Material considerations in reflow soldering techniques
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In previous articles we have discussed that the main elements of technology integration are design, materials, process, equipment, and quality (note 1). In this issue we will look at the materials section.

In SMT reflow soldering, there are three main materials involved, namely PCB, device and solder paste solder. With the advent of lead-free technology, the variety of materials currently used in the SMT industry has become more complex. These materials, though, can be used in reflow soldering. But their requirement to craft and individual character expression are not identical completely, some even have considerable difference. Some require a lot of heat (higher temperature, longer time), while others require little or no heat at all. In this case, if we want to ensure that the welding work can be done well, we must make appropriate selection and matching of materials, so that different or contradictory materials will not be used at the same time. This is known as the welding technology of material considerations.

In the second article in this series, we talked about a variety of reflux technologies. For the sake of space, we consider the material here only in relation to the most commonly used hot air reflux technique. Also because of the space, I do not want to analyze the characteristics of various materials carefully here, but to provide a direction and help readers do better through the description and explanation of some considerations.

Three modes of material welding:
In the process of welding, we usually use one or more of the following three characteristics of the mixed mode to make the two welding surfaces and solder to form a solder joint.
1. Dissolve the Melting;
2. The corrosion Dissolution;
3. Diffusion
Which kind of principle is adopted under actual circumstance, it is the characteristic that sees material (metal), for example the melting point of this metal, relative dissolve corrode rate to wait a moment and decide. Different metal characteristics and modes determine the process we must adopt and set the value of process parameters. For example, when we use the first dissolution principle, we must ensure that the welding temperature reaches or exceeds the melting point of the two metals (for example, the melting point of Sn37Pb tin lead alloy is 183oC), and the formation of the welding is very rapid and can be completed in one or two seconds (note 2). If the second principle is used, the welding temperature must reach or exceed the melting temperature of the metal with low melting point, and sufficient time is provided for the dissolution of the material with low melting point to complete the corrosion of the material with high melting point. In this process, the temperature determines the rate of dissolution, so the higher the temperature, the shorter the welding time. The combined temperature and time control of the whole process is more demanding than the first mode.
The first two modes are generally used on the surface protection layer of the welded surface, while the third infiltration and diffusion mode is a necessity for every welded surface. Good welding depends on this third principle, which is to form a thin layer of intermetallic alloy between the welding surfaces (IMC, note 3).

Due to the different characteristics of various modes, we must consider the material and welding principle when designing the welding system. In this way, there will not be too much demand difference on the same PCBA and it will be difficult to weld. It should be noted that for solder (solder paste), since it is also a filler (filling the gap between welding surfaces), we all require it to melt. So in the application of temperature requirements, as long as it is above the melting point of the temperature can be (note 4). After reaching the melting temperature, it does not have as strict requirements on temperature and time as the component soldering materials or PCB soldering materials. Therefore, it is not correct to set reflow soldering process only according to the temperature curve standard provided by solder paste supplier. In terms of traditional Sn37Pb solder, after 183oC (melting point temperature of Sn37Pb), the welding process parameters should take PCBA devices and pad materials into consideration.

Materials in reflow welding technology:
During welding, the entire material system consists of three main parts. This is the device, the substrate (PCB), and the solder that binds them together (solder paste commonly used in reflux technology). To ensure that welding can be done with high quality results, they must have a good match for the process requirements. Therefore, these materials should not be selected independently. In general, the types of materials used in devices vary the most. This is not only because there are many types of materials available, but also because users choose different suppliers. The second aspect is PCB. The least change should be in solder, in general, except for the processing industry (CM or EMS) to meet the requirements of individual customers may use more kinds of solder paste, manufacture their own products OEM or ODM only choose one or two kinds of solder.

From a process and quality control perspective, the less variety and variety of materials, the better. So unless there is significant pressure on electrical performance, reliability or cost, the design department should minimize changes in material selection. This is an important guideline for DFM/DFR management.

Material characteristics consideration:
From the perspective of reflow soldering, we are most concerned about the following three properties of materials.

