Lead-free reflow soldering - flux

Status of lead-free welding technology

The standardization of the alloy composition of lead-free solder is not clearly stipulated at present. The opinion of most business associations such as IPC: Alloys with lead content <0.1-0.2WT % (tend to < 0.1%, and do not contain any other toxic elements are called lead-free solder alloys.
Lead-free solder alloy
The core and first task of lead-free is lead-free solder. According to statistics, more than 100 kinds of lead-free solder paste, wire and wave soldering bar have been developed worldwide, but only a few are truly recognized as usable.
At present, the most likely alternative to Sn/Pb solder alloy materials
The most likely non-toxic alloy to replace Sn/Pb solder is the SN-based alloy. Based on Sn, metal elements such as Ag, Cu, Zn, Bi, In, and Sb are added to form binary, ternary or multicomponent alloys. Metal elements are added to improve the alloy performance and improve the weldability and reliability. Mainly include: Sn-Bi solder alloy, Sn-Ag eutectic alloy, SN-Ag-Cu ternary alloy, Sn-Cu solder alloy, Sn-Zn solder alloy (only developed and applied in Japan), Sn-Bi solder alloy, Sn-In and Sn-Pb alloy.
At present, the most widely used lead-free solder alloys Sn95.8Ag3.5Cu0.7 in ternary eutectic form (USA) and Sn96.5Ag3.0Cu0.5 in ternary near-eutectic form (Japan) are currently the most widely used lead-free solder for reflow welding. Its melting point is about 216-220℃.
Since Sn95.8Ag3.5Cu0.7 lead-free solder has been patented in the United States, and because the solder with 3.0WT % Ag content is not patented, the price is cheaper, and the solder joint quality is better, IPC recommends Sn-Ag-Cu solder with 3.0WT % Ag content (weight percentage).
Sn-0.7Cu-Ni solder alloy is used for wave soldering. Its melting point is 227 ° C.
Although Sn base lead-free alloy has been more widely used, compared with Sn63Pb37 eutectic solder, lead-free alloy solder still has the following problems:
(A) The melting point is about 34℃ high.
(B) High surface tension and poor wettability.
(C) High prices
2, PCB pad surface coating material
Lead-free welding requires that the PCB pad surface coating material should also be lead-free, and the lead-free coating of the PCB pad surface is easier than the lead-free surface of the components. At present, non-lead metal or lead-free solder alloys are mainly used to replace Pb-Sn hot air polishing (HASL), electroless Ni plating and gilding (ENIC), Cu surface coating OSP, silver dipping (I-Ag) and tin dipping (I-Sn).
At present, the lead-free standard is not perfect, so there are many types of lead-free components welding end surface coatings. There are more pure Sn and Sn/Ag/Cu plating in the United States and Taiwan Province, while there are more types of component welding end plating in Japan, and each company is different, in addition to plating pure Sn and /Sn/Ag/Cu, there are Sn/Cu, Sn/Bi and other alloy layers. Due to the relatively low cost of Sn plating, Sn plating processes are often used. However, because Sn surface is easy to oxidize to form a thin oxide layer, pressure will be generated after adding electricity, and Sn will be pushed out where there is no uniformity, forming Sn whisk. Sn is easy to cause short circuit in components such as QFP with narrow spacing, which affects reliability. For low-end products and components with a life requirement of less than 5 years, pure Sn can be plated. For highly reliable products and components with a life requirement of more than 5 years, a layer of Ni with a thickness of about 1µm is first plated, and then Sn with a thickness of 2-3µm is plated.
At present, lead-free welding technology is in transition and initial stage
Although lead-free technology is applied in different degrees at home and abroad, it is still in the transition and initial stage, and it is not mature from theory to application. There is no unified standard, there is no unified understanding of the reliability of the solder joints of lead-free welding, so regardless of the international and domestic lead-free application technology is very confusing, most enterprises although the welding material is lead-free, but the components still have lead welding end. Which kind of lead-free solder is better? Which PCB pad coating is more favorable for lead-free welding? Which component end material is better for lead-free solder joint reliability? What is the most reasonable temperature curve? What are the requirements of lead-free welding for printing, welding, testing and other equipment? There is no clear explanation. In short, there are different opinions on lead-free welding technology, and each has a set of statements and practices. This state is very unfavorable to the reliability of lead-free welding products. Therefore, it is urgent to accelerate the research of lead-free welding technology from theory to application.

Characteristics and countermeasures of lead-free welding
Main features of lead-free welding and solder joints
Main features of lead-free welding
(A) High temperature, melting point is about 34℃ higher than the traditional lead eutectic solder.
(B) High surface tension and poor wettability.
(C) The process window is small and the quality control is difficult.
Characteristics of lead-free solder joints
(A) Poor infiltration and scalability.
(B) Lead-free solder joints are rough in appearance. Traditional inspection standards and AOI need to be upgraded.
(C) There are more pores in the lead-free solder joints, especially when the lead solder end is mixed with lead-free solder, the lead solder on the solder end (ball) will melt first, cover the pad, and the flux cannot be discharged, resulting in pores. But the porosity does not affect the mechanical strength.
(D) Many defects - due to poor infiltration, the self-positioning effect is weakened.
Lead-free solder joints have rough appearance, porosity, wetting Angle, and no half-moon shape, because the appearance of lead-free solder joints is significantly different from that of lead solder joints, if there are original lead inspection standards, it can even be considered unqualified, but for the general requirements of civilian electronic products these do not affect the quality of use. Therefore, it is necessary to convince customers to understand that this is caused by poor wettability of lead-free welding. With the deepening and development of lead-free technology, due to the improvement of flux and the progress of process, the rough appearance of lead-free solder joints has had some changes, and I believe that there will be better progress in the future.
The characteristics and countermeasures of lead-free welding are analyzed from the reflow welding temperature curve, and the characteristics and countermeasures of lead-free welding are analyzed by comparing the temperature curves of lead and lead-free:
In the heating zone, lead welding from 25℃ to 100℃sec only takes 60-90sec; And lead-free welding from 25 ° C to 110 ° C needs 100-200 sec, the heating time is twice as long as the lead, when the multi-layer board, large board and complex printed circuit board with large heat capacity components, in order to make the entire PCB temperature uniform, reduce the temperature difference of PCB and size components Δt, lead-free welding needs to slowly heat up. It can be seen that lead-free welding requires the length of the heating and preheating zone of the welding equipment to be extended.
In the rapid heating zone (flux wetting zone), lead welding rises from 150 ° C to 183 ° C, with a temperature rise of 33 ° C, which can be completed between 30-60sec, and its heating rate is 0.55 − 1 ° C /sec; And lead-free welding from 150 ° C to 217 ° C, warming 67 ° C, only allowed to be completed between 50-70sec, the heating rate is 0.96-1.34 ° C /sec, the heating rate is required to be about 30% higher than lead, the higher the temperature, the more difficult it is, if the heating rate can not be raised, Long-term exposure to high temperature will cause the flux in the solder paste to end the activation reaction in advance, and in serious cases, the PCB pad, component pins and solder alloy in the solder paste will be reoxidized at high temperature, resulting in poor welding. In order to improve the heating slope of the flux wetted area, the number of reflux areas should be increased or the heating power should be increased. Due to the high temperature and poor wettability, it is necessary to increase the activation temperature and activity of the flux in the solder paste.
Looking at the reflux area, the peak temperature with lead is 210-230 ° C, and the peak temperature without lead is 235-245 ° C, because the limit temperature of FR-4 substrate PCB is 240 ° C, it can be seen that when there is lead welding, a fluctuation range of 30 ° C is allowed, and the process window is relatively loose; In lead-free welding, only a fluctuation range of 5 ° C is allowed, and the process window is very narrow. If the PCB surface temperature is uniform, the actual process allows for an error of 5 ° C. If the PCB surface has a temperature error Δt>5 ° C, then the PCB somewhere has exceeded the FR-4 substrate PCB limit temperature 240 ° C, will damage the PCB. This margin is only suitable for simple products, for complex products with large heat capacity. It may take 260 degrees to weld. Therefore, FR-4 substrate PCB can not meet the requirements.
In the actual reflow welding, on the same PCB, due to the different distribution area of copper at different locations, the size of components and the density of components at different locations, the temperature of the PCB surface is not uniform. If the minimum peak temperature during reflow soldering is 235 ° C, the maximum peak temperature depends on the temperature difference Δt of the plate surface, which depends on the plate size, thickness, layer number, component layout, Cu distribution, component size and heat capacity. Large, thick printed boards with large and complex components (such as CBGA, CCGA, etc.) typically have Δt as high as 20-25 ° C. In order to reduce the Δt on the PCB surface, a small lead-free process window is met. The heat capacity and transverse temperature difference of the reflow welding furnace are also important factors to ensure the quality of lead-free welding. It is generally required that the transverse temperature difference of the reflow welding furnace is less than 2℃. In order to reduce the transverse temperature difference Δt of the furnace, in addition to taking better heat preservation measures, the method of heating the guide rail can also be used. Because the guide rail is easy to dissipate heat, the temperature near the guide rail is generally slightly lower. Since the melting point of lead-free is 34 ° C higher than that of lead, lead-free welding equipment is required to be resistant to high temperature and corrosion. For large-size PCBS, intermediate support is required for the guide rail of the equipment.
In the cooling zone, due to the high peak temperature of the reflux zone, in order to prevent the long solidification time of the solder joint, the crystallization particles of the solder joint will grow up. In addition, accelerated cooling can prevent segregation and avoid the formation of dendritic crystals, so welding equipment is required to increase the cooling device to quickly cool the solder joint.
Lead-free solder paste printing and mounting process countermeasures
Differences in physical properties between lead-free and leaded solder paste
The wettability and spreadability of lead solder paste is far lower than that of lead solder paste, where there is no printing solder paste on the pad, the molten solder can not spread to those places, then, after welding, the bare brass pad that is not covered by solder will be exposed to the air for a long time, in the harsh environment such as moisture, high temperature, corrosive gas, causing the solder joint to be corroded and failed. Affect product life and reliability.
