FR2740720A1 - MOLD ASSEMBLY FOR PROTECTIVE BODY OF SEMICONDUCTOR PASTILATE HOUSING - Google Patents

Abstract

A semiconductor wafer package mold assembly is disclosed. It relates to an assembly comprising two upper and lower frames (90a, 90b) each having a mold cavity, two upper and lower plates (80a, 80b), a pin (4) with an identification mark and an upper pin ( 9) ejection attached to the upper plate (80a), and a first and a second lower ejection pin (8) attached to the lower plate (80b). A lower center eject pin (13) is placed between the first and second lower eject pin (8), the lower center eject pin (13) being fixed to the lower plate (80b) and protruding towards the cavity . Application to the manufacture of semiconductor components.

Description

The present invention relates generally to a set of molds

  intended to form a housing body

  protector of a semiconductor chip. More precise-

  it relates to a set of molds which can be used in a transfer molding process and which includes central ejection pins for reducing the molding stress applied to the semiconductor wafer and preventing the formation of cracks in the

  the pellet and the body of the case.

  A semiconductor chip package is not only

  intended to form a protective body ensuring the func-

  and an integrated circuit device, but is also an electrical interconnection device of the integrated circuit device with an external electronic system. The molding technique of plastic material, in particular thermosetting and thermoplastic materials, in the protective body, has advantages of low cost and mass production, but with a poor reliability compared to that which gives the

  ceramic packaging technology.

  The transfer molding process is widely used

  for forming a plastic case body

  tick. The method begins by injecting a liquid and hot molding composition from a reservoir into a number of cavities in which is placed a set of wire frame frames and semiconductor wafers, and it ends with the ejection of the completed part. A set of molds can be divided into two parts, an upper mold and a lower mold. The upper and lower molds are open when the entire web formed by the lead wire frames and the semiconductor wafers is put in place and during the ejection step, and they are closed for

  injecting the molding composition into the cavities.

  The realization of the upper and lower molds is

  for the mounting performance and reliability of the

conditioned product.

  FIG. 1A is a section showing that the upper and lower molds are open and that a set of

  semiconductor wired and semi-wired frame band

  1B indicates that the upper and lower molds are closed and a molding composition is injected into a cavity, and FIG. 1C represents a step of ejecting the molded component out of the cavity. cavity. The upper mold 100 and the

  lower mold 102 have two chassis 90 and trays 80.

  The trays 80 are solid blocks of steel bolted to the upper and lower portions of a molding press (not shown). The lower plate 80b comprises a driving plate 1b and a plate 2b with ejection pins, and two lower ejection pins 8 are fixed to the plate 2b. The lower ejection pins 8 are intended to push the molded part out of the cavity 6. The driving plate 1b is heated, as is the frame 90, so that a molding temperature remains constant. The upper plate 80a also has, like the lower plate

  80b, a plate leading to and a plate 2a with ejector pins

  tion. However, an identification marking pin 4 and an upper ejection pin 9 are attached to the tray 2a. The trays are intended to accommodate a pressure such that the clamping force applied by the press can

  to reach tens of thousands of newtons.

  The frames 90 are actually in contact with the molding composition and give it its shape. The frames comprise two parts 90a and 90b, each having housings 3a and 3b and bars 5a and 5b forming cavities. The liquid and hot molding composition is injected into the cavity 6 by a pouring jet and an orifice (not shown) formed in the bars 5a and 5b and the air contained in the cavity is driven by ventilation holes (not shown ) who are also trained in

bars.