1. Solderability.
2. Heat resistance;
3. Inventory life.

All materials must be certified for the above characteristics to qualify (suitable for reflow soldering).
The definition must be complete when considering the solderability of materials such as components or PCB. Weldability is not just the wettability, or the fact that molten tin can bond to the weld surface. In terms of process and quality, a complete definition of weldability must include the following points.
• rapid wetting formation and adequate wetting force;
• can be completed in less time with lower welding temperature;
• IMC with good mechanical properties;
• it is not easy to have process faults during welding (such as air holes, welding balls, tin absorption, etc.)
The following factors determine the above characteristics:
• materials for each layer of the welding surface;
• thickness of each layer of material on the welding surface;
• the process of forming various layers of materials (such as electroless plating, dip plating, etc.);
• shape and structure of device packaging and welding end;
In other words, if we want to ensure good reflow soldering quality, we must evaluate each component or material according to the four definitions of weldability described earlier.
In the process of reflow welding, the material to be welded must be processed for a long time and at a relatively high temperature. So the heat resistance of the material is also an important consideration. Since lead-free technology generally requires higher temperature treatment, the problem of heat resistance of materials becomes more important after lead-free technology comes. Some organizations in the industry have also suggested clearer norms. For example, figure 1 shows the standard for testing the heat resistance of lead-free devices recommended jointly by IPC and JEDEC (note 5).

FIG. 1
In my personal experience, such a specification, while useful in most situations, is not a precise insurance specification. There are two main considerations. One is that the upper and lower limits should not be provided in the standards, and the heat-resistant properties should have only the lowest standards and no highest standards. Providing a cap only creates confusion in reference and execution. The second is that the pattern of thermal failure is actually related to three different thermal reactions, and such a specification cannot accurately assess the capability of these three aspects. For those users who need to attach great importance to quality, the author suggests that the following three kinds of evaluation should be made in the material evaluation to be more accurate.
1. Maximum withstand temperature (must include time concept);
2. Maximum heat shock resistance (rate of heating and cooling);
3. Maximum allowable total thermal energy (relation of total temperature and time)
The failure modes of the above three thermal reactions are different and there is no certain relationship between them. In other words, a device or material with a high temperature bearing capacity may not have a high thermal shock bearing capacity. Therefore, we must test and confirm when we evaluate the materials. The problem that users are likely to encounter is that many material or device suppliers do not use similar authentication methods. So there is no way to provide such indicators. That is to say, in many cases, to grasp these characteristics of the material, to ensure quality, the user must rely on their own certification. And attestation needs resource investment, this is in this area more sensitive and management experience is weaker. My suggestion is that users should selectively evaluate the key products and new packaging devices according to the quality responsibility of their own products or services. This may lead to a better balance.
A third consideration for device materials is inventory requirements and lifetime. Although JIT management is a better management practice in terms of both efficiency and quality, it can eliminate (or at least greatly reduce) inventory problems. But in fact for many users, the implementation of JIT conditions do not exist. Especially in an increasingly fast-changing competitive landscape. Therefore, it is an important work to study and deal with the inventory problem of device materials.
When we study or consider inventory life, must have the factor of inventory condition inside. Inventory conditions refer to the environment (temperature, humidity) and packaging protection. Both the device solder end and PCB pad coatings have protective coatings to ensure a long inventory life, but these coatings degrade over time. So inventory has a life, not an indefinite life. Generally speaking, during the inventory period, the surface of the welding end will oxidize and form an oxidation layer which is adverse to welding. However, since the formation of oxide layer will reduce the oxidation rate in the later stage, and the flux we use in welding can remove this oxide layer, or in more serious cases, we can restore its weldability through rework or surface treatment, so the oxidation problem in the inventory is not the most serious. On the other hand, during the inventory period, if the alloy layer (IMC) on the inner layer of the coating at the welding end increases excessively and the surface is exposed or leaves the surface only to a very thin degree, the welding surface will not be well formed when the solderable material on the surface is exposed to the IMC after dissolution. In this case, we are unable to rework, and devices that cannot be welded can only be discarded.
Whether the above IMC growth problem will appear depends on the material, thickness and inventory conditions of the welding end coating. Choosing materials with slow IMC growth such as NiAu, thicker plating, and lower stock temperature will increase the stock life of the device weldability. But as has been the case with other technologies, there are good and bad aspects to the solutions that test us at the same time. Thicker plating, while good in stock, brings us other problems. For example, the cost of Au is high, and the content is too high (thick coating) will bring the reliability problem of gold embrittlement to the solder joints after welding. Thicker deposits also require more heat energy to weld, meaning higher temperatures and/or longer times. This is detrimental in any welding process. Therefore, we must give integrated analysis when considering and selecting materials.
Solder paste considerations:
When choosing solder paste, we also take into account the characteristics mentioned above in terms of weldability, heat resistance, inventory demand and service life. However, we generally use the failure mode in its welding to describe the first two capabilities. For example, thermal collapse, ball/bead degree, wettability, etc. A good solder paste formula, we use in the prevention of its process fault in a better treatment, this is due to its' tolerance 'of heat change is strong. Understanding the 'tolerance' of individual solder paste is what we must do before choosing solder paste. The original solder paste features should be provided to the user in full and detail by the supplier. Unfortunately, due to issues such as proprietary secrets, users are generally not able to get very accurate information. This makes it necessary for users to authenticate themselves. Even if the above problems do not exist, users are generally unable to know or verify the technical data sources of suppliers, which may cause problems in application. For example, when I was assisting a customer in east China to improve their welding process capability at the beginning of the year, I modified several indexes in the standards provided by the supplier. For example, the supplier's index suggests that the heating rate in the heating zone should be 2.5oc, but within the design specification of this customer, this index will cause excessive thermal collapse. After the actual test certification, the heating rate of 1.6oC is a safe practice for users. And we also found that the solder paste of this model, as long as the heating speed is limited in the heating zone, the thermal collapse is very minor after the reflux enters other heating zones. Such discoveries and knowledge of the paste's properties can help us better develop our process specifications and troubleshoot problems.
From the perspective of welding, the author suggests that users evaluate and certify the following characteristics before selecting solder paste (note 6).
• wettability
• cold collapse, heat collapse
• antioxidant capacity
• flux flux capability
• tin splashing characteristics
• quantity and characteristics of residues
• thermal volatility
• service life (life after opening, recovery times, life after printing, etc.)

