RU2528553C2 - Method of converting matrix-arranged microcircuit ball leads made of lead-free solder into tin-lead leads of near-eutectic composition and solder paste therefor - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: invention relates to radio electronics and can be used to convert matrix-arranged ball leads of microcircuits from lead-free solder into tin-lead leads of a near-eutectic composition with further surface mounting of radio components and integrated circuits on printed-circuit boards and forming reliable and high-quality soldered connections designed to operate in harsh operating conditions. A microcircuit with matrix-arranged ball leads made of lead-free solder based on tin and silver is placed on a flat substrate of non-solder wettable material, on which certain solder paste doses are first deposited through a meal stencil, said doses having high lead content, while providing overlapping and contacting of the ball leads and solder paste doses, further heating to a peak temperature not higher than 230°C and cooling while holding at temperature higher than 180°C until formation, during crystallisation, of new, larger ball leads consisting of a near-eutectic tin-lead solder, close to the composition of the eutectic three-component alloy Sn₆₂Pb₃₆Ag₂.

EFFECT: invention provides said conversion with minimal mechanical and thermal action on a microcircuit in order to preserve full operating capacity thereof after conversion.

3 cl, 9 dwg, 1 tbl

Description

The proposal relates to electronics and can be used to convert matrix-spaced ball leads of microcircuits from lead-free solder to tin-lead near-eutectic composition with further surface mounting of radio electronic elements and integrated circuits on printed circuit boards and the formation of reliable and high-quality soldered joints designed for operation in harsh operating conditions .

A known method of removing the matrix-located ball leads from the solder of microcircuits in the BGA case by mechanical means according to the patent KR 200443484 (Y1) dated 02/12/2009, IPC H01L 21/02, "Brush for Reballing Pre-treatment Removing Lead and Other Material of Ball Grid Array Package ”(“ Brush for removing lead and other materials from BGA during the preliminary stage of reballing ”) - [1].

A disadvantage of the known mechanical removal of ball leads from solder [1] is the complexity and complexity of the process, which subsequently significantly reduces the reliability of the chip itself due to mechanical stresses that lead to damage to the BGA pads.

Known methods for replacing the matrix-positioned ball leads of microcircuits from solder to new ball leads from solder of a different composition by the thermomechanical method according to patents:

- JP 2001345401 (A) dated 12/14/2001, IPC H01L 21/60, H01L 23/12, “Ball alignment junction method” (“The alignment method when connecting balls”) - [2];

- KR 20030070364 (A) dated 08/30/2003, IPC H01L 21/6, “Method for fabricating repairing and reballing ball grid array package” (“Method for manufacturing, repairing and reballing BGA”) - [3];

- KR 20090078417 (A) from 07.20.2009, IPC H01L 21/60; H01L 23/48; H05K 3/34, “Reballing method of ball grid array” (“BGA reballing method”) - [4].

A disadvantage of the known thermomechanical methods for replacing ball leads from solder according to [2, 3 and 4] is the multi-stage process, which includes

- thermal heating to the melting point of the old ball leads;

- mechanical removal of molten solder balls;

- cooling microcircuit;

- installation (preliminary fixing) of new ball leads from solder of a different composition, for example, eutectic tin-lead solder, in place of the removed ones;

- heating to the melting temperature of new balls (formation, for example, tin-lead leads) with subsequent cooling of the chip.

Such a multi-stage process is complex and time-consuming, and in addition to mechanical impact on the microcircuit, it requires double heating, which significantly reduces the reliability of the microcircuit and makes it unsuitable for installation in devices designed to operate in harsh operating conditions.

Known devices for implementing the methods of [2, 3 and 4], for example:

- KR 100831167 (B1) dated May 26, 2008, IPC H01L 21/60, H01L 23/48, “Solder ball reballing device for BGA type sub-strate” (“Device for reballing solder balls from the substrate of BGA type components”) - [5];

- KR 100860944 (B1) dated 09/29/2008, IPC H01L 21/60, “Solder ball re-balling device for BGA type sub-strate” (“Device for reballing solder balls from the substrate of BGA type components”) - [6 ];

- KR 20100008409 (U) dated 08.25.2010, IPC H01L 21/60, Н05К 3/34, "Apparatus For Solder Ball Reballing" ("Apparatus for reballing solder balls") - [7].

The disadvantage of devices according to [5, 6 and 7] is their complexity and high cost. In addition, on these devices, only the installation of new ball leads from the solder is performed, and the removal of old balls is not provided.

There are also known methods of installing ball leads from solder according to patents and application:

- KR 20050011454 (A) dated January 29, 2005, IPC H01L 21/60, “Reballing method using smd print manner for reducing contact failure between flux and solder ball and arranging exactly solder balls” (“Reballing method using a screen printer for reduce bad contact between the flux and the solder ball and install directly the solder balls ") - [8];

- US 2005189402 (A1) dated September 1, 2005, IPC B23K 1/00, V23K 3/06, H05K 3/34, “Area array and leaded SMT component stenciling apparatus and area array reballing method” (“Stencil for components with matrix arrangement of conclusions and the reballing method ”) - [9];

- Application WO 2011006482 (A1) dated January 20, 2011, IPC Н05К 3/34, “Device for heating smd components for reballing” ([10].

However, materials [8, 9, and 10] do not describe methods for removing solder balls, which also provides for a multi-stage replacement process with mechanical stresses and double thermal heating, which leads to disadvantages of the methods [2, 3, and 4].

The prototype of the proposed technical solution (method) is the method according to patent US 2011132972 (A1) dated 06/09/2011, IPC BK 31/02, "Manual method for reballing using a solder preform" ("Manual method of reballing using a soldering device") - [eleven].

The method [11] of manually re-connecting the solder balls to the matrix-located contact pads of the microcircuit, which consists in the following:

first, new balls of tin-lead solder can be installed in the holes of the device, made in exact accordance with the matrix-located contact pads of the microcircuit, from which the old balls of lead-free solder have already been removed. The balls are fixed in the device using the adhesive layer located in the device.

Further, solder paste or flux-gel can be applied to the contact pads of the microcircuit, and the balls in the device are combined and connected to the contact pads of the microcircuit. Then, the melting of the solder paste and / or balls of solder is performed, and then cooling is carried out with the formation of ball terminals of the microcircuit. In the future, the device is removed, the ball terminals of the microcircuit are cleaned of glue and flux residues.

The disadvantages of the prototype method are the same as analogues [2, 3, 4, 8, 9 and 10].

The indicated drawbacks of the analogues and the prototype set the task of significantly reducing the technological effects on the microcircuit in the BGA (CSP) package - the microcircuit with matrix-spaced ball leads by eliminating the preliminary operation of removing balls from lead-free solder and creating a method for converting the same matrix-spaced ball leads of microcircuits from lead-free solder in tin-lead near-eutectic composition for one cycle of heat exposure (low-temperature, with a melting point of about-eut kticheskogo solder) on the chip, to maintain its full efficiency after conversion.

The object is achieved by the fact that (the essence of the invention) the proposed method for converting matrix-spaced ball leads of a microcircuit from lead-free solder to tin-lead near-eutectic composition consists in the fact that the microcircuit with a matrix-arranged ball leads from lead-free solder based on tin and silver (SAC 305, SAC 405 and others) are installed on a flat substrate of non-wettable solder material (ceramics, glass, non-foamed glass fiber laminate, etc.), on which they are preliminarily applied. es metal stencil, in strict accordance with a matrix arrangement of ball pin chips certain dose solder paste being composed of high lead content (the metal component is a solder paste in its composition has a lead content of 75 ... 95% by weight). This ensures the combination and contacting of the ball leads and doses of solder paste. Next, they produce heating to a peak temperature of no more than 230 ° C and subsequent cooling with holding a certain time interval at a temperature above 180 ° C until the formation of new larger ball pins during crystallization, consisting of near-eutectic tin-lead solder, similar in composition to a three-component eutectic Sn ₆₂ Pb ₃₆ Ag ₂ alloy. At a temperature of 183 ° C, the eutectic tin-lead solder Sn ₆₃ Pb ₃₇ in the deposited solder paste melts, which initiates the contact melting of the ball leads and the metal component of the deposited solder paste in the contact zone due to the diffusion of tin and lead into the molten solder, accelerated in process of further increase in temperature. As a result, for a short time the lead-free solder of the ball leads and the tin-lead solder with a high lead content included in the composition of the applied solder paste pass from solid to liquid, mix with each other due to the mutual diffusion of tin and lead in the melt with the formation of alloy with decreasing temperature to a solidus temperature (about 180 ° C) during the crystallization of new larger-sized ball leads having a uniform fine-grained structure and consisting of a near-eutectic tin-lead solder. The conversion process is completed by removing the microcircuit from the substrate, removing (for example, washing) the flux residues from the ball terminals of the microcircuit, drying and control.

For matrix-positioned ball leads, microcircuits from lead-free solder pre-determine the mass of the dose of solder paste (with a high content of lead), the composition of which is calculated on the basis of the mass-overall dimensions (diameter, height, volume, mass), chemical composition of lead-free solder of the ball leads and their pitch matrix arrangement on the chip. In this case, taking into account the necessary and possible increase in the volume of the converted ball lead, the masses of lead, tin and other components are calculated, which should be introduced into the lead-free lead solder (so that during the conversion a ball lead from the solder Sn ₆₂ Pb ₃₆ Ag _{2 is} obtained). The composition and percentage of lead, tin and silver in the metal component, its volume fraction in the solder paste, as well as the volume and overall dimensions of the aperture of the metal screen (for high-quality application of doses of solder paste) are determined, while the manufacture of solder paste with a high lead content of the calculated composition and a metal screen with apertures having design dimensions that take into account the requirements of a certain ratio of the area of the aperture to the area of the walls of the aperture, makes it possible to realize the claimed method.

The prototype of the solder paste is the solder paste according to patent RU 2450903 C2 dated 05.20.2012, IPC V23K 35/36, V23K 35/363, V23K 35/26, “Solder paste” (applicant of Avangard OJSC) - [12], which contains 80 ... 91 wt.% powder of low-temperature solder and 9 ... 20 wt.% flux-binder, while the flux-binder includes components in the following ratio, wt.%:

modified rosin based natural components 30 ... 60 a mixture of organic solvents in fatty acids and esters 35 ... 70 activators in the form of a mixture of organic acids 1 ... 8

A disadvantage of the known solder paste [12] is that it is in the present form without the powder of low-temperature solder with a high content of lead cannot be used to implement the proposed method.

The solder paste for converting the matrix-positioned ball leads of the microcircuit from lead-free solder to tin-lead near-eutectic composition according to the above method consists of solder powder and flux binder and contains low-temperature solder powder from 88 ... 92 wt.% And flux binder 8 ... 12 wt. %, wherein:

the low-temperature solder powder consists of a mixture of tin-lead solders powders and has the following ratio of components, wt.%:

tin-lead solder powder Sn ₆₃ Pb ₃₇ or Sn ₆₂ Pb ₃₆ Ag ₂ 1 ... 30 a mixture of powders of tin-lead solders with a high content of lead (for example, Sn ₅ Pb ₉₅ , Sn ₁₀ Pb ₉₀ or others, or one of them) 70 ... 99

and the flux bundle consists of the following ratio of components, wt.%:

modified rosin based on natural components 30 ... 60

a mixture of organic solvents in the form of fatty

acids and esters 35 ... 70 activators in the form of a mixture of organic acids 1 ... 8

Figure 1 presents a diagram of the application of doses of the inventive solder paste on a substrate.

Figure 2 - diagram of the installation (alignment) of the chip with a matrix located ball leads from lead-free solder dose of solder paste deposited on a substrate.

Figure 3 - the process of heating the substrate with the installed chip before the conversion of the ball leads.

