How does the cleaning process after flip-flop welding affect the product?

Soldering package is to arrange the active surface of the entire chip according to the pin array, and prefabricate solder bumps, and then interconnect them through the flip soldering process. Compared with traditional wire bonding technology, it has a higher assembly density and signal transmission rate, and is one of the key technologies to realize the miniaturization, lightweight, and multi-functionality of electronic products. For small-size, micro-pitch flip soldering chips, it is relatively difficult to clean after welding, so cleaning technology is also an important factor affecting the flip soldering process. The cleaning effect of different cleaning methods and parameters was compared, and the effect of flux residue on the underfilling effect was studied, so as to optimize the flip soldering cleaning technology. The test results show that the vacuum vapor cleaning process of pre-cleaning (≥3 min), formal cleaning (≥3 min), steam rinsing (≥3 min) and vacuum drying (≥4 min) can fully clean the flux in the fine spacing between the flip solder chip and the substrate without cleaning liquid residue, so as to ensure the rapid flow and complete curing of the underfill, and the void rate of the filler can reach less than 5%.

I.. Introduction As with the common surface mount soldering process, flip soldering also requires the use of flux, so the flux needs to be cleaned after soldering. If not cleaned, the residual flux will not only cause surface contamination, which can easily lead to oxidation or corrosion of solder joints, but also hinder the flow and curing of flip chip underfill, affecting the performance and reliability of the product.

Wire bonding technology and flip soldering are the two main ways to interconnect integrated circuits. Wire bonding technology requires the distribution of pads around the chip, and the interconnection density of the wire to the substrate is limited by factors such as the size of the chip, the routing capacity of the substrate, and the bonding process, and the number of I/Os that can be achieved by the bonding process is also limited. The flip soldering technology can be used to route and array I/O arrangement on the entire active surface of the chip, and use the flip soldering technology to interconnect with the substrate or directly with the tube shell through the micro-bumps on the surface of the chip, which can not only be used as mechanical support, but also realize electrical interconnection, which significantly improves the number of I/Os per unit area of the chip and the assembly density; At the same time, the horizontal signal transmission direction is partially converted to the vertical direction of the chip, which greatly shortens the signal transmission distance and lays the foundation for the realization of high-speed signal transmission. Therefore, flip welding can greatly promote the miniaturization, lightweight, and multi-function of electronic products, and meet the needs of high integration of integrated circuits and high-performance, high-density, and high-reliability microelectronic devices in important fields. For the traditional PCB surface mount soldering process, the flux is usually cleaned directly in the cleaning solution, scrubbed or brushed with a brush after welding. The surface stickers on the printed board are generally plastic and ceramic sealing devices, and the spacing between the devices is large, and the brush brushing can achieve a better cleaning effect, and the devices are not susceptible to mechanical damage. However, for flip soldering bare chips, on the one hand, bare chips are easy to damage and are not suitable for brushing; On the other hand, flux is more likely to remain in the gap between the chip and the substrate, and the gap width is smaller than the diameter of the chip bump, and brushing can only clean the surface of the device or substrate, but cannot clean such a fine gap. Therefore, traditional dip or brushing is not suitable for the flip welding process, although it is simple to operate and has low equipment requirements. The vapor phase cleaning and vacuum vapor phase cleaning are different from the traditional mechanical cleaning, which can be heated by heating the vapor phase to boil, and the solvent vapor condenses and takes away the pollutants after contacting the cleaned device, and circulates in the equipment, using the always pure steam to achieve the purpose of cleaning the device, which has the advantages of less vibration, less damage and high efficiency, and is more suitable for high-reliability, high-density, and small-gap electronic product cleaning.

As with the common surface mount soldering process, flip soldering also requires the use of flux, so the flux needs to be cleaned after soldering. If not cleaned cleanly, residual flux will not only cause surface contamination and easy oxidation or corrosion of solder joints, but also hinder the flow and curing of flip chip underfill, affecting the performance and reliability of the product.

If not cleaned, residual flux can not only cause surface contamination, but also cause oxidation or corrosion of solder joints, and may also affect the performance of the entire circuit system. Just like a delicate machine, even a small part can be contaminated or damaged, which can affect the overall operation. For example, in the production of circuit boards for some high-end electronic products, the residue of flux may change the resistance value of the circuit, thus affecting the accuracy of signal transmission. This requires that appropriate cleaning methods and equipment must be used when cleaning the flux, such as using special cleaning solvents, and cooperating with ultrasonic cleaning technology to ensure that the flux is completely removed and ensure the stability and reliability of the solder joint and the entire circuit system.

2. Test materials and methods

The commonly used post-weld cleaning methods in the microelectronics industry include cleaning liquid immersion or brushing, vapor phase cleaning, vacuum vapor phase cleaning and other processes. According to the solubility of contaminants such as fluxes of different components, the cleaning liquid used in cleaning can be divided into two categories: water-based and CFC-based, alcohol-based, hydrocarbon-based, hydrocarbon-based, hydrocarbon-based and other organic solvents. Firstly, the flip soldering equipment is used to pick up the chip, dip the paste flux, and then complete the flip soldering alignment of the chip and the silicon substrate through the alignment marking, and then use the vacuum brazing furnace to reflow the flip soldering components that have completed the alignment, and form a connection with the chip bump through the melting and solidification process of the SnAg boss.

For the traditional PCB surface mount soldering process, the flux is usually cleaned directly in the cleaning solution, scrubbed or brushed with a brush after welding. The surface of the printed board is generally plastic and ceramic sealing devices, and the spacing of the devices is large, and the brush brushing can achieve better cleaning results, and the devices are not easy to be damaged by machinery. However, for flip soldering bare chips, on the one hand, bare chips are easily damaged and are not suitable for brushing; On the other hand, flux is more likely to remain in the gap between the chip and the substrate, and the gap width is smaller than the diameter of the chip bump, and brushing can only clean the surface of the device or substrate, and cannot clean such a fine gap. Therefore, although the traditional dip or brush cleaning is simple to operate and has low equipment requirements, it is not suitable for the flip welding process. The vapor phase cleaning and vacuum vapor phase cleaning are different from the traditional mechanical cleaning, they can be heated by heating the vapor phase to clean the solvent to boiling, the solvent vapor condenses and takes away the pollutants after contacting the cleaned device, circulates in the equipment, and uses the always pure steam to achieve the purpose of cleaning the device, which has the advantages of small vibration, small damage and high efficiency, and is more suitable for high reliability, high density and small gap cleaning of electronic products. The vapor phase cleaning process is divided into two stages:

In this paper, two methods of vapor phase cleaning and vacuum vapor phase cleaning are used to clean the flux of flip soldered chip/silicon substrate components, and the cleaning effect under different process parameters is compared, the influence of flux residue on the flow and curing process of underfill adhesive is studied, and the effective cleaning technology is explored for multi-bump and fine-gap flip welding structures. (2) Rinse with cleaning solution in the rinse area. The boiling zone time of 2 groups of 5 min and 10 min and the time of rinsing zone of 1 min, 5 min and 10 min were compared.

2. Test materials and methods

(1) Pre-cleaning cycle;

In the test, silicon chips and silicon substrates arranged in a bump array of solder balls were selected for interconnection to form flip soldered components. The chip size is 9.7 mm×10.1 mm and contains a total of 2975 bumps with a bump diameter/pitch of φ90 μm/180 μm. The silicon substrate size is 15 mm×23 mm, and the SnAg boss, chip and silicon substrate are fabricated on the surface of the pad with a diameter of 90 μm by electroplating process

(2) Final steam rinsing;

Firstly, the flip soldering equipment is used to pick up the chip, dip the paste flux, and the flip soldering alignment of the chip and the silicon substrate is completed by the alignment marker, and then the flip soldering assembly that has completed the alignment is reflow soldered by the vacuum brazing furnace, and the connection with the chip bump is formed through the melting and solidification process of the SnAg boss. After cleaning, take 1 sample, remove the chip, and observe the flux residue on the silicon substrate and the surface of the chip. Another sample was taken and underfilled with filler, the flow of filler was observed, and the curing effect and filling effect of the filler were compared under the same conditions to study the effect of residual flux or cleaning solution on the underfill process. The filler used has a viscosity of 7.5 Pa·s and excellent flowability.

The soldered chip/silicon substrate components are grouped, and the components are cleaned by two processes, vapor phase cleaning and vacuum vapor phase cleaning, and different cleaning parameters are used.

3. The influence of different cleaning methods and parameters

The vapor phase cleaning process is divided into two stages: the difference between the two is that the vacuum vapor phase cleaning equipment can provide a vacuum environment, and multiple cleaning steps can be parameterized separately, which can realize the fine control of the cleaning process. Firstly, in the pre-cleaning stage, the steam formed by the solvent heating is used to preheat the device, and at the same time, the steam has a softening and degreasing effect on the surface of the device, which greatly reduces the follow-up effect

(1) Steam cleaning in boiling area; Finally, clean steam is introduced to clean the device for vapor phase cleaning and drying in a vacuum. All cleaning processes are completed under vacuum, which not only ensures the safety of the equipment, but also reduces the surface tension of the solvent and realizes the cleaning of ultra-fine gap flip welded structures. THE SURFACE OF EACH SAMPLE CLEANED BY DIFFERENT METHODS OR PARAMETERS WAS OBSERVED UNDER AN OLYMPUS SZ61 MICROSCOPE, AND THE GAP BETWEEN THE CHIP AND THE SUBSTRATE WAS OBSERVED FROM THE SIDE OF THE SAMPLE, AND FINALLY THE FLIP SOLDERED SAMPLE WAS TRUNCATED ALONG THE CENTER LINE OF THE CHIP TO CONFIRM THE CLEANING EFFECT OF THE BOTTOM OF THE CENTER PART OF THE CHIP

(2) Rinse with cleaning solution in the rinse area. The boiling zone time of 5 min and 10 min and the rinsing zone time of 1 min, 5 min and 10 min were compared. The surface of the flip soldering chip and the silicon substrate can be basically cleaned by vacuum vapor phase cleaning, and the edge of the gap between the flip soldering chip and the substrate can be observed from the side of the flip soldering chip, and it can be found that except for the sample with the shortest cleaning time, the rest of the samples have no residual flux in the gap, and then the chip is cut off from the middle, and the morphology in the gap in the center of the chip can be observed through vacuum vapor phase cleaning, even the center of the chip where the cleaning liquid or steam is most difficult to enter can be cleaned. No flux or cleaning fluid carryover issues were observed.

