CN111725081A - A SOP preparation method of different sizes for plastic packaged flip-chip substrates - Google Patents

Abstract

The present invention provides a method for preparing SOPs of different sizes for plastic-encapsulated flip-chip soldering substrates, comprising the following steps: step 1, performing flux printing on all pads; step 2, selecting the required size of solder balls according to the size of the pads, The pads are divided into multiple groups; for a group of pads, place the substrate with the printed flux under the ball-mounting stencil corresponding to the group of pads, and pass the solder balls corresponding to the group of pads through the corresponding ball-mounting stencil Leakage is placed on the set of pads; Step 3, after the solder balls are leaked and placed on the pads, place the substrate in a reflow oven for reflow soldering to form spherical solder joints; Step 4, Repeat steps 1 to 3 until traversed For each group of pads, corresponding spherical solder joints are formed on each group of pads; step 5, cleaning the substrate on which the solder joints are prepared to remove residual flux; step 6, leveling all spherical solder joints. The method of the invention improves the welding quality of the high-density plastic packaging flip-chip welding process, and ensures the long-term reliability of the plastic packaging flip-chip welding device.

Description

A SOP preparation method of different sizes for plastic packaged flip-chip substrates

technical field

The invention relates to a method for preparing an SOP of a packaging substrate, in particular to a method for preparing an SOP on a flip-chip soldering pad of a plastic-encapsulated flip-chip soldering substrate, and belongs to the technical field of semiconductor packaging.

Background technique

Flip-chip soldering technology is to prepare interconnect bumps on the active surface of the chip. The bumps are arranged in a surface array, which has a series of advantages of high interconnect density and high reliability. It has been used in electric power, military industry, aerospace, etc. large-scale application in the field. With the continuous improvement of the performance of plastic packaged flip-chip devices, the density of interconnect bumps has increased sharply, and the total number of bumps on a single chip will reach more than 10,000. It is difficult to guarantee the soldering quality and soldering consistency of the bumps during the reflow soldering process. The phenomenon of virtual soldering is very easy to occur, which brings serious hidden dangers to the long-term reliability of plastic packaged flip-chip devices.

In addition, the increase in interconnect density will inevitably lead to a sharp decrease in the size of bumps, and bumps with small dimensions will face more serious reliability problems after interconnection. On the one hand, the shrinking bump size will lead to a decrease in the soldering strength of the bumps, which will affect the overall soldering quality of the chip, especially in the areas where the stress and strain are concentrated at the corners of the device. Reliability; on the other hand, the decrease in bump size will make the size effect prominent, and the proportion of IMC at the solder interface to the bump will increase sharply, especially in thermal environments such as high temperature storage, which may cause all solder joints to be converted to IMC. It will seriously affect the long-term reliability of bumps.

At present, the industry usually uses screen printing to prepare the SOP on the flip-chip bonding pads of the plastic packaging substrate, so that the chip bumps and the SOP are welded, so as to reduce the difficulty of the process and improve the welding quality. However, the screen printing method cannot prepare SOPs of different sizes, and it cannot solve the problem of low reliability of small-sized bumps in individual areas. Therefore, developing a method for preparing SOPs of different sizes for flip-chip substrates can improve the process quality of plastic-encapsulated flip-chip devices. and long-term reliability is extremely important.

SUMMARY OF THE INVENTION

The technical problem solved by the present invention is: to overcome the above-mentioned deficiencies of the prior art, and to provide a different size SOP preparation method for plastic packaging flip-chip soldering substrates, which is suitable for the plastic packaging flip-chip soldering packaging process, and is compatible with various package sizes, different The flip-chip packaging shell of SOP material has the characteristics of simple process and high dimensional accuracy, which can significantly improve the welding quality of high-density flip-chip welding process, solve the problems of low welding strength and poor reliability of small-sized bumps, and then Long-term reliability of flip-chip devices is guaranteed.