Figure 4 - substrate with the installed microcircuit after the conversion of the conversion of the ball terminals of the microcircuit from lead-free solder to tin-lead near-eutectic composition.

Figure 5 - types of microcircuits: a) with ball leads from lead-free solder (smaller size); b) with ball leads of solder from tin-lead near-eutectic composition (larger size).

Figure 6 - temperature-time profile of heating in the process of converting the matrix-positioned spherical microchips from lead-free solder to tin-lead near-eutectic composition.

Figure 7 is a microsection view of a converted ball terminal of a CSP132 chip simulator consisting of tin-lead solder of a near-eutectic composition Sn ₆₄ Pb ₃₄ Ag ₂ (Sn64% Pb34% Ag2%) according to x-ray microanalysis (× 200).

In Fig.8 - the microstructure of the solder in Fig.7 at the lower boundary of the ball output (× 1000).

In Fig.9 - the microstructure of the solder in Fig.7 in the center of the ball terminal (× 1000).

According to the schemes shown in figures 1, 2, 3, 4 and 5, the sequence of implementation of the proposed method is clearly presented, where: 1 - a flat substrate of non-wettable solder material, for example, ceramics, glass, non-foamed fiberglass and other material; 2 - solder paste for converting the ball leads of the microcircuit from lead-free solder to tin-lead near-eutectic composition; 3 - squeegee for applying solder paste; 4 - a metal stencil, the apertures on which are located in exact accordance with the matrix arrangement of the ball terminals of the microcircuit; 5 - doses of solder paste deposited on a flat substrate 1; 6-microcircuit in the BGA (CSP) package - a microcircuit with matrix-spaced ball leads; 7 - contact pads of the chip; 8 - ball output from lead-free solder (soldered to the contact pad 8); 9 - ball output from tin-lead solder of near-eutectic composition (soldered to the contact pad 8); 10 - flux residues on the ball terminals of the chip.

In addition, in FIG. 7 and FIG. 8, zones are identified in which a chemical analysis of the ball lead from tin-lead solder of a near-eutectic composition was performed: Z ₁ — zone 1 of the ball lead 9; Z ₂ - zone 2 of the ball terminal 9; Z ₃ - zone 3 of the ball terminal 9; Z ₄ - zone 4 of the ball terminal 9 - near the contact pad 7.

The introduction of eutectic tin-lead solder powder into the solder paste is necessary to significantly reduce the temperature of the beginning of the conversion process, at which the eutectic tin-lead solder melts and the contact melting process is initiated, in which lead-free solder ball leads and tin-lead solder with a high content of lead pass from solid to liquid at a temperature substantially lower than their melting points.

The introduction into the solder paste of a mixture of powders of tin-lead solders with a high content of lead (or one powder of tin-lead solder with a high content of lead) is necessary to ensure that the converted ball lead has a limited possibility of increasing its volume with a certain ratio of tin, lead and silver close to composition to the eutectic ternary solder Sn ₆₂ Pb ₃₆ Ag ₂ .

The rosin based on natural components used in the proposed technical solution is a modified highly hydrogenated rosin, the melting point of which is stable and lies in a narrow range of 80 ... 82 ° C, and the acid number is 175 ... 180. Its application can significantly stabilize the properties of the solder paste and improve the quality of reflow with the conversion of the ball terminals of the chip from lead-free solder to tin-lead near-eutectic composition.

Activators in the form of a mixture of organic acids based on carboxylic acid, which, on the one hand, acts as an activator, and on the other, as a thixotropic additive, providing improved rheology of solder paste. The good rheological properties of the solder paste are also provided by the basis of the flux binder: modified highly hydrogenated rosin.