  When the upper mold 100 and the lower mold 102 are opened as shown in FIG. 1C, an assembly comprising a semiconductor chip 10 and a wire-frame strip 7 is aligned and positioned in the cavity 6. The chip 10 is interconnected electrically to the frame by a wire 11. When the upper and lower molds are closed as shown in Fig. 1B, a molding composition 104 is injected into

  the cavity 6 by a casting jet and the introduction opening

  tion. Since the molding composition has adhesive properties that increase its bond to itself and to the wafer 10 and to the frame 7, it also adheres to the surface of the upper and lower bars 5a and 5b giving the shape of the cavity 6. Accordingly, additional ejection pins 4, 8 and 9 must be used for separation of the molded part from the cavity. The ejection step is performed as follows. First, after injection of the molding composition, the upper plate 80a and the

  upper frame 90a move upwards and simultaneously

  Incidentally, the pin 8 forming an identification mark and the upper ejection pin 9 push the body 104 of the molded case. Then, by raising the lower platen b with the lower frame 90b which remains fixed, the lower ejection pins 8 push the body 104 of the casing and separate it from the contact surface of the bar b. The arrangement of the pin 4 forming the identification mark and the upper ejection pin 9 protruding from the upper bar 5a appears in FIG. 2A, while the lower ejection pins 8 protruding from the lower bar 5b are placed as shown on the

Figure 2B.

  The conventional mold assembly having the construction described above poses certain problems due to the bonding property of the molding composition on the surface of the mold. Although the surface finish of the

  cavity corresponds to titanium nitride or an electro-

  deposition of hard chromium and thus facilitates the ejection step, the bonding of the molding composition to the surface of the cavity can not be totally eliminated at

  because of the excellent adhesive properties of the

  molding. Moreover, the casing body, just after molding, is still relatively soft, and an additional hardening step in an oven is used

  then for hardening the body of the housing. As a result

  Therefore, when the lower ejection pins 8 are used to push the body of the case upwards while it is still soft, this case body

  flexes so well that the body and the lozenge can present

  to crack. This cracking occurs in the direction of the shortest edge in the central region of the body or pellet. Bending of the housing body may cause separation of the housing body and the leadframe frame or bottom surface of the pad. The invention therefore relates to a transfer mold assembly which makes it possible to prevent cracking of the body of the

  housing and a semiconductor chip.

  It concerns such an assembly that makes it possible to avoid the separation of the body and the assembly formed by a frame to

  connecting wires and a semiconductor chip.

  The invention thus relates to a set of molds des-

  to form a protective body of semiconductor wafer package, the set of molds comprising a

  upper mold assembly and a lower mold assembly

  the upper and lower mold assemblies comprising A) two upper and lower frames each having a cavity in which a hot and liquid molding composition is injected and which gives shape to the housing body, B) two upper and lower trays for opening and closing the upper frame of the upper mold assembly and the lower frame of the lower mold assembly; C) an identification pin and an upper ejection pin attached to the upper tray and protruding towards the cavity for ejection of the housing body just after the molding composition has been fully injected into the cavity, and D) a first and a second lower ejection pin attached to the lower tray and protruding towards the cavity so that they eject the body of the housing after the molding composition has been totally injected into the

  cavity, in which a lower center pin of ejection

  is placed between the first and the second lower ejection pin, the lower central ejection pin being fixed to the lower plate and protruding

towards the cavity.

  Other features and advantages of the invention

  will be better understood by reading the description that will

  follow of embodiments, with reference to the accompanying drawings, in which: FIG. 1A is a section illustrating conventional open top and bottom molds and a semiconductor wafer and connecting wire frame set between the molds; upper and lower; FIG. 1B is a section showing molds

  upper and lower class closed with a

  injection molding injected into a cavity; Figure 1C is a section showing the conventional step of ejecting the molded part out of a cavity; Fig. 2A is a bottom view showing the location of the ejection pins of a conventional upper mold; Figure 2B is a top view showing the location of the ejection pins of a conventional lower mold; Fig. 3A is a section of a set of molds according to the present invention; Figure 3B is a bottom view of the upper mold according to the invention, showing the location of the ejection pins; Figure 3C is a top view of a lower mold according to the invention showing the location of the ejection pins; Figure 3D is a bottom view of the lower mold according to the invention, showing the location of the ejection pins; and Fig. 3E is a plan view of a lower mold according to the invention, showing the location of the ejection pins. The bending amplitude of a still soft body body is proportional to the bonding force between a molding composition and the surface of the cavity. This is because, when the bonding force is increased, the lower ejection pins must push the body

  soft case up with greater force.