In order to improve the wettability, the flux content of lead-free solder paste is usually higher than that of leaded solder paste.
Due to the lack of lead lubrication, the filling and film removal properties of solder paste printing are poor.
Lead-free template opening design
In view of the poor wettability and spread of the lead-free solder paste, the opening design of the lead-free template should be larger than that of the lead, so that the solder paste can cover the pad completely. Specific measures can be taken as follows.
For devices with Pitch > 0.5mm, openings of 1:1.02-1:1.1 are generally adopted.
For devices with Pitch≤0.5mm, 1:1 openings are usually used, and in principle, there is at least no need to shrink.
For the Chip components of 0402, 1:1 openings are usually used, and in order to prevent the displacement of components during the erection and reflow, the inside of the pad opening can be modified into a sharp Angle or bow.
In principle, the thickness of the template is the same as that of the lead template, because the flux content in the lead-free solder paste is higher, that is, the alloy content is less, so the thickness of the template can also be appropriately increased.
Lead-free formwork width to thickness ratio and area ratio.
In order to correctly control the printing amount of solder paste and the quality of solder paste graphics, it is necessary to ensure that the ratio of the minimum opening width on the template to the thickness of the template is greater than 1.5, and the ratio of the opening area of the template to the inner wall area around the opening is greater than 0.66, which is the IPC7525 standard and the most basic requirement for the design of lead template openings.
Due to the poor filling and stripping ability of lead-free solder paste, the requirements for the opening width to thickness ratio and area ratio of the template are higher:
Lead-free width to thickness ratio: Opening width (W)/template thickness (T) > 1.6
Lead-free area ratio: opening area (W×L)/hole wall area [2× (L+W) ×T] > 0.71
(F) Selection of lead-free formwork manufacturing methods
For products of general density, traditional laser, corrosion and other methods can be used. For high-density components such as 0201, laser + electropolishing, or electrocasting should be used, which is more conducive to improving the filling and stripping capacity of lead-free solder paste.
Lead-free requirements for printing accuracy and mounting accuracy
Because lead-free infiltration force is small. Self-correcting at reflux (Selfalign) is less effective, so printing accuracy and patch accuracy are more required than when lead is present.

3. Problems in the special stage of the transition from lead to lead-free welding
Lead-free processes challenge components
High temperature resistance
The impact of high temperature on component packaging should be considered. Since the packaging material of traditional surface mount components can meet the welding temperature of lead solder as long as it can withstand 240 ° C high temperature, and the welding temperature of lead-free welding is as high as 260 ° C for complex products, it is necessary to consider whether the component packaging can withstand high temperature.
In addition, the effect of high temperature on the internal connection of the device should be considered. The internal connection methods of the IC are gold wire ball welding, ultrasonic pressure welding, and reverse loading welding methods, especially BGA, CSP and combined composite components, modules and other new components, their internal connection materials are also the same solder with the surface assembly, but also with the reflow welding process. Therefore, the internal connection materials of lead-free components should also meet the requirements of lead-free welding.
Solder ends are lead-free
The welding end of lead components is mostly Sn/Pb coating, and there are many types of welding end surface coatings of lead-free components. There is no conclusion as to which coating is the best, so the standard for lead-free components has yet to be improved.
The challenge of lead-free processes to PCBS
Lead-free process requires PCB good heat resistance, high glass transition temperature Tg, low thermal expansion coefficient, low cost.
Lead-free processes require a high glass transition temperature (Tg)
Tg is a unique property of polymers and is the critical temperature that determines the properties of materials. In the SMT welding process, the welding temperature is much higher than the Tg of the PCB substrate, and the lead-free welding temperature is 34℃ higher than that of the lead, which is more likely to cause thermal deformation of the PCB and damage components during cooling. The base PCB material with higher Tg should be appropriately selected.
Low Coefficient of Thermal Expansion (CTE) required
When the welding temperature increases, the CTE mismatch between the Z axis of the multi-layer PCB and the XY direction of the laminated material, glass fiber, and Cu will produce a great deal of stress on the Cu, which will cause the metallized pore coating to break and fail in serious cases. This is a rather complex problem, because it depends on many variables, such as the number of PCB layers, thickness, laminate material, welding curve, as well as the distribution of Cu, the geometry of the hole (such as aspect ratio), etc.
Measures to overcome the fracture of metallized holes in multi-layer plates:
Etching Process 1 - Remove the resin/glass fiber inside the hole before electroplating.
The bonding force between the hole wall and the multilayer plate is metallized.
The erosion depth is 13-20µm.
High heat resistance
The limit temperature of FR-4 substrate PCB is 240℃, for simple products, the peak temperature of 235-240℃ can meet the requirements, but for complex products, it may need 260℃ to weld well. Therefore, high temperature resistant FR-5 is required for thick plates and complex products.
Low cost
Due to the relatively high cost of FR-5, for general consumer products can use composite CEMn to replace FR-4 substrate, CEMn is the surface and core of different materials composed of rigid composite copper clad foil laminate, referred to as CEMn represents different models.
(2) Tin whisker problem
SN grows under compression