4. The process of vacuum vapor phase cleaning is as follows: the vacuum vapor phase cleaning of the 50 μm gap flip welding structure is carried out by using the process parameters of pre-cleaning (≥3 min→), formal cleaning (≥3 min → steam rinsing (≥3 min) → vacuum drying (≥4 min), which can achieve good cleaning effect, and there is no flux or cleaning solution residue in the gap between the chip and the substrate.

(1) Pre-cleaning cycle; The stress due to the difference in the coefficient of thermal expansion between the chip, the substrate, and the bump is one of the key factors that lead to the failure of flip welded structures, and in order to eliminate this stress, underfill is widely used in the flip soldering process to improve its reliability. The underfill process is to first inject the underfill along the edge of the chip, with the help of the capillary action of the liquid, the underfill is sucked into the gap between the chip and the substrate and flows to the center and the non-sizing edge until it flows out from the non-sizing edge, and then the filler is cured at high temperature after filling.

(2) Formal cleaning cycle; In addition, sufficient drying time is also one of the key factors affecting the final cleaning effect. Observing the morphology of the underfill of the chips after cleaning in the 2-1 group of samples before and after curing, it can be found that the non-sizing edge of the chip of the 2-1 sample after only 2 minutes of vacuum drying was partially underfilled, and it was translucent in color different from the black filler before curing (see Fig. 5(a)), and the translucent liquid volatilized to form a cavity after curing [Fig. 5(b)]. Since the flux does not decompose or vaporize at the curing temperature, and the vaporization temperature of the cleaning fluid is lower than the curing temperature of the filler, it means that there is a residual cleaning solution that will block the flow of the filler, resulting in underfilling due to vaporization after curing. However, the 2-5 groups of samples did not have this problem, the four sides of the chip were well filled, the gel shape was uniform, and the 2-5 groups of samples were detected by ultrasonic scanning

(3) Final steam rinsing; Finally, in order to study the effect of flux or cleaning solution residue on the curing effect of underfill, tungsten was used to test the surface of the filler after curing, and it was found that the filler near the flux residue and the cavity formed after the volatilization of the cleaning solution was still soft and gel-like, as shown in Fig. 7(a). Tungsten needles can form deep scratches on their surface, see Figure 7(b); The rest of the filler has formed a solid with high hardness, and the tungsten needle scratches on its surface and will not leave any traces, see Figure 8. Tested by a triple LX-D Shore hardness tester, the cured filler has a hardness of 98 (Shore D). The sample curing was performed under the same conditions required for the underfill to be fully cured, indicating that the composition of the flux or cleaning fluid would cause the underfill to cure poorly, which could not eliminate the structural stress of the flip weld and improve the long-term reliability of the flip weld.

(4) Vacuum drying. Both pre-cleaning and formal cleaning are three-step cycle processes of steam degreasing, spraying, and immersion spraying, and the time of pre-cleaning cycle and formal cleaning cycle can be controlled by adjusting the duration and number of cycles of each step. Due to the longer the post-soldering flux placement time, the more it affects the cleaning effect, in order to eliminate other factors, the post-soldering placement time of each flip soldering component sample is controlled within 1h. Compared with conventional vapor phase cleaning, vacuum vapor phase cleaning technology is more suitable for the cleaning of flip welding structures with fine gaps, because the vacuum environment can reduce the surface tension of the cleaning solution and steam, so that the cleaning liquid can enter the fine gap at the bottom of the flip welding chip for effective cleaning; The vacuum vapor cleaning process of pre-cleaning (≥3 min→), formal cleaning (≥3 min), → steam rinsing (≥3 min→ vacuum drying (≥4 min) can fully wash the flux in the gap between the flip solder chip and the substrate about 50 μm, and there is no flux residue in the bottom center of the flip soldering chip within 10 mm × 10 mm after cleaning. The underfill process will be affected by the residual flux or cleaning fluid in the gap, and the filler may cause problems such as blocked flow, insufficient filling, poor curing, etc., while when the flip soldering component has been cleaned in a vacuum vapor phase for enough time and fully dried, the void rate of the filler is ≤5%, and a good underfill effect can be obtained.

If not cleaned, residual flux can not only cause surface contamination, but also cause oxidation or corrosion of solder joints, and may also affect the performance of the entire circuit system. Just like a delicate machine, even a small part can be contaminated or damaged, which can affect the overall operation. For example, in the production of circuit boards for some high-end electronic products, the residue of flux may change the resistance value of the circuit, thus affecting the accuracy of signal transmission. This requires that appropriate cleaning methods and equipment must be used when cleaning the flux, such as using special cleaning solvents, and cooperating with ultrasonic cleaning technology to ensure that the flux is completely removed and ensure the stability and reliability of the solder joint and the entire circuit system.