  Table 1 below shows the experimental results

  obtained by measuring the bonding force between the

  molding surface and the cavity surface which varies depending on the nature of the molding composition and the

number of molded lots.

Table 1

Composition of Lot of Difference

  molding molding Min. Max. Average type

1-10 0.38 0.84 0.63 0.161

11-20 0.72 1.18 0.88 0.165

21-30 0.90 1.34 1.11 0.151

  TYPE "A"! 31-40 0.96 1.58 1.18 0.199

41-50 0.96 1.78 1.34 0.252

51-60 1.12 1.90 1.48 0.258

1-10 0.35 1.60 1.00 0.391

11-20 1.90 2.95 2.23 0.348

  TYPE "B" E 21-30 i 1.75 3.96 2.94 0.804

31-40 2.40 4.70 3.44 0.722

41-50 1.80 4.65 3.55 0.908

51-60 2.85 4.70 4.06 0.619

  In Table 1, the unit of adhesive force is indicated by the required pressure expressed in MPa, and the two molding compositions are of type "A" and "B" which correspond to an epoxy resin molding composition containing a bisphenol. In type "B", an inert material called "filler" is modified so that the viscosity of the molding composition is increased. The measurement of the bonding force is carried out with a push-pull gauge. As indicated in the table, the bonding force increases with the number of molded batches, and the bending of the soft body of the casing becomes important. So that the case body and the semiconductor chip do not crack, the bonding force must correspond to a lower pressure

at 0.5 MPa.

  Table 2 below shows the relationship between the flexural amplitude and the cracks in the body of the

casing and the pellet.

  Table 2 Pressure Flexion Cracks Cracks (N) (mm) pellet casing

0.19 3 3

0.17 3 3

40 0.15 2 2

0.13 1 2

0.11 0 1

0.09 0 1

0.07 0 1

15 0.05 0 0

0.03 0 0

0.01 0 0

  In this experiment, a semiconductor chip of 243.5 x 592 μm and 300 μm thickness and a 5.6 mm TSOP LOC housing are used. The experimental results in Table 2 were obtained by application

  increased pressure of 5 N to six packaged devices.

  As shown in Table 2, the pressure force and bending amplitude which can cause cracking of the semiconductor wafer are 20 N and 0.07 mm respectively whereas, for cracking of the body of the housing, it is necessary to minimum values of 35 N and 0.13 mm for bending. The experimental results of Table 2 can be described experimentally with the following equation: K P1 [29 (a) 1 / 2-46 (a) 3/2 + 218 (a) 5 / 2-37.6 () / 2 + 38.7 () 9 /] wt2 / 3 ttttt

  In this case, t is the thickness of the semi-

  conductive, w its width, 1 its length, t the pressure force applied to the pellet in the direction of the thickness, to the scratch depth and K the intensity of the stress. The scratch is formed during a polishing operation of the back of the wafer when the back side of the wafer is polished with a grinding wheel so that the wafer has a thinner thickness. The stripe having a V-shaped section may be the origin of cracking of the pellet. When a / t << 1, we have K = P (1 / w) a / t2. At this

  moment, the growth rate of the crack in the pas-

  is proportional to K at the power n, n depending on the material of the pellet, for example, when the pellet is formed of silicon, n is between 1 and 2. In conclusion, the intensity K of the stress, which presents a close relationship with cracking of the pellet, is proportional to the pressure, the length of the pellet and the root of the length a of the stripe, but varies in inverse proportion to the width and thickness of the

semiconductor chip.

  As previously described, cracking of the pellet can be avoided either by reducing the bonding force by changing the molding composition, or by reducing the intensity of the stress applied to the pellet. In order for the bonding force to be reduced, it is necessary to consider improvements in the molding composition and surface finish of the cavity. However, the invention relates to reducing the intensity of the stress so that pellet cracking and case cracking are avoided by changing the structure of the transfer mold assembly as a whole.

described below.