Understanding the above characteristics of solder paste helps us to set the correct temperature curve and reduce process failures. Although the work in the early investment of resources more, but a very worthwhile investment.
Device considerations:
The consideration of device material focuses on the two parts of welding end and packaging body. The three aspects of solderability, heat resistance, and inventory mentioned above when we talked about material characteristics considerations are also important for device selection considerations. But there are details to be added to these considerations.
The definition of weldability must be made more detailed when applied to device selection. We must fully take into account the following five requirements in the welding process.
1. Good wetting;
2. A suitable amount of tin;
3. Correct molten tin flow direction;
4. The welding end is stationary during the welding process;
5. Form IMC of appropriate thickness.
The overall structure of the device, including the welding end and package body, welding end materials and coating thickness, will have a certain impact on the above requirements. Careful consideration should be given when selecting or certifying the manufacturability of the device. The more complete and accurate this work is, the closer we are to zero defects. This work is part of manufacturability design in technology integration, which I will share with you in a more detailed paper in the future. I want to mention that the semiconductor industry, which is responsible for packaging technology, is mainly concerned with electrical considerations. Because 'our devices are the most electrical' is certainly a better selling point than 'our devices are the easiest to assemble.' In addition to engaged in packaging design for SMT assembly process may not know enough, so the design of packaging is sometimes not conducive to the assembly work. For example, BGA, DirectFETTM, some DFN packaging and so on, their design has a good electrical performance, but for assembly engineers, but they bring difficulties and challenges.

At present, many devices used in the electronic industry are still non-airtight packaging. Non-hermetically sealed packaging materials will absorb moisture during the inventory process. This characteristic brings the risk of popcorn failure to the welding process. So in the selection of materials we have to give

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