The technical solution solved by the present invention is: a method for preparing SOPs of different sizes for plastic packaging flip-chip substrates, the steps are as follows:

(1) Place the substrate under the flux printing screen, a large number of pads of different specifications are arranged on the substrate, apply flux on the flux printing screen, and perform flux printing on all the pads;

(2) According to the pad size, select the required solder ball size, and divide the pads into multiple groups; each group of pads is of the same specification, and the solder balls of the same size are set on it, and each specification pad corresponds to a set of balls Stencil; for a set of pads, place the substrate with the printed flux under the stencil corresponding to the set of pads, and place the solder balls corresponding to the set of pads through the corresponding stencil. on the set of pads;

(3) After the solder balls are leaked and placed on the pads, the substrate is placed in a reflow furnace for reflow soldering to form spherical solder joints;

(4) Repeat steps (1) to (3) until each group of pads is traversed, and corresponding spherical solder joints are formed on each group of pads;

(5) Clean the substrate on which the solder joints are prepared to remove the residual flux.

(6) Flatten all spherical solder joints.

Preferably, a large number of pads of different specifications are arranged on the substrate, specifically: circular pads are arranged on the substrate, the pads are prepared by electroless plating of nickel-gold or nickel-palladium-gold, and the diameter of the pads is preferably 70 μm to 150 μm , the pad spacing is preferably in the range of 120 μm to 150 μm, which further improves the preparation effect.

Preferably, each flux printing screen is provided with a plurality of circular holes of uniform specifications, which can leak the flux to the pads.

Preferably, the substrate is a multilayer composite structure organic flip-chip substrate, including a core board, a wiring layer and a solder resist layer; the thickness of the core board is preferably in the range of 400 μm to 800 μm, the core board is a wiring layer, and the number of layers of the wiring layer is There are 3 to 6 layers, the thickness of each layer is preferably 15 μm to 30 μm, the wiring layer is a solder mask layer, the height of the solder mask layer is preferably 30 to 50 μm, and the solder mask layer is provided with an opening for a pad, and the shape of the opening is a circle. The diameter is preferably 50-80 μm, which further improves the preparation effect.

Preferably, the pad has a Ni-Au structure, including a Ni layer and an Au layer, and is prepared by an electroplating process, the Ni layer is an Au layer, the thickness of the Ni layer is preferably more than 2.54 μm, and the thickness of the Au layer is preferably 0.02 μm to 0.3 μm. μm to further improve the preparation effect.

Preferably, the flux is a water-soluble rosin-based flux;

Preferably, the flux printing screen is prepared by an electroforming process, and the screen is machined with the same circular holes of a certain size. The diameter is preferably 70% to 90% of the diameter of the pad;

Preferably, the components of the solder balls preferably include tin-based materials such as PbSn, SnAg, and SnAgCu, and the diameter of the solder balls is preferably 80% to 120% of the diameter of the pads to further improve the preparation effect.

Preferably, the ball stencil is prepared by an electroforming process, the side facing the substrate is coated with a protective coating, and the stencil is machined with the same circular hole of a certain size, and the position of the circular hole needs to be soldered on the substrate. The pad positions of the balls correspond one by one. The diameter of the round hole is 110% to 130% of the diameter of the solder ball. When placing the ball, the pressure of the ball placement device and the distance between the ball placement screen and the substrate should be adjusted. The ball placement device pressure is preferably 0.1 MPa to 0.5 MPa, and the distance between the ball screen and the substrate is preferably 30 to 50 μm, which further improves the preparation effect.

The advantages of the present invention compared with the prior art are:

(1) The present invention adopts the process of "flux printing-ball placement-reflow-leveling" to prepare SOP, with simple process and high maturity;

(2) In the present invention, stencils of different specifications are used to place the balls, the distance between the stencils and the base plate for placing the balls is adjustable, and the preparation of SOPs of different sizes is realized by repeatedly placing the balls, without affecting the existing size SOPs.

(3) The present invention uses a leveling method to process the surface of the SOP to ensure that the coplanarity of the SOPs of different sizes meets the requirements of the process index.

Description of drawings

FIG. 1 is a schematic diagram of the structure of a plastic-encapsulated flip-chip substrate;

Figure 2 is a schematic diagram of flux printing;

Figure 3 is a schematic diagram of solder ball leakage;

Figure 4 is a schematic diagram of the solder balls after reflow;

Figure 5 is a schematic diagram of another specification of solder ball leakage;

FIG. 6 is a schematic diagram of another specification solder ball after reflow;

Fig. 7 is the schematic diagram of SOP after leveling;

