CN113172291B - Preparation method of low-temperature high-strength connecting welding spot in PoP packaging process - Google Patents
Preparation method of low-temperature high-strength connecting welding spot in PoP packaging process Download PDFInfo
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- CN113172291B CN113172291B CN202110384732.5A CN202110384732A CN113172291B CN 113172291 B CN113172291 B CN 113172291B CN 202110384732 A CN202110384732 A CN 202110384732A CN 113172291 B CN113172291 B CN 113172291B
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K1/00—Soldering, e.g. brazing, or unsoldering
- B23K1/0008—Soldering, e.g. brazing, or unsoldering specially adapted for particular articles or work
Abstract
A method for preparing a low-temperature high-strength connecting welding spot in a PoP packaging process. The invention belongs to the technical field of stacked packaging. The invention aims to solve the technical problem that the performance of a connected welding spot is improved slightly by the conventional method for carrying out surface treatment on the welding spot. The method comprises the following steps: step 1: mixing SAC305 solder and Sn-58Bi solder on the surface of the bonding pad according to different proportions; step 2: and (3) performing fusion welding on the system in the step (1) to realize the connection of the composite welding spot and the welding pad, and obtaining a low-temperature high-strength connection welding spot in the PoP packaging process. The invention strictly controls the proportion of the two solders and simultaneously matches with the melting welding process parameters of the invention, so that the shearing strength of the composite welding point on the bonding pad is up to 75.1MPa and is far higher than the bonding pad connection strength of the surfaces of the ENIG and the OSP.
Description
Technical Field
The invention belongs to the technical field of stacked packaging, and particularly relates to a preparation method of a low-temperature high-strength connecting welding spot in a PoP packaging process.
Background
As an advanced packaging technology, multi-pass reflow soldering is necessary to achieve stacking of packaged devices, and thus, the PoP has a need for a low-temperature bonding technology. With the continuous development of packaging technology, the packaging density is continuously improved, the packaging process window is narrower and narrower, the packaging structure is more and more complex, the performance of welding spots can not meet the requirements, and meanwhile, the improvement of the performance of the welding spots is also an important development direction.
For the low-temperature bonding technology, low-temperature solder and metal nano-particle sintering are commonly used. For the low-temperature brazing filler metal, connection is realized due to different melting points of alloys with different components, but the SAC305 brazing filler metal is still the most commonly used alloy brazing filler metal in the industry at present, and other alloy brazing filler metals with different components have performance defects (SnBi eutectic brazing filler metal is brittle and cannot resist impact load; SnIn alloy brazing filler metal is soft and is easy to deform). The nano-particle sintering based on metal materials (Ag, Cu and the like) has the defects that the nano-particle has large surface energy, and can be diffused to form connection under the condition of being lower than the melting point of the nano-particle to realize the low-temperature service of high-temperature connection, but the nano-particle has high cost, is easy to oxidize, needs pressurization in the connection process, has low connection strength and the like, and limits the application of the nano-particle.
In order to improve the performance of the welding spot, the surface treatment of the welding spot is a common method, and although the method can improve the oxidation resistance of the welding spot and improve the wetting effect of the welding flux, the improvement of the performance of the connected welding spot is small.
Disclosure of Invention
The invention provides a preparation method of a low-temperature high-strength connecting welding spot in a PoP packaging process, aiming at solving the technical problem that the performance of the connected welding spot is improved slightly by the existing method for carrying out surface treatment on a welding spot.
The preparation method of the low-temperature high-strength connecting welding spot in the PoP packaging process is carried out according to the following steps:
step 1: mixing SAC305 solder and Sn-58Bi solder on the surface of the bonding pad according to different proportions;
step 2: and (2) performing fusion welding on the system in the step (1) to realize the connection of the composite welding point and the welding pad, wherein the fusion welding comprises the following treatment processes: the temperature is increased from the room temperature to 180-220 ℃, the temperature is kept for 75-85 s, then the temperature is reduced from 180-220 ℃ to the room temperature, wherein the temperature increased from the room temperature to 138 ℃ is taken as a first node, the temperature reduced from 180-220 ℃ to 138 ℃ is taken as a second node, and the temperature is shared between the two nodes for 300-620 s, so that the fusion welding treatment is completed, and the low-temperature high-strength connecting welding spot in the PoP packaging process is obtained.
Further, the mass ratio of the SAC305 solder to the Sn-58Bi solder in the step 1 is defined to be 1: 0.5.
Further, the mixed loading form of the SAC305 solder and the Sn-58Bi solder on the surface of the pad in the step 1 is solder paste solder ball connection or laminated stacking.
Further limiting, in the step 1, the bonding pad is an electroless nickel-palladium gold-immersion bonding pad or an electroless nickel-gold-immersion bonding pad.
Further limiting, in the step 2, the temperature is firstly increased from the room temperature to 200 ℃, the temperature is kept for 80s, and then the temperature is reduced from 200 ℃ to the room temperature.
Further limiting, in step 2, the two nodes share the same time within 380 s-540 s.