So, when conducting an experimental verification of the method for converting matrix-positioned spherical-shaped ball leads of a microchip from lead-free solder based on tin and silver SAC 305 with a diameter of 0.3 mm (ball leads of a CSP132 microchip simulator) into tin-lead near-eutectic composition, in one of the many options solder paste was chosen containing a powder of low-temperature solder from 90 wt.% and a flux binder of 10 wt.%, while:

the low-temperature solder powder consisted of a mixture of tin-lead solders powders and had the following ratio of components, wt.%:

tin-lead solder powder Sn ₆₃ Pb ₃₇ 5 high tin-lead solder powder lead content Sn ₅ Pb ₉₅ 95

and the flux bundle consisted of the following ratio of components, wt.%:

modified rosin based on natural components 60

a mixture of organic solvents in the form of fatty acids and complex

esters: butyric acid one decanol 7 dibutyl phthalate (phthalic acid dibutyl ether) thirty activators in the form of a mixture of organic acids: salicylic acid one adipic acid one

In the composition of the solder paste, the metal component had the composition: Sn _7.9 Pb _92.1 . The conversion of the ball terminals of the microcircuit from lead-free solder to tin-lead near-eutectic composition (for the formation of Sn ₆₄ Pb ₃₄ Ag ₂ solder) was carried out in the temperature-time profile of the heating shown in Fig.6.

Next, an x-ray microanalysis was performed with a 200-fold increase. A microsection of a transformed ball outlet consisting of tin-lead solder with a near eutectic composition (Sn64% Pb34% Ag2%) is shown in Fig. 7, and with a magnification of 1000 times - in Fig. 8 and Fig. 9.

Table 1 shows the distribution of the chemical elements of the obtained ball output (PMA data in wt.%) For zones Z ₁ , Z ₂ , Z ₃ and Z ₄ .

Table 1 Zone Ni Cu Ag Sn Pb W ₁ - - 2.0 63.6 34.4 H ₂ - 0.2 * 2.1 64.4 33.3 ₃ 0.2 * 0.3 2.0 63.3 34.2 W ₄ - 0.2 * - 16.8 83.0 * - data at the level of statistical error.

A fine-grained structure of tin-lead solder is observed, which is a mechanical mixture of tin-lead eutectic, a solid solution of lead in tin, and minor inclusions of a solid solution of tin in lead and intermetallic compounds of copper, nickel and silver with tin.

Thus, the method of converting the matrix-mounted spherical microcircuit ball leads from lead-free solder to tin-lead near-eutectic composition in one heat exposure cycle, excluding mechanical stresses on the microcircuit and its contact pads and preserving the microcircuit's full performance after conversion, allows microcircuits during subsequent surface mounting ( or microcircuits) with converted ball leads on a printed circuit board to form reliable and high-quality soldered soybeans Densities designed to operate in harsh environments.

The high fatigue life of soldered joints, provided by tin-lead solder and the increased height of the matrix-located ball (coplanar) terminals of the microcircuit, is necessary in devices for medicine, transport equipment (automobile, railway, aviation and marine), as well as in military and space technology.

We believe that the proposed "Method for converting a matrix-mounted spherical pin of a microcircuit from a lead-free solder to a tin-lead near-eutectic composition and solder paste for its implementation" have all the criteria of the invention, since:

- The inventive solutions in combination with restrictive and distinctive features of the claims is new to well-known methods and compositions for their implementation and, therefore, meets the criterion of "novelty";

- The totality of the features of the claims of the claimed invention is unknown at this level of technological development and does not follow well-known methods (methods and rules) of converting ball terminals of a microcircuit from lead-free solder to tin-lead near-eutectic composition, which proves compliance with the criterion of "inventive step";

- The implementation of the claimed invention does not represent any structural, technical and technological difficulties, which implies compliance with the criterion of "industrial applicability".

Literature

1. KR patent 200443484 (Y1) dated 12.02.2009, IPC H01L 21/02, "Brush for Reballing Pre-treatment Removing Lead and Other Material of Ball Grid Array Package".

2. Patent JP 2001345401 (A) dated 12/14/2001, IPC H01L 21/60, H01L23 / 12, "Ball alignment junction method".

3. Patent KR 2003 00703 64 (A) dated 08/30/2003, IPC H01L 21/6, "Method for fabricating repairing and reballing ball grid array package".