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

The present invention provides a method for preparing SOPs of different sizes for plastic-encapsulated flip-chip soldering substrates, comprising the following steps: Step 1. Place the substrate under a flux printing screen, a large number of pads of different specifications are arranged on the substrate, and the flux printing is performed on the substrate. Apply flux on the stencil, and perform flux printing on all pads; step 2. According to the size of the pads, select the required solder ball size, and divide the pads into multiple groups; each group of pads is of the same specification, and the Set solder balls of the same size, and each type of pad corresponds to a ball stencil; for a set of pads, place the substrate with the printed flux under the ball stencil corresponding to the set of pads, and place the The solder balls corresponding to the set of pads are leaked on the set of pads through the corresponding ball stencil; step 3. After the solder balls are leaked and placed on the pads, the substrate is placed in a reflow oven for reflow soldering to form a spherical Solder joints; step 4, repeat steps 2 and 3 until each group of pads is traversed, and corresponding spherical solder joints are formed on each group of pads; step 5, clean the substrate on which the solder joints are prepared to remove residual flux ; Step 6. Flatten all spherical solder joints. The method of the invention improves the welding quality of the high-density plastic packaging flip-chip welding process, and ensures the long-term reliability of the plastic packaging flip-chip welding device.

The invention is a SOP preparation method on the flip-chip welding pad of a plastic-packaged flip-chip welding substrate, which is used to realize the interconnection between the chip bump and the substrate in the flip-chip welding device. The traditional screen printing method can only prepare SOPs of the same specification, and the present invention can realize the preparation of SOPs of different specifications on the plastic packaging substrate, the preparation process is simple, the yield is high, and the small-sized protrusions in individual areas of the plastic packaging flip-chip devices can be solved. The problem of low point reliability is improved, and the process quality and long-term reliability of plastic packaged flip-chip devices are improved.

The present invention is a method for preparing SOPs of different sizes for plastic-encapsulated flip-chip soldering substrates, and the preferred solution steps are as follows:

(1) A circular pad is arranged on the substrate. The pad is prepared by chemical plating such as nickel-gold or nickel-palladium-gold. . Place the substrate under the flux printing stencil, as shown in Figure 2, the distance between the substrate and the flux printing stencil is 30-50 μm, and then apply the flux on the surface of the printing stencil to ensure the coating quality and consistency. The amount of flux is preferably greater than 100g, the printing pressure is preferably 0.1-0.5MPa, and the printing speed is preferably 10-20mm/s.

(2) According to the pad size, select the required solder ball size, and divide the pads into multiple groups, preferably 2 to 3 groups according to product requirements. Each group of pads is of the same specification, and the solder balls of the same size are set on it, and the pads of each specification correspond to a ball screen;

For a group of pads, place the substrate with the printed flux under the ball screen corresponding to the group of pads, as shown in Figure 3, so that the holes on the ball screen and the substrate need to be leaked solder balls The positions of the pads correspond one-to-one, the distance between the ball screen and the substrate is preferably 30-50μm, and the pressure of the ball-setting device is preferably 0.1MPa-0.5MPa, so that the solder balls corresponding to the group of pads pass through the corresponding The ball mesh board is placed on the set of pads to achieve ball placement;

(3) After the solder balls are leaked on the pads, place the substrate in a reflow furnace for reflow soldering, control the reflow peak temperature, reflow time, and reflow atmosphere, and set the peak temperature according to the composition of the solder balls. The peak temperature is preferably high. When the melting point of the solder ball is above 30°C, the reflow time is preferably 60-90s to ensure that the solder ball is fully melted and soldered to the pad well. The reflow atmosphere is nitrogen, which improves the wettability of the solder ball and the substrate after melting. , after reflow, spherical solder joints are formed, and the prepared SOP (ball solder joints on pads) is obtained, and the height is H, as shown in Figure 4;

(4) Repeat steps (1) to (3). During the second flux printing and ball placement, the height between the flux printing stencil and the ball placement stencil and the substrate should be adjusted to the height of the prepared SOP. 3 to 5 times of the SOP to prevent the prepared SOP from being damaged by the flux printing stencil or the ball stencil, until each group of pads is traversed, and the corresponding spherical solder joints are formed on each group of pads, as shown in Figure 5- As shown in Figure 6;

(5) Clean the substrate on which the spherical solder joints have been prepared, and spray the substrate with a water-based cleaning solution in a heated state. ~50 Psi, the spraying time is preferably 10-15 minutes, and then drying is performed, the drying temperature is preferably 50-70 °C, and the drying time is preferably 5-10 minutes.

(6) All spherical solder joints are leveled, and a leveling tool is used to contact the spherical solder joints. The diameter of the tool is preferably 30-50 μm, and the contact pressure is preferably 0.1-0.5 MPa, as shown in Figure 7.

Through the above steps, SOP preparation of various specifications on the plastic packaging substrate is realized, the consistency of the SOP process is effectively guaranteed, the preparation process is simple, and the yield is high.

The present invention achieves a further scheme for improving the quality of SOP preparation: when repeating steps (1) to (3) in step (4), when performing secondary flux printing and ball placement, a printing screen or a ball screen and a substrate ( The distance between the surface of the solder mask layer) is L, the height of the prepared SOP is H, and the ball setting pressure is P (the ball setting pressure means that the solder balls must pass through the brush that entrains the solder balls, and the balls are lost in the setting balls. On the screen, the pressure exerted by the brush on the ball-setting screen is defined as the ball-setting pressure), preferably satisfying P ² ×L<H/L<1, and satisfying the optimal constraint condition can further improve the quality of SOP preparation, further Improve the quality and reliability of the flip-chip soldering process.

In a method for preparing SOPs of different sizes for plastic-encapsulated flip-chip substrates of the present invention, the further preferred solution steps are as follows:

(1) As shown in FIG. 1 is a schematic diagram of the structure of the plastic packaging substrate in the embodiment of the present invention, wherein 1 is the pad of the substrate, which is prepared by an electroless plating process, and has a Ni-Au structure, and 2 is a solder resist layer, which protects the pad. Function, 3 is the whole of the substrate;

Place the substrate 3 under the flux printing screen 4 so that the circular holes 5 on the flux printing screen 4 correspond to the positions of the pads 1 one-to-one, and adjust the distance between the substrate 3 and the flux printing screen 4 , so that all the pads 1 on the substrate 3 are evenly coated with flux;

(2) Remove the flux printing stencil 4, place the substrate 3 with the printed flux under the ball stencil 6, and make the round holes 7 on the ball stencil 6 correspond to the positions of the pads 1 one-to-one , the solder ball 8 of a size specification is placed on the pad 1 through the circular hole 7;

(3) Place the ball-mounted substrate 3 in a reflow oven for reflow soldering. After the soldering is completed, a spherical solder joint 9 is formed on the pad 1, and the height of the spherical solder joint is set to H, (after the height of the solder joint 9 is detected Height is 90µm ;)

(4) Repeat steps (1) to (3) until each group of pads is traversed, and corresponding spherical solder joints are formed on each group of pads. The preferred solution is as follows:

Place the substrate on which the solder joints 9 are formed under the flux printing screen 4 of another specification, so that the circular holes 5 on the flux printing screen 4 correspond one-to-one with the positions of the pads 1 of the other specification. The distance between the substrate 3 and the flux printing screen 4 is set to 400 μm, and the flux printing pressure is set to 0.1 MPa, so that the pad 1 of another specification on the substrate 3 is evenly coated with flux;

Place the substrate 3 under the ball screen 6 of another specification, make the round holes 7 on the ball screen 6 correspond to the positions of the pads 1 of the other specification, and set the gap between the substrate 3 and the ball screen 6. The distance between them is 400 μm, the ball placement pressure is set to 0.1Mpa, and the solder balls 8 of another specification are placed above the pads 1 through the circular holes 7 on the ball placement screen 6 .

When repeating steps (1) to (3) in step (4), and performing more than two times of flux printing and ball placement, the preferred solution is: the height of the solder joint 9 and the distance between the flux printing screen 4 and the substrate 3 , The distance between the ball screen 6 and the substrate 3 and the ball pressure value satisfy the optimal formula P ² ×L<H/L<1, which effectively ensures that the prepared solder joints 9 are not damaged, and ensures the quality of SOP preparation and the finished product Rate.

The appearance and height of the formed solder joints 9 are inspected, which are consistent with the height of the spherical solder joints 9 in step (3). Damage occurs during the second ball placement;

Until each group of pads is traversed, the corresponding spherical solder joints are formed on each group of pads;

(5) Clean the substrate on which the spherical solder joints have been prepared, and spray the substrate with a water-based cleaning solution in a heated state. ~50 Psi, the spraying time is preferably 10-15 minutes, and then drying is performed, the drying temperature is preferably 50-70°C, and the drying time is preferably 5-10 minutes.