Further limited, in step 2, the two nodes share the same time 460 s.
Compared with the prior art, the invention has the following advantages:
1) the invention uses two solders of Sn-3Ag-0.5Cu (SAC305) and Sn-58Bi (SnBi) as the basis to carry out mixed loading, and connects the composite welding spot on an ENEPIG processing welding spot, the SnBi melts after the temperature rises to be higher than the melting point of the SnBi, the SAC305 dissolves into the SnBi to finally form a uniform Sn-Ag-Cu-Bi welding spot, wherein, elements such as Au, Pd and the like in the welding spot dissolve in the solder within seconds after the SnBi melts.
2) The invention strictly controls the proportion of the two solders and simultaneously matches with the melting welding process parameters of the invention, so that the shearing strength of the composite welding point on the bonding pad is up to 75.1MPa and is far higher than the bonding pad connection strength of the surfaces of the ENIG and the OSP.
Drawings
FIG. 1 is a schematic view of the mixing and charging of example 1;
FIG. 2 is a schematic view of the mixing and charging of example 4;
FIG. 3 is the weld spot cross-sectional texture of example 1; FIGS. 3a-300s, FIGS. 3b-380s, FIGS. 3c-460s, FIGS. 3d-540s, FIGS. 3e-620 s;
FIG. 4 is a sectional structure of a welding spot according to example 2; FIGS. 4f-300s, FIGS. 4g-380s, FIGS. 4h-460s, FIGS. 4i-540s, FIGS. 4j-620 s;
FIG. 5 is a sectional structure of a welding spot according to example 3; FIGS. 5k-300s, FIGS. 5l-380s, FIGS. 5m-460s, FIGS. 5n-540s, FIGS. 5o-620 s;
FIG. 6 is a schematic diagram of ENEPIG surface IMC growth;
FIG. 7 is a graph of the shear strength and fracture morphology of the weld point of example 2;
FIG. 8 is a microstructure of a weld fracture in example 2; FIGS. 8a-300s, FIGS. 8b-380s, FIGS. 8c-460s, FIGS. 8d-540s, FIGS. 8e-620 s;
FIG. 9 is the welding spot cross-sectional structure of example 4;
FIG. 10 is a schematic diagram of IMC growth on a Ni surface;
FIG. 11 is a graph of the shear strength and fracture morphology of the solder joint of example 4;
FIG. 12 is a microstructure of a weld fracture in example 4; FIGS. 12a-300s, FIGS. 12b-380s, FIGS. 12c-460s, FIGS. 12d-540s, and FIGS. 12e-620 s.
Detailed Description
Example 1: the preparation method of the low-temperature high-strength connecting welding spot in the PoP packaging process is carried out according to the following steps:
step 1: according to the mixed loading schematic diagram shown in FIG. 1, SAC305 solder and Sn-58Bi solder are stacked on the surface of an electroless nickel-plated palladium-impregnated gold (ENEPIG) pad with the diameter of 1.8mm in a laminar manner according to the mass ratio of 1: 0.5;
step 2: and (2) performing fusion welding on the system in the step (1) to realize the connection of the composite welding point and the welding pad, wherein the fusion welding comprises the following treatment processes: the temperature is increased from room temperature to 180 ℃, the temperature is kept for 80s, then the temperature is reduced from 180 ℃ to room temperature, wherein the temperature increased from room temperature to 138 ℃ is taken as a first node, the temperature reduced from 180 ℃ to 138 ℃ is taken as a second node, and 300s, 380s, 460s, 540s or 620s is shared between the two nodes, so that the fusion welding treatment is completed, and the low-temperature high-strength connecting welding spot in the PoP packaging process is obtained.
Example 2: this example differs from example 1 in that: the melting welding process comprises the following steps: the temperature is raised from the room temperature to 200 ℃, the temperature is preserved for 80s, and then the temperature is lowered from 200 ℃ to the room temperature. The other steps and parameters were the same as in example 1.
Example 3: this example differs from example 1 in that: the melting welding process comprises the following steps: the temperature is firstly increased from the room temperature to 220 ℃, the temperature is preserved for 80s, and then the temperature is reduced from 220 ℃ to the room temperature. The other steps and parameters were the same as in example 1.
Example 4: this example differs from any of examples 1-3 in that: the pad is an Electroless Nickel Immersion Gold (ENIG) pad with a diameter of 1.8 mm. Other steps and parameters were the same as in any of examples 1 to 3.
Example 5: the cross-sectional structure of the solder joint in example 2 is detected, and the fracture cross-sectional structure morphology shown in fig. 3-5 is obtained, it can be seen that intermetallic compound (IMC) grows in a needle shape to the solder ball, the needle-shaped grain structure is not oriented, the needle-shaped IMC mainly grows axially and continuously increases in length along with the increase of the holding time and the holding temperature, and simultaneously, IMC grains are found to enter the solder joint close to the interface, because the IMC with too large length is broken by the force, fig. 6 is an ENEPIG surface IMC growth schematic diagram, and it can be known that the intermetallic compound is (Cu, Ni, Pd)6Sn5 through energy spectrum analysis and reference literature, and the needle-shaped growth is because the crystal energy is affected by the addition of Pd element.
