form and a meandering form or a zigzag form of the cross-section are possible.
In order to produce a certain independence between the uneven structure and the direction of the press-on areas of the encapsulation mold, another variant of the method according to embodiments of the invention provides for the surface of the grid structure to be produced in a manner such that first depressions and elevations lying alongside one another, which run in a first direction, cross second depressions and elevations lying alongside one another, which run in a second direction. In this variant, a projecting elevation of the soldering resist will always be able to be pressed into an adjacent depression, thereby considerably improving the seal.
A third possibility for the configuration of the unevennesses in the substrate surface makes use of a disordered unevenness by virtue of the surface being textured. In this case, it assumes an orange peel form, which can give rise to elevations and depressions alongside one another in a disordered manner.
This textured surface may be produced, in one instance, by the soldering resist mask being exposed to a thermal process after application. On account of this thermal process, surface alterations take place in a manner such that parts of the soldering resist contract. This contraction then gives rise to a nonuniformly uneven surface.
Another variant of the production of a textured surface consists in the fact that a chemical or mechanical admixture that brings about the unevennesses in the surface is added to the soldering resist before application.
In the case of a chemical admixture, this admixture will quite generally be distributed nonuniformly in the soldering resist and then lead to nonuniform applications of resist or contraction of the resist in regions.
The admixture of mechanical means may involve flexible admixtures that have a certain granularity. As a result of this admixture, on the one hand the surface becomes uneven, and, on the other hand, the flexibility of these admixtures has the effect that the surface of the soldering resist mask approximates very well to the encapsulation mold.
Finally, there is also the possibility of the soldering resist being applied in uneven fashion. It will be endeavored, quite generally, to configure the soldering resist with a certain planarity, for example by means of a spin-off method or a uniform injection method. In the case of the deliberately uneven application of the resist, the injection application of the resist, in particular, will be successful. This is because if the injection is performed with a resist having higher viscosity, then the individual injected droplets are compensated for inadequately on the surface and an unevenness thereby arises which, for its part, in turn leads to the compensation of the soldering resist surface upon contact of the encapsulation mold in the region of the sealing area.
Another possibility for the configuration of the sealing area consists in the substrate surface requiring a layer made of elastic material in the sealing region. The layer may be applied as an elevation. A silicone, which has a very good elasticity, is particularly suitable in the case of this sealing elevation. A rubberlike seal is thus provided directly in the region where the encapsulation mold makes contact with the substrate surface, so that even extremely low pressure forces already lead to an excellent seal.
All the above-mentioned measures do not disturb subsequent processes, in the case of which, after all, it is merely important that the solder balls are impeded from flowing away when melting by the soldering resist. The surface structure, which the soldering resist, has is independent for this outcome. Uneven elevations, as provided in the last-mentioned variant of the method, also effect just as little disturbance.
Another aspect of the invention provides a further method wherein the substrate surface on the solder ball side between the region in which the encapsulation mold is placed onto the substrate surface of the solder ball side and the ball pads, a trench is introduced into the substrate surface in such a way that this trench prevents a flow of material from the mold compound in the direction of the ball pads.
Firstly, mold compound or material secreted from the mold compound is collected by the trench and thus prevented from continuing to flow. It has additionally been shown that the edges of the trench, on account of the adhesion forces acting, constitute a flow resistance for the flowing material, with the result that flowing is impeded.
A further aspect provides that the substrate surface on the solder ball side, at least in the region in which the encapsulation mold is placed onto the substrate surface of the solder ball side, is formed as a sealing region in such a way that the substrate surface is provided with seal elements, and at the substrate surface on the solder ball side between the region in which the encapsulation mold is placed onto the substrate surface of the solder ball side and the ball pads, a trench is introduced into the substrate surface in such a way that this trench prevents a flow of material from the mold compound in the direction of the ball pads.
Consequently, the seal elements make it more difficult for material to emerge from the mold compound. If “bleeding” should nevertheless occur, the flowing of the material that has emerged in the direction of the ball pads is stopped by the trench.
In terms of an arrangement, embodiments of the invention also provide a substrate of the type mentioned in the introduction in which the substrate surface, on the solder ball side, has a sealing region for an encapsulation mold that is to be emplaced during production, in which region the substrate surface is provided with seal elements for sealing connection to the encapsulation mold. With a substrate of this type, the substrate manufacturer can already take care to ensure that the tightness between the encapsulation mold and the substrate surface is ensured in a later encapsulation process.
In one refinement of the substrate according to embodiments of the invention, it is provided that the conductor structure is patterned in such a way that there is a minimum lateral distance between the individual conductive tracks in the sealing region. In the case of this refinement, the conductive tracks, as an essential part of the substrate on the solder ball side, provide a smoothest possible surface below the soldering resist mask, with the result that the soldering resist mask is configured in essentially planar fashion on its top side. Additional depressions, which are caused by distances between the elevated conductive tracks and are imaged in the soldering resist mask, are thus avoided.
