1、Impact of Voiding Rate on Product Reliability
With the increasing functions of electronic products, the integration degree of printed circuit boards is getting higher and higher, and the unit power of devices is getting larger and larger. Especially in the fields of communication, automobile, rail transportation, photovoltaic, military, aerospace, etc., the use of devices such as high-power transistors, RF power supplies, LEDs, IGBTs, MOSFETs, etc., is increasing, and the packages of these components are usually in the form of BGA, QFN, LGA, CSP, TO packages, etc. Their common feature is that the power consumption of devices is high, which requires high heat dissipation performance, These components are usually packaged in BGA, QFN, LGA, CSP, TO packages, etc., and their common feature is that they consume large amounts of power and require high thermal performance, and the void rate of the thermal pads directly affects the reliability of the product.
After reflow soldering, some voids are usually left in the solder joints of SMD devices, and the greater the size of the joint, the greater the size of the void; this is due to the fact that when the molten solder cools down and solidifies, the air created in the solder does not escape and “freezes” down to form the void. The factors affecting the creation of voids are multifaceted and relate to the choice of solder paste, the device package form, the pad design, the PCB pad surface treatment method, the grid pattern, the reflow profile setting, and so on.
As a result of voids, the mechanical strength of the solder joint decreases and the thermal resistance increases, reducing the current path and affecting the thermal and electrical conductivity of the solder joint, thereby reducing the electrical reliability of the device. Studies have shown that temperature increases are responsible for approximately 60% of electronic product failures, and that the failure rate of a device increases exponentially with temperature, doubling for every 10°C increase in temperature.
In specifications such as IPC-A-610, IPC7095, and IPC7093, solder joint voiding is described in detail for BGA and BTC type packages, with a 30% voiding rate standard for BGA type devices with collapsible solder balls and no standard for all other cases, which are to be negotiated between the manufacturer and the customer; and for the ground pads of high power devices, some users of high reliability products have expressed concern about voiding. For high-power device ground pads, some users of high-reliability products often require a higher void ratio than the industry standard, further reducing it to 10% or even lower.
Reducing voiding in the solder joints of such SMT devices is therefore one of the key issues in improving product quality and reliability. Various solutions have been used so far in the industry, such as the use of low voiding paste, optimisation of PCB pad design, use of dot-matrix grid openings, soldering in a nitrogen environment, the use of preforms, and so on. However, the final result is not ideal, and it is very difficult to stabilise the voiding rate at less than 10 per cent for large-area grounded pads.
Vacuum soldering is a very effective means of solving the void rate problem, as it can stably achieve a void rate of less than 5%; the vacuum phase soldering technology, due to its technical principles and equipment structure, is not very suitable for mass production; therefore, we will discuss the vacuum reflow soldering technology that has emerged in recent years.
2、Vacuum Reflow Soldering Technology
Vacuum reflow soldering is a reflow soldering technique that introduces a vacuum environment into the reflow soldering process. In contrast to conventional reflow soldering, vacuum reflow soldering creates a vacuum environment at the back end of the product’s entry into the reflow zone, where the force of the atmosphere can be reduced to less than 5 mbar (500 pa) for a certain period of time in order to realise the combination of vacuum and reflow soldering, whereby the joints are still in a molten state and the environment outside the joints is close to a vacuum, thus creating an environment that is not only a vacuum, but also an environment that is not suitable for mass production. The environment outside the joint is close to a vacuum, and due to the difference in pressure strength between the inside and outside of the joint, the bubbles inside the joint can easily escape from it, resulting in a significant reduction in the voiding rate of the solder joint, see Fig. 1. A low voiding rate is especially important for power devices with large-surface area pads, as these large-surface area pads are required for the conduction of current and heat for high power devices, so reducing the number of voids in the solder joints can fundamentally improve the conductivity and thermal conductivity of the device. thermal conductivity of the device.
Figure 1
Compared with traditional reflow soldering, the technical parameters of vacuum reflow soldering technology include four additional vacuum parameters on top of the parameters of temperature and chain speed, including vacuum level, vacuum extraction time, vacuum holding time and atmospheric pressure charging time (see Fig. 2), in which the vacuum can also be extracted in steps to gradually reduce the atmospheric pressure, so as to prevent abnormalities in the solder joints in the molten state due to the impact of vacuum on the devices, and at the same time to prevent the solder in the molten state from being damaged or damaged. Keeping the pressure difference between the internal bubble and the vacuum chamber from changing too quickly and too much when the solder is in the molten state can lead to blowing up of the solder, which in turn can lead to beads of solder around the device.
Figure 2
3、Structure analysis of vacuum reflow soldering equipment
Vacuum reflow ovens are based on traditional reflow ovens with an additional vacuum chamber located at the end of the high temperature reflow zone. Currently, the mainstream domestic vacuum reflow oven brands are SMT and REHM, with differences in equipment structure between the two, with SMT using a three-section, spliceable split structure and REHM using a one-piece structure, which is analysed below using the SMT brand as an example.
Figure 3
As seen in Figure 3, the vacuum reflow oven consists of a three-section structure. The first section is the preheating reflow module, which is generally divided into 6-8 temperature zones; the second section is the vacuum zone, which is divided into two zones; and the third section is the cooling zone, which is divided into 2-5 zones that can be configured according to the soldering process of different products. The size of the vacuum chamber can also be selected according to the size of the product.
The vacuum chamber of the vacuum reflow oven is structured as shown in Figure 4 below. The lower part of the chamber is connected to the base of the machine and to a chain rail system, while the upper cover can be lifted vertically up and down to open and close the chamber, and the holes in the side wall of the chamber are connected to an external vacuum pump for evacuation and return pressure; the heating of the chamber relies on the two sets of hot-air heaters at the upper part of the chamber and in the neighbouring areas.
Figure 4
The length of the vacuum zone is available in two sizes, 320 mm and 450 mm, and the width of the track can be adjusted automatically in the programmable range of 65-510 mm. Since PCBs need to be held in the vacuum zone to evacuate, maintain vacuum, and return to atmospheric pressure, dedicated sensors are installed at the front and rear of the vacuum zone chain track to prevent transmission problems from occurring. At the outlets of the vacuum reflow oven, SMEMA signals and a blocking mechanism are used to control the board feed intervals and prevent PCBs from crashing while in transit.
4、 Vacuum Reflow Oven Temperature Profile Features
01、Measurement Method of Oven Temperature Profile
During the actual soldering process in a vacuum reflow oven, the PCB boards need to stay in the vacuum zone for about 10-30 seconds, so the temperature measurement process in a vacuum reflow oven differs from that of a conventional reflow oven. A dedicated temperature measurement mode is built into the software. When this mode is activated and the board reaches the vacuum zone, the chain stops running, the vacuum chamber lid does not drop (to avoid compressing the pyrometer or the temperature measurement wire), the vacuum pump does not start, and the chain resumes running when the board’s dwell time reaches the cumulative time set by the vacuum parameter to simulate a test of the reflow profile.
For more accurate furnace temperature testing, a special fixture can be used, in which case the vacuum pump is activated to perform the actual test without the use of the temperature measurement mode, closing the vacuum chamber; the overall length of the pyrometer and the plate must be considered in order to match the length of the vacuum chamber.
02、Extended Reflow Time
PCB boards need to stay in the vacuum zone for vacuum soldering, and the cycle time is generally about 30 seconds before they can be transferred to the cooling section; therefore, the overall reflow time will be longer than that of ordinary reflow soldering, and the TAL time will be about 100 seconds, as shown in Fig. 5, which shows the temperature profile of a typical vacuum reflow oven. Components that are sensitive to reflow time pose a risk and should be avoided in the process design.
Figure 5
03、Three-stage Chain Conveyor Rail
The conveyor chain of the vacuum reflow oven is divided into three sections, namely the reflow section, the vacuum section, and the cooling section. In general, the chain speed of the three sections of the track is set to be the same by default; after the vacuum soldering function is turned on, the chain speed of the cooling section can be set individually, so as to allow for different speeds of the PCB boards in the front and rear sections of the conveyor chain, which will change the parameters of the reflow profile of the oven, and also affect the slope of the cooling slope, which can also reduce the product temperature out of the oven. The temperature of the product can also be reduced.