(6) All spherical solder joints are leveled, and a leveling tool is used to contact the spherical solder joints. The diameter of the tool is preferably 30 to 50 μm, and the contact pressure is preferably 0.1 to 0.5 MPa. As shown in Figure 7, the flip chip is further improved. Welding quality and reliability.

Any modification or equivalent replacement to the solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

The present invention adopts the process of "flux printing-ball placement-reflow-leveling" for SOP preparation, the process is simple, and the maturity is high; The distance is adjustable, and the preparation of SOPs of different sizes can be realized by repeatedly placing the balls, which will not affect the existing size SOPs. In addition, the present invention uses a leveling method to process the surface of the SOP, so as to ensure that the coplanarity of the SOPs of different sizes meets the requirements of the process index.

Claims (10)

1. a different size SOP preparation method for plastic-encapsulated flip-chip substrates, characterized in that the steps are as follows: (1) Place the substrate under the flux printing screen, a large number of pads of different specifications are arranged on the substrate, apply flux on the flux printing screen, and perform flux printing on all the pads; (2) According to the pad size, select the required solder ball size, and divide the pads into multiple groups; each group of pads is of the same specification, and the solder balls of the same size are set on it, and each specification pad corresponds to a set of balls Stencil; for a set of pads, place the substrate with the printed flux under the stencil corresponding to the set of pads, and place the solder balls corresponding to the set of pads through the corresponding stencil. on the set of pads; (3) After the solder balls are leaked and placed on the pads, the substrate is placed in a reflow furnace for reflow soldering to form spherical solder joints; (4) Repeat steps (1) to (3) until each group of pads is traversed, and corresponding spherical solder joints are formed on each group of pads; (5) Clean the substrate on which the solder joints are prepared to remove residual flux; (6) Flatten all spherical solder joints. 2. A method for preparing a SOP with different sizes for a plastic-encapsulated flip-chip substrate according to claim 1, characterized in that: a large number of pads of different specifications are arranged on the substrate, specifically: a circular pad is arranged on the substrate , the pad is prepared by electroless plating of nickel-gold or nickel-palladium-gold. 3. A kind of SOP preparation method for plastic-encapsulated flip-chip soldering substrates of different sizes according to claim 1, characterized in that: each flux printing screen is provided with a plurality of circular holes of uniform specifications, which can drain onto the pad. 4. The method for preparing a SOP with different sizes for a plastic-encapsulated flip-chip substrate according to claim 1, wherein the substrate is a multilayer composite structure organic flip-chip substrate, comprising a core board, a wiring layer and a resistor Soldering layer; the core board is a wiring layer, the wiring layer is a solder resist layer, and the solder resist layer is provided with an opening for a pad. 5. The method for preparing a different size SOP for a plastic-encapsulated flip-chip substrate according to claim 1, wherein the pad is a Ni-Au structure, comprising a Ni layer and an Au layer, and is prepared by an electroplating process, On the Ni layer is an Au layer. 6 . The method for preparing a SOP with different sizes for a plastic-packaged flip-chip soldering substrate according to claim 1 , wherein the flux is a water-soluble rosin-based flux. 7 . 7. The method for preparing a different size SOP for plastic-encapsulated flip-chip soldering substrates according to claim 1, characterized in that: the flux printing screen is prepared by an electroforming process, and a certain size is processed on the screen. For the same round hole, the position of the round hole corresponds to the position of the pad on the substrate that needs to be printed with flux, and the diameter of the round hole is 70% to 90% of the diameter of the pad. 8. The method for preparing a SOP with different sizes for plastic packaging flip-chip substrates according to claim 1, wherein the components of the solder balls include: tin-based materials such as PbSn, SnAg, and SnAgCu, and the diameter of the solder balls is 80% to 120% of the diameter of the disc. 9 . The method for preparing a SOP with different sizes for plastic-encapsulated flip-chip substrates according to claim 1 , wherein the ball stencil is prepared by an electroforming process, and a protective coating is applied to the side facing the substrate. 10 . . 10. A method for preparing SOP with different sizes for plastic-encapsulated flip-chip substrates according to claim 1, characterized in that: the same circular holes of a certain size are processed on the stencil, and the positions of the circular holes and the substrate need to be leaked. The positions of the pads for placing the solder balls correspond one-to-one. When placing the balls, it is necessary to adjust the pressure of the ball placement device and the distance between the ball placement screen and the substrate.

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