In a further refinement of the substrate according to embodiments of the invention, it is provided that the surface of the soldering resist is patterned in uneven fashion at least in the sealing region. This uneven patterning of the soldering resist exploits the plastic property thereof and the soldering resist can be deformed below the encapsulation mold so as to give rise to a best possible sealing area between the substrate surface, that is to say between the surface of the soldering resist, and the encapsulation mold.
The best possible sealing function can be supported by virtue of the fact that the surface is provided with a groove structure having longitudinally extended depressions and elevations lying alongside one another, which run in one direction.
It is particularly expedient in this case that in regions of the sealing region that have a longitudinal extent, the depressions and elevations are introduced transversely with respect to the direction of the longitudinal extent. In the region of these longitudinal extents, the encapsulation mold will also have a longitudinally extended sealing area. Consequently, the unevennesses will run transversely with respect to its longitudinal extent and the compensation between the most elevated part of the surface and the part of the surface lying at the lowest level is achieved in a greatest possible form.
The groove structure may have various cross-sections. An undulatory, a meandering or a zigzag cross-section is thus possible. All three cross-sectional forms or any further different cross-sectional form that has elevations and depressions has the effect that the material in the elevations passes into the depressions if the encapsulation mold presses onto the surface of the soldering resist.
Another possibility, in particular for the configuration of the directional independence of the sealing region consists in the fact that a grid structure is introduced in the surface, first depressions and elevations lying alongside one another, which run in a first direction crossing second depressions and elevations lying alongside one another, which run in a second direction.
In a further refinement, it is provided that the surface is either textured or applied in uneven fashion. In any event a nonuniform distribution of unevennesses and depressions is thus produced, as is known from the surface of orange peel.
A further embodiment, which enables a seal independently of the elasticity of the soldering resist, consists in the fact that the substrate surface has a layer made of elastic material in the sealing region. In particular, the layer may be arranged as an elevation. The elevation makes it possible for the seal also to be able to yield laterally, thereby supporting the sealing effect.
In principle, the elastic layer may be applied directly on the substrate, that is to say below the surface on which the soldering resist is applied. Another possibility consists in the fact that the layer made of elastic material is arranged on the surface of the soldering resist.
The aspect according to embodiments of the invention also provide a substrate in which, at the substrate surface on the solder ball side between the region in which an encapsulation mold that is to be emplaced during production can be placed on to the substrate surface of the solder ball side and the ball pads, a trench that prevents a material flow—occurring during production—from the mold compound in the direction of the ball pads is introduced into the substrate surface.
For security, it is also possible to realize both solutions, that is to say the arrangement of a sealing element and the arrangement of a trench in a substrate.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:
The following list of reference symbols can be used in conjunction with the figures:
In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention.
As illustrated in
That side of the substrate 1 on which the semiconductor chip 2 is situated is referred to as chip side 9. That side of the substrate 1 on which the solder balls 8 are situated is referred to as solder ball side 10.
Alongside the solder balls 8, the conductor structure 3 is also arranged on the solder ball side 10. In order that the bonding wires 4 can be drawn from the semiconductor chip 2 to the bonding islands 6, provision is made of a bonding channel 11.
In order to prevent solder from subsequently flowing away from the solder balls 8, the solder ball side 10 of the substrate 1 is provided with a mask 12 made of a soldering resist.
For closure, the FBGA housing has an upper housing part 13 and a lower housing part 14.
As can be seen in
After these two encapsulation molds 15 and 16 have been emplaced, they are filled with an encapsulation material that is self-curing. As a result, in particular, the bonding wires 4 are protected by the lower encapsulation mold 16 or the lower housing part 14.
A pressure force FA is exerted during the application of the lower encapsulation mold 16. The pressure force FA serves to ensure that the sealing webs 17 of the lower encapsulation mold bear as tightly as possible on the substrate surface of the substrate 1 on the solder ball side 10.
As illustrated in
A further problem is illustrated in
An increase in the pressure force FA will not lead to successful sealing in this case, particularly not when the sealing web 17 is supported on a boundary line between an edge of the conductor structure 3 and a region without a conductor structure.
However, even an increase in the pressure force will at best only intensify a break along the breaking lines 18.
As specified in
As illustrated in
As illustrated in
If the sealing web 17 is then pressed against the surface of the soldering resist mask 12 with the pressure force FA, it is possible for resin constituents 20, as illustrated in
As illustrated in
At places at which the sealing web 17 does not bear completely on the sealing area, this may lead to “bleeding,” as is illustrated in
In order, then, to prevent the situation where the resin constituents emerging in this case flow as far as the ball pads and make it more difficult to effect soldering there, a trench 27 is introduced into the soldering resist mask 12 alongside the region 26, to be precise on that side of the region 26, which is remote from the lower housing part 14.
As illustrated in
Number | Date | Country | Kind |
---|---|---|---|
10 2005 002 862.4 | Jan 2005 | DE | national |