  FIG. 3A is a sectional view of a set of molds according to the invention, FIG. 3B a bottom view of the upper mold according to the invention showing the location of the ejection pins, and FIG. 3C a plan view of the lower mold according to the invention, indicating the location of the ejection pins. The structure of the elements of the mold assembly, such as the frames 90 and the plates 80, with the exception of the ejection pins, is the same as that of the conventional mold assembly with the exception of the pins ejection, and therefore not described in detail. The upper center pin 12 ejection, in addition to the

  pin 4 of the identification mark and the upper pin

  9 ejection, exceeds to the bar 51 of the upper cavity. A lower central ejection pin 13 placed between two lower ejection pins 8 protrudes from the lower bar 5b. The upper and lower central injection pins 12 and 13 push the central portions of the body of the housing when it is soft during the ejection step so that the bending amplitude of the body can be reduced and the intensity of the the constraint

  applied to the pellet can also be reduced.

  FIG. 3D is a plan view of the lower mold according to the invention, intended to represent the location of the ejection pins, and FIG. 3E is a plan view of the lower mold according to the invention indicating the location of the pins. ejection. Although various provisions of the ejection pins are possible, it is preferable to place the center pins 12 and 13 halfway between the pin of the identification mark and the upper ejection pin and between the two lower pins. ejection respectively. Of course, various modifications may be made by those skilled in the art to the assemblies which have just been described solely by way of nonlimiting example.

  without departing from the scope of the invention.

Claims (9)

  1. Set of molds intended to form a protective body   The mold assembly comprises an upper mold assembly and a lower mold assembly, the upper and lower mold assemblies comprising A) two upper and lower chassis (90a, 90b) having each a cavity in which a hot and liquid molding composition is injected and which gives shape to the protective body of the housing, B) two upper and lower trays (80a, 80b) for opening   and close the upper frame of the upper mold assembly   the lower frame of the lower mold assembly   C) a pin (4) with identification mark and an upper pin (9) for ejection fixed to the upper plate (80a) and protruding towards the cavity for ejection of the body of the housing just after the composition of molding has been totally injected into the cavity, and D) a first and a second lower ejection pin (8) attached to the lower plate (80b) and protruding towards the cavity so as to   that they eject the body of the case after the   molding unit has been completely injected into the cavity, characterized in that a lower central ejection pin (13) is placed between the first and the second lower ejection pin (8), the lower ejection central pin being attached to the lower plate (80b) and protruding towards the cavity.   2. Set of molds according to claim 1, characterized   characterized in that an upper central pin (12) for ejecting   is placed between the pin (4) of the identification mark   and the upper ejection pin (9), the upper central pin (12) ejection being fixed to the tray   superior and protruding towards the cavity.   3. Set of molds according to claim 1, characterized   characterized in that the pin (4) of the ejection mark, the upper central pin (12) for ejection and the pin   upper (9) ejection are placed on a straight line.   4. Set of molds according to claim 2, characterized   characterized in that the first and the second lower ejector pin (8) and the lower central pin (13)   ejection are placed on a straight line.   5. Set of molds according to claim 3, characterized in that the first and the second lower ejector pin (8) and the lower central ejection pin (13) are arranged parallel to the long side of the ejector. lower plateau.   6. Set of molds according to claim 3, characterized   characterized in that the first and the second lower ejection pin (8) and the lower central ejection pin (13) are disposed diagonally in relation to each other.   at the side of the lower plate (80b).   7. Set of molds according to claim 4, characterized   in that the pin (4) of the identification mark   the upper central pin (12) for ejection and   the upper spindle (9) are arranged parallel to each other.   same as a large side of the upper plate (80a).   8. Set of molds according to claim 4, characterized   in that the pin (4) of the identification mark   the upper central pin (12) for ejection and the upper pin (9) for ejection are arranged diagonally with respect to the sides of the upper plate (80a).   9. Set of molds according to claim 1, characterized   characterized in that the molding composition is a resin of plastic material.