How to choose solder paste in SMT product?

As known, SMT manufacturing industry is a process that is soldering various components to PCB, so solder paste choice and quality become more important.

To begin, I will introduce some simple knowledge about solder paste. Solder paste is a new material accompanying SMT production development.

It is a complex system, made of solder powder, flux, and other additives and blended. