With the continuous miniaturization of SMT electronic components and the increasing integration of chips, whether it’s notebooks, smartphones, medical devices, automotive electronics, military, or aerospace products, there is an increasing application of devices such as BGA and CSP in array packaging, along with rising quality requirements for products.
The term “5G” was a buzzword in 2019, and now the era of 5G has unveiled its curtain. Looking inside the PCBA circuit board of a mobile phone, besides the baseband chip, the design difficulties of 5G smartphones mainly lie in the radio frequency (RF), antennas, and other components, compared to 4G smartphones. Since 5G has at least double the frequency of 4G, five times the bandwidth, up to 29 frequency bands, five times the power, ten times the speed, and dozens of times more antennas, this requires us to continuously improve our process capabilities, increase high-end equipment, and ensure high-reliability products through high-quality welding.
Analysis of Various High-Reliability Welding Processes for PCBA
In high-precision electronic manufacturing processes, there are many SMT production equipment, with the main automated equipment including SMT automatic X-RAY component counters, SMT first-article inspection machines, fully automatic paste printers, inline 3D-SPI paste inspection machines, placement machines, reflow ovens, inline AOI optical inspection machines, and inline PCBA automatic milling cutter depaneling machines.
Each type of equipment has specific functions and uses. The reflow oven is the final process in the SMT production line, responsible for melting the solder on the already mounted PCB and components to bond them to the motherboard.
Reflow soldering has become the mainstream process in SMT. Most of the components on the smartphone board are welded to the circuit board through this process, relying on the action of hot air on the solder joints. The colloidal flux undergoes a physical reaction at a certain high temperature to achieve SMD welding. It is called “reflow soldering” because the gas circulates within the welding machine to generate high temperatures for the welding purpose. Reflow ovens also come in many varieties, such as hot air reflow ovens, nitrogen reflow ovens, vapor-phase reflow ovens, and vacuum reflow ovens.
SMT reflow soldering is a crucial process in SMT assembly. The quality of the solder joints in PCBA welding completely depends on the performance of the reflow soldering equipment and the setting of the temperature profile.
Reflow soldering technology has undergone various developmental stages, including plate radiant heating, quartz infrared tube heating, infrared hot air heating, forced hot air heating, and forced hot air heating with nitrogen protection.
(The following is an introduction to several reflow soldering processes, with their respective characteristics and applications.)
1. Hot Plate/Push Plate Conductive Reflow Soldering
This type of reflow oven relies on the heat source under the conveyor belt or push plate for heating, heating the components on the substrate through thermal conduction. It is used for single-sided assembly of ceramic (Al2O3) substrate thick-film circuits. Only when the ceramic substrate is placed on the conveyor belt can it receive sufficient heat. It has a simple structure and is inexpensive. Some thick-film circuit factories in China imported such equipment in the early 1980s.
2. Infrared Radiant Reflow Soldering
Most of these reflow ovens are also conveyor belt-type, but the conveyor belt only serves to support and convey the substrate. The heating method mainly relies on infrared heat sources to heat by radiation. The temperature inside the furnace chamber is more uniform than the previous method, with larger mesh holes, suitable for reflow soldering and heating of double-sided assembled substrates. This type of reflow oven can be considered the basic model of reflow ovens. It is widely used in China and is relatively inexpensive.
3. Infrared Heating + Hot Air Reflow Soldering
This type of reflow oven adds hot air to the IR oven to make the temperature inside the oven more uniform. When infrared radiation heating is used alone, it is found that within the same heating environment, different materials and colors absorb heat differently, meaning the Q value in formula (1) is different, resulting in different temperature rises ΔT. For example, the packaging of ICs and other SMDs is black phenolic or epoxy, while the leads are white metal. When heated alone, the temperature of the leads is lower than that of the black SMD body. Adding hot air can make the temperature more uniform and overcome differences in heat absorption and shading issues. IR + Hot air reflow ovens were once widely used internationally.
4. Nitrogen Reflow Soldering
With the increase in assembly density and the emergence of fine-pitch assembly technology, nitrogen-filled reflow soldering processes and equipment have been developed, improving the quality and yield of reflow soldering and becoming a development trend for reflow soldering.
Nitrogen reflow soldering has the following advantages:
- Prevents and reduces oxidation.
- Improves welding wettability and accelerates wetting speed.
- Reduces the generation of tin balls, avoids bridging, and obtains better welding quality.
Particularly importantly for obtaining better welding quality, low-activity flux pastes can be used, which can also improve the performance of solder joints and reduce substrate discoloration. However, its disadvantage is a significant increase in cost, which increases with nitrogen consumption. When you need to achieve 1000 ppm oxygen content versus 50 ppm oxygen content inside the oven, the demand for nitrogen is vastly different. Currently, flux paste manufacturers are working to develop no-clean flux pastes that can perform well in higher oxygen atmospheres, which can reduce nitrogen consumption.
For introducing nitrogen into reflow soldering, a cost-benefit analysis must be conducted. Its benefits include product yield, quality improvement, and reduced rework or maintenance costs. A thorough analysis often reveals that introducing nitrogen does not increase the final cost; instead, we can benefit from it.
Most of the ovens currently in use are forced hot air circulation types, and it is not easy to control nitrogen consumption in such ovens. There are several ways to reduce nitrogen consumption, such as reducing the opening area of the oven inlet and outlet, and importantly, using partitions, roller shutters, or similar devices to block the unused portion of the inlet and outlet space. Another method is to utilize the principle that the hot nitrogen layer is lighter than air and not easily mixed. When designing the oven, the heating chamber is made higher than the inlet and outlet, so that a natural nitrogen layer forms inside the heating chamber, reducing the compensation amount of nitrogen and maintaining the required purity.
05
Through-hole Reflow Soldering
Through-hole reflow soldering, also sometimes referred to as reflow soldering for through-hole components, is gradually gaining popularity. It can eliminate the wave soldering process and serve as a step in PCB mixed-assembly technology. One advantage is that it allows for the use of through-hole components to achieve better mechanical connection strength while leveraging the benefits of surface mount manufacturing processes. For larger PCB boards, the flatness may not allow all the leads of surface-mounted components to make contact with the soldering pads. Even if the leads and soldering pads do make contact, the mechanical strength provided may often be insufficient, leading to potential failure points where the components may detach during product use.
Despite the benefits of through-hole reflow soldering, there are still several drawbacks in practical applications. The large amount of solder paste increases the degree of machine contamination due to flux volatilization and cooling, necessitating an effective flux residue removal device. Another issue is that many connectors are not designed to withstand the temperatures of reflow soldering. Early furnaces based on direct infrared heating are no longer suitable, lacking the effective heat transfer efficiency required to handle both general surface-mounted components and through-hole connectors with complex geometric appearances on the same PCB. Only high-capacity forced convection furnaces with high heat transfer capabilities can potentially achieve through-hole reflow, and it has been proven through practice that the remaining challenge is ensuring that the solder paste in the through-holes and component leads have an appropriate reflow soldering temperature profile. With advancements in processes and components, through-hole reflow soldering will increasingly be applied.
In traditional electronic assembly processes, wave soldering technology is generally used for the soldering of printed circuit board assemblies with through-hole mounted components (THD). However, wave soldering has many shortcomings: it is not suitable for the soldering of high-density, fine-pitch components; there are many bridging and missing solder joints; flux needs to be sprayed; and the printed circuit board undergoes significant thermal shock and warping deformation. Therefore, wave soldering cannot adapt to the development of electronic assembly technology in many aspects.
To adapt to the development of surface mount technology and address the above soldering difficulties, the measure adopted is through-hole reflow soldering technology (THR, Through-hole Reflow), also known as pin-in-hole reflow (PIHR). The principle of this technology is that after the surface mount components are placed on the printed circuit board, a special template with many needle tubes is used. The template position is adjusted so that the needle tubes align with the through-hole soldering pads of the through-hole mounted components. A squeegee is used to print the solder paste on the soldering pads from the template, and then the through-hole mounted components are installed. Finally, both the through-hole mounted components and the surface mount components are simultaneously soldered through reflow soldering.
It can be seen from this that pin-in-hole reflow soldering has advantages over traditional processes: firstly, it reduces the number of processes by eliminating the wave soldering process, saving costs and reducing the required workforce while also improving efficiency; secondly, compared to wave soldering, reflow soldering has a much lower likelihood of bridging, thus improving the first-pass yield. Pin-in-hole reflow soldering has significant advantages over traditional processes in terms of economy and advanced technology.
Through-hole reflow soldering technology originated from Sony Corporation in Japan and began to be applied in the early 1990s, but it was mainly used in Sony’s own products, such as TV tuners and CD Walkmans. In the mid-1990s, China introduced this technology from Japan. At that time, several tuner manufacturing factories in China, such as Wuxi Radio Factory No. 6, Shanghai Jinling Radio Factory, Chengdu 8800 Factory, Chongqing Testing Instrument Factory, and Shenzhen Dongguan Tuner Factory, applied this technology and achieved good returns. Currently, it has been widely used in fields such as CD, DVD laser pickup servo boards, DVD-ROM servo boards, and laptop motherboards.
(1) High reliability, good soldering quality, with a defective parts per million (DPPM) rate below 20.
(2) Less soldering defects such as cold solders and bridging, reducing the workload of board repair.
(3) The PCB surface is clean, with a significantly better appearance than wave soldering.
(4) Simplifies the process. By eliminating the steps of dispensing (or printing) adhesive, wave soldering, and cleaning, both operation and management are simplified. It is easier to manage when fewer materials and equipment are used in the same product. Moreover, the operation of the reflow oven is much simpler than that of the wave soldering machine, with no issue of tin slag and lower labor intensity.
(5) Reduces costs and increases benefits. After adopting this process, it eliminates the need for wave soldering equipment, cleaning equipment, wave soldering and cleaning workshops, wave soldering and cleaning personnel, as well as a large amount of wave soldering and cleaning materials. Although the price of no-clean solder paste is slightly higher than that of non-no-clean solder paste, the overall cost can be greatly reduced and benefits increased.
Through-hole Reflow Soldering Production Process:
The production process is extremely similar to the SMT process, namely, solder paste printing – component insertion – reflow soldering. The process is the same for both single-sided and double-sided mixed boards.
Solder Paste Printing Process:
Selection of Solder Paste: The solder paste used for through-hole reflow has low viscosity and good fluidity, facilitating its flow into through-holes. Generally, through-hole reflow is performed after the SMT process. If the solder alloy composition of the solder paste used in SMT is 63Sn37Pb, then to ensure that the SMT components do not melt again and fall off during through-hole reflow, the solder alloy composition in the solder paste can adopt 46Sn46Pb8Bi (178°C) with a slightly lower melting point. The solder particle size should be less than 25 μm for less than 10%, 25 to 50 μm for more than 89%, and above 50 μm for less than 1%.
As electronic products increasingly prioritize miniaturization and multifunctionality, the component density on circuit boards is getting higher and higher, with many single-sided and double-sided boards dominated by surface-mounted components. However, due to factors such as connection strength, reliability, and applicability, some through-hole components still cannot be chip-based, especially perimeter connectors. In traditional SMT mixed-assembly processes, through-hole mounted components are mostly soldered using wave soldering, selective wave soldering, soldering robots, or manual soldering. These traditional methods, especially wave soldering and manual soldering, are far inferior in quality to reflow soldering. Currently, in many electronic products, the proportion of through-hole components only accounts for 10%-5% or even less of the total number of components. The assembly costs using methods such as wave soldering, selective wave soldering, automatic soldering robots, manual soldering, and crimping far exceed this proportion, with high costs per soldering joint. Therefore, through-hole component reflow soldering technology is becoming increasingly popular, and the use of reflow soldering to replace wave soldering for through-hole mounted components (i.e., pure reflow soldering processes) has become one of the current developments in SMT technology.
06
Vertical curing reflow soldering
With the popularization of 5G, the vertical consolidation furnace is the first choice for the dispensing and curing process of high-reliability and high-quality terminal products! Almost all dispensing packaging materials require a long curing time, so it is not practical to use an online continuous production curing oven. Usually, we often use “batch baking ovens”, but the technology of vertical baking ovens is also becoming more and more perfect, especially when the heating curve is simpler than that of a reflow oven. Vertical baking ovens are fully capable of handling it.
Chip bottom filling & component precision encapsulation dispensing process
The vertical oven uses a vertical lifting conveyor system as a “buffer and accumulator”, and each PCB must pass through this process cycle. The result is a long enough curing time while reducing the footprint.
The market’s demand for smaller size has led to the increased application of CSPs such as FLIP CHIP, which have a smaller footprint and higher signal transmission rate after component placement. Filling or glue pouring is used to strengthen the solder joint structure to withstand the stress caused by the inconsistency in thermal expansion coefficients between the silicon wafer and the PCB material. Typically, the upper drop or encirclement method is used to seal the wafer with glue.
Many of these encapsulation adhesives require long curing times, which are not practical for online production furnaces. Batch processing ovens are often used, but vertical ovens have been proven to successfully perform the curing process, and their temperature profile is simpler than ordinary reflow ovens. Vertical ovens use a PCB transport system to act as a buffer/stacking area, which extends the time that PCB boards can remain in a small footprint oven.
Schematic diagram of the working principle of the vertical curing oven
The fully automatic vertical reflow furnace is mainly used for thermosetting production processes in high-end manufacturing industries such as automotive electronics, chip bonding in the 5G communication product manufacturing industry, underfilling, component packaging, dispensing and sealing, precision encapsulation, underfilling + silver paste/thermal glue, and high-precision non-contact jet underfill dispensing for assembly operations. The application industry covers SMT/EMS, home appliances, solar energy, automotive electronics, military, semiconductors, medical equipment, etc.
07
Vacuum reflow soldering
With the advent of 5G, many industries are undergoing significant technological reforms, such as in the fields of intelligent driving for automobiles, smart homes, and remote intelligent surgery for medical treatment… All of these fields require high-quality and high-reliability welding. Due to the fact that the data transmission capacity of 5G communication is much greater than that of commercial 4G communication, the reliability requirements for welding will be higher. Due to the negative impact of bubble voids on heat dissipation and high-frequency attenuation, it is necessary to effectively control the void rate of semiconductor components themselves, the void rate during the soldering process of 5G communication base station boards, and the void rate during the soldering process of 5G mobile phone motherboards.
After reflow soldering, SMT patch PCBA usually has some voids remaining in the solder joints. The larger the solder joint area, the larger the void area will be; The reason is that when the molten solder cools and solidifies, the gas generated in the solder does not escape and is “frozen” to form a void. The factors that affect the generation of voids are multifaceted, including the selection of solder paste, the form of device packaging, the design of solder pads The surface treatment method of PCB pads, the hole pattern of the stencil, and the reflow curve settings are all related.
The vacuum reflow soldering process is a reflow soldering technique that introduces a vacuum environment during the reflow soldering process. Compared to traditional reflow soldering, vacuum reflow soldering creates a vacuum environment after the product enters the reflow zone, where the atmospheric pressure can be reduced to below 5mbar500pa and maintained for a certain period of time, thus achieving the combination of vacuum and reflow soldering. At this point, the solder joint is still in a molten state, while the external environment of the solder joint is close to vacuum. Due to the pressure difference between the inside and outside of the solder joint, bubbles inside the solder joint are easily overflowed, resulting in a significant reduction in the void rate of the solder joint.
Schematic diagram of the formation of reflow soldering voids under non-vacuum and vacuum conditions
Industry professionals analyzed that, for example, the traditional welding bubble void rate is within 20% to 25%, and the IPC standard for void rate is also set at 25%. However, the void standard for power devices now requires ≤5%, and some devices require ≤3%. The PCB & copper substrate & aluminum substrate welding void rate requirement is ≤10~15%.
The vacuum furnace is a weapon to solve the problem of bubbles and voids. Through high-quality, high-stability, energy-efficient welding equipment, it helps electronic companies improve product quality while reducing their daily energy and nitrogen costs, as well as their downtime and maintenance costs.
Schematic diagram of vacuum reflow furnace
Vacuum reflow soldering, also known as vacuum/controlled atmosphere eutectic furnace, has a large thermal capacity and small temperature difference on the PCB surface, and has been widely used in the fields of aviation, aerospace, military electronics, and other fields in Europe and the United States. It adopts the principle of infrared radiation heating, with characteristics of uniform temperature, ultra-low temperature safe welding, no temperature difference, no overheating, reliable and stable process parameters, no need for complex process testing, and low environmental cost operation. It meets the needs of military products with multiple varieties, small batches, and high reliability welding.
Comparison of non-vacuum and vacuum soldering bubbles in BGA solder joints
The vacuum reflow soldering system is used for soldering under relatively closed and vacuum-assisted conditions, which has a good advantage for product quality. Under this condition, the vacuum reflow soldering system can effectively discharge the bubbles generated by the volatilization of flux in the solder, effectively reducing the void rate of the product’s soldering surface, thereby effectively improving the soldering quality of the product.
Comparison of non-vacuum and vacuum solder joints in large areas
With the development of the electronics industry, more and more customers have increasingly high requirements for product reliability. In order to effectively reduce the void rate and promote the use of updated equipment to meet this requirement, the use of new equipment will inevitably require adjustments to our existing models and process windows, and may also present many new problems that require in-depth exploration and discovery.
We have introduced the development history of equipment improvement and reflow soldering equipment. In fact, the development of reflow soldering technology has been driven by the following two aspects:
- Electronic products are developing towards short, small, light and thin. The assembly of high density, SMC/SMD micro-pitch, SMC/SMD variety and specification series, especially the increasing number of special-shaped components and electromechanical components, have forced the reflow soldering process, an important process in SMT, to face challenges. It needs to be continuously developed and improved to improve soldering quality and yield.
With the development of human civilization, it has become a consensus to control the three wastes of waste gas, waste materials, and waste water to protect the environment. Traditional solder paste contains flux, and its residues after soldering need to be cleaned with solvents such as Freon CFC and acetone, which can cause environmental pollution. To avoid pollution, water cleaning processes, no-clean processes, and new solder paste have emerged.