5、Removal of bubbles is effective
Vacuum reflow can theoretically remove all voids in the solder, but in practice, the vacuum parameters must be adjusted according to the PCB and device conditions.
The void ratio of normal reflow pads is around 25%, but with vacuum soldering, the void ratio of solder joints is significantly reduced; the void ratio can reach less than 5% at different vacuum levels; the lower the vacuum level, the lower the void ratio; and the longer the vacuum holding time, the lower the void ratio. The lower the vacuum degree, the lower the void rate; the longer the vacuum holding time, the lower the void rate. For details, please refer to the following table for comparison.
Vacuum reflow soldering has a significant advantage in removing solder joint voids, which greatly helps to improve the reliability of solder joints. On the other hand, however, component manufacturers generally do not perform targeted reliability verification of the vacuum reflow soldering process in their,and in actual production, there are certain process risks that need to be optimised and avoided in process design.
01、Risk of Device Packaging Failure
Vacuum reflow soldering is tolerated by most components, however, there are still a very small number of devices that are at risk of failure.
In non-airtight components with internal cavities, the air in the cavity is subjected to thermal expansion at high temperatures, and after superposition with the vacuum environment, the pressure difference between the inside and outside of the device is greater than that under normal reflow soldering conditions; at the same time, when the ambient temperature is greater than the Tg temperature of the material, the CTE of the material will increase significantly, and the mechanical strength indicators have decreased dramatically; in the thermal strength of the material and the internal and external air pressure, which can lead to package cracking. The thermal strength of the material itself and the strength of the internal and external air pressure may lead to cracking of the package.
Figure 6 shows the surface heat distortion measurements of a QFN package in a simulated reflow soldering environment (at atmospheric pressure). It can be seen that two out of the five samples had distortion of more than 140µm, and that in a vacuum reflow environment the distortion would be further increased and cracking would eventually occur at the substrate-to-cover bonding point.
Figure 6
02、Excessive Reflow Time
The reflow time for vacuum reflow soldering is longer than that for normal reflow soldering, generally 80 seconds or more, and for some components more than 100 seconds. For some devices with shorter TAL specifications, the specifications are exceeded and there is a risk of damage to the device. These devices should be accurately measured during furnace temperature control and measures should be taken to avoid this.
03、Solder Joint Risks
Vacuum reflow soldering affects the solder joints of BTC-type devices in that the stand-off height of the solder joints is significantly reduced, causing the solder to spread in all directions and creating a risk of bridging the solder joints; therefore, the grid apertures for some of the pads need to be narrowed, if necessary.
When soldering BGA devices with a pitch of 0.4 mm or less, the vacuum process is likely to cause bridging of the solder joints, so it is not recommended to use the vacuum process when the pitch of the solder balls is too small or too dense. The risk of BGA bridging can also be reduced by appropriately narrowing the grid aperture, but the ratio of grid area to surface area should also be taken into consideration.
In the case of large-area ground pads, the final solder coverage may be reduced due to a significant reduction or even elimination of voids; in this case, it is necessary to appropriately enlarge the aperture area of the grid board for the ground pads.
04、Equipment Risks
The equipment risk of vacuum reflow soldering is mainly due to the three-stage conveyor chain system and the vacuum chamber. Since there is a gap between the vacuum section chain and the front and rear sections of the chain (Figure 7), with a distance of about 20-30 mm and a rotational radius of the chain of about 15 mm, when the PCB passes through the gap, there is a 50-60 mm gap between the chain and the PCB contact edge, which increases the chance of board jamming for boards smaller than 100 mm in size, and may also result in vibration of the PCB, resulting in shifting, dropping of reverse side components, or even the possibility of board jamming. PCB vibration may also occur, resulting in defects such as device displacement, falling components on the opposite side, or even short-circuiting of BGA solder balls. It is recommended that the risk be greatly reduced by using a jigger oven.
Figure 7
Secondly, as the vacuum zone has many moving parts and operates at high temperatures (above 250 degrees) for long periods of time, the requirements for maintenance and servicing of the equipment in the vacuum zone should be strictly adhered to, in particular the chain system, sensors, seals, etc., should all be in good condition, or else precise control of the vacuum parameters may be compromised or problems may occur such as jamming or transmission failures.
05、Operational Risks
In the course of a vacuum reflow oven’s growth, the circuit boards remain in the vacuum zone for a period of time while the chain in the front preheating zone continues to travel, so it is important to ensure that the circuit boards are fed into the oven at a strict distance apart; although the equipment hardware itself controls the signalling of the feeder rail via the SMEMA connector, in practice, operators sometimes use manual pushing of the boards to feed them into the oven, so that if the distance between boards falls below the minimum interval set by the equipment, the circuit boards will be fed into the oven. If the distance between plates is less than the minimum interval set by the equipment, a “plate crash” or plate jam may occur in the vacuum zone, causing unnecessary damage.
7、Summary
Vacuum reflow soldering has a significant effect on the removal of voids from solder joints, and under vacuum conditions and with reasonable settings of the process parameters, it is possible to achieve stable batch production with a void rate of less than 3%.
The risks associated with the vacuum reflow process in actual production need to be identified and avoided by technical staff, who can ensure the quality of the final product by conducting screening experiments on the device package structure and process thresholds, optimising the grid aperture and process parameters, and tightening control over equipment maintenance and staff operation.
It is believed that the demand for vacuum reflow soldering technology in the field of high-reliability products will become more and more widespread, and that related technological research will become more and more in-depth.
Why is there a jamming phenomenon in vacuum reflow soldering? How can it be avoided? What precautions should be taken?
Cardboard is in the electronic assembly (PCBA) manufacturing reflow soldering process production practice is relatively common failure phenomenon, but for the vacuum reflow soldering, the impact of cardboard phenomenon than ordinary reflow soldering is much larger, so Tortai according to its in the actual production of the cardboard encountered in the production of the problem and preventive measures and combined with the experience of peers to write the following:
Cardboard phenomenon refers to the welding process, electronic components or PCB boards stuck in the welding platform can not move normally, resulting in poor welding or welding quality is not stable. Cardboard phenomenon is mainly due to the following reasons:
1、 the welding platform is not flat: the welding platform surface is not flat or there are raised parts, which will lead to electronic components or PCB board stuck on it can not move.
2、welding platform surface contaminants: welding platform surface oil, dust and other pollutants, will also lead to electronic components or PCB board stuck on it can not move.
3、welding platform temperature is too high: welding platform temperature is too high, will make the electronic components or PCB board and welding platform between the adhesion force increases, resulting in cardboard phenomenon.
4、welding platform vacuum is not enough: vacuum reflow soldering needs to be carried out under a certain degree of vacuum, if the vacuum is not enough, it will make the electronic components or PCB boards and the adhesion between the welding platform to increase, resulting in cardboard phenomenon.
Preventive measures
In order to avoid the occurrence of cardboard phenomenon, you can take the following preventive measures:
1、The welding platform to keep flat: before using the welding platform, should check whether its surface is flat, if there is a raised part should be trimmed in time.
2、Welding platform to keep clean: before using the welding platform, its surface should be cleaned to avoid the presence of oil, dust and other pollutants.
3、Control the temperature of the welding platform: when welding, the temperature of the welding platform should be controlled to avoid too high a temperature resulting in increased adhesion between the electronic components or PCB board and the welding platform.
4、Improve the vacuum: in the vacuum reflow soldering, should ensure that the vacuum of the welding platform meets the requirements, to avoid the occurrence of insufficient vacuum leading to cardboard phenomenon.
5、The use of appropriate welding process: in the choice of welding process, should be based on the characteristics of electronic components and PCB boards to choose the appropriate welding process, to avoid over-welding or under-welding leading to the occurrence of cardboard phenomenon.
In summary, the card board phenomenon is a common problem in vacuum reflow soldering, but can be avoided by taking appropriate preventive measures. In actual production, attention should be paid to the details to ensure that the welding platform is flat, clean and temperature control, and to ensure that the vacuum level meets the requirements, in order to improve the welding quality and productivity.