4. Patent KR 20090078417 (A) of July 20, 2009, IPC H01L 21/60; H01L23 / 48; H05K 3/34, “Reballing method of ball grid array”.

5. Patent KR 100831167 (B1) dated 05/26/2008, IPC H01L 21/60, H01L 23/48, “Solder ball reballing device for BGA type sub-strate”.

6. Patent KR 100860944 (B1) dated September 29, 2008, IPC H01L 21/60, “Solder ball reballing device for BGA type sub-strate”.

7. Patent KR 20100008409 (U) dated 08.25.2010, IPC H01L 21/60, H05K 3/34, "Apparatus For Solder Ball Reballing".

8. Patent KR20050011454 (A) of January 29, 2005, IPC H01L 21/60, “Reballing method using smd print manner for reducing contact failure between flux and solder ball and arranging exactly solder balls”.

9. Patent US 2005189402 (A1) dated September 1, 2005, IPC B23K 1/00, B23K 3/06, H05K 3/34, “Area array and leaded SMT component stenciling apparatus and area array reballing method”.

10. Application WO 2011006482 (A1) dated 01.20.2011, IPC Н05К 3/34, “Device for heating smd components for reballing”.

11. Patent US 2011132972 (A1) dated 06/09/2011, IPC B23K 3 1/02, "Manual method for reballing using a solder preform" - a prototype of the method.

12. Patent RU 2450903 C2 dated 05/20/2012, IPC V23K 35/36, V23K 35/363, V23K 35/26, “Solder paste” is a prototype of solder paste.

Claims (3)

1. A method of converting a matrix-positioned ball leads of a chip from a lead-free solder to a tin-lead near-eutectic composition, characterized in that the chip with a matrix-arranged ball leads of a lead-free solder based on tin and silver is mounted on a flat substrate of non-wettable solder material, which is previously applied through a metal stencil, in exact accordance with the matrix arrangement of the ball terminals of the microcircuit, certain doses of solder paste having the composition contains a high lead content, while combining and contacting the ball leads and doses of solder paste, then produce heating to a peak temperature of not more than 230 ° C and subsequent cooling with a certain time interval being kept at a temperature above 180 ° C until it forms during crystallization new larger-sized ball leads, consisting of a near eutectic tin-lead solder, similar in composition to the eutectic three-component alloy Sn ₆₂ Pb ₃₆ Ag ₂ . 2. The method according to claim 1, characterized in that for the matrix-positioned ball leads of the microcircuit from the lead-free solder, the dose mass of the solder paste with a high lead content is preliminarily determined, the composition of which is calculated on the basis of the overall dimensions and chemical composition of the lead-free solder of the ball leads and the steps of their matrix arrangement on the microcircuit, while taking into account the necessary and possible increase in the volume of the converted ball output, the masses of lead, tin, and other components are calculated, which are up to zhny be introduced in the output lead-free solder for forming solder Pb Sn ₆₂ Ag _{36 2} and determine the composition and the percentage of lead, tin and silver in a metallic constituent, its volume fraction in the solder paste, the volume and dimensions of the aperture of the metal stencil. 3. Solder paste for converting the matrix-positioned ball leads of the microcircuit from lead-free solder to tin-lead near-eutectic composition according to claim 1, consisting of solder powder and flux bundle, characterized in that it contains low-temperature solder powder from 88 ... 92 wt.% and flux bundle 8 ... 12 wt.%, while:
the low-temperature solder powder consists of a mixture of tin-lead solders powders and has the following ratio of components, wt.%:
tin-lead solder powder Sn ₆₃ Pb ₃₇ or Sn ₆₂ Pb ₃₆ Ag ₂ 1 ... 30 a mixture of tin-lead solders with a high content of lead or one of them 70 ... 99
and the flux bundle consists of the following ratio of components, wt.%:
modified rosin based on natural components 30 ... 60 a mixture of organic solvents, in the form of fatty acids and esters 35 ... 70 activators in the form of a mixture of organic acids 1 ... 8

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