Example 6: the shear test was performed on the solder joints of example 2, and the results are shown in fig. 7, in which the solder joint strength decreased first and then increased and then decreased as the holding time increased, the solder joint strength reached 75.1MPa after 300s holding, and the solder joint strength reached 68.5MPa again after 540s holding. And the welding spot macro fracture shows a mixed fracture except 460s heat preservation samples.
Example 7: the microscopic morphology of the welding spot fracture of the embodiment 2 is detected, and the result is shown in fig. 8, and it can be seen that the fracture is in a composite form when the heat preservation time is 300s and 380s, and the existence of the acicular IMC and the solder fracture structure can be obviously observed; when the heat preservation time is 460s, the fracture presents internal fracture of the solder; the heat preservation time is 540s, when 620s, the fracture is in composite fracture, the IMC interface at the fracture is thick, and the fracture section is different from the acicular morphology with short heat preservation time.
Example 8: the solder joint of example 4 was subjected to cross-sectional structure inspection to obtain the cross-sectional structure morphology as shown in fig. 9, from which the intermetallic compound cross-sectional morphology was found to be in the shape of triangle, trapezoid, pentagon, etc., from which reference literature comparison could find the sample IMC to be in the shape of prism, pyramid, etc., from which reference literature comparison could find the intermetallic compound to be (Cu, Ni)6Sn5, and since a Cu6Sn5 type compound was grown on the Ni surface, in which the Cu element originated from SAC305, unlike growth on a copper pad, the IMC did not completely cover the pad but grew in discrete grains. Under the same temperature condition, the IMC particles grow continuously and the volume increases continuously along with the increase of the heat preservation time; the same heat preservation time is adopted, and different temperature gradients have smaller influence on the structure of the intermetallic compound. FIG. 10 is a schematic diagram of the growth of IMC on the Ni surface, wherein the IMC performs heterogeneous nucleation on the Ni surface, and Cu and Sn atoms in the liquid phase are gathered to the Ni surface and grow into (Cu, Ni)6Sn5 with the increase of the holding time and the holding temperature. Since the growth of (Cu, Ni)6Sn5 on the Ni surface is not affected by the orientation of Ni grains, its growth tends to be the case of the smallest surface energy, i.e., it is a hexagonal lattice structure in relation to its crystal type, so that the IMC particles grow as 12-sided or prismatic shapes.
Example 9: the shear test was performed on the weld of example 4, and the results are shown in fig. 11, in which the weld strength increased first and then decreased as the holding time increased, the weld strength reached 63.7MPa after 460s holding, and the weld strength reached 51.1s after 620s holding. Solder joint macro-fractures all exhibit mixed fractures, mainly because the IMC does not completely cover the pad in the Au/Ni/Cu interface.
Example 10: the microscopic morphology of the solder joint fracture in example 4 is detected, and the result is shown in fig. 12, it can be seen that the fracture is in a composite form, prismatic IMC and lamellar solder fracture structures exist, and when the heat preservation time is 300s, the Ni pad is exposed at the fracture, because the short heat preservation time results in smaller IMC particles and more direct contact area between the Ni pad and the structures.
Claims (4)
1. A preparation method of a low-temperature high-strength connecting welding spot in a PoP packaging process is characterized by comprising the following steps:
step 1: mixing SAC305 solder and Sn-58Bi solder on the surface of the bonding pad according to different proportions; the bonding pad is a chemical nickel-plating palladium gold-dipping bonding pad or a chemical nickel-plating gold-dipping bonding pad, and the mass ratio of the SAC305 solder to the Sn-58Bi solder is 1: 0.5;
step 2: and (2) performing fusion welding on the system in the step (1) to realize the connection of the composite welding point and the welding pad, wherein the fusion welding comprises the following treatment processes: the temperature is increased from room temperature to 200 ℃, the temperature is kept for 80s, then the temperature is reduced from 200 ℃ to room temperature, wherein the temperature increased from room temperature to 138 ℃ is taken as a first node, the temperature reduced from 180-220 ℃ to 138 ℃ is taken as a second node, the two nodes share the same time for 300 s-620 s, the fusion welding treatment is completed, the low-temperature high-strength connecting welding spot in the PoP packaging process is obtained, and the strength of the welding spot reaches 75.1MPa at most.
2. The method for preparing a low-temperature high-strength connecting solder joint in a PoP packaging process as claimed in claim 1, wherein the mixed loading form of the SAC305 solder and the Sn-58Bi solder on the surface of the pad in step 1 is solder paste ball connection or lamellar stacking.
3. The method for preparing a low-temperature high-strength connection solder joint in a PoP packaging process as claimed in claim 1, wherein the time for sharing between two nodes in step 2 is 380-540 s.
4. The method according to claim 1, wherein 460s is used for the common use of two nodes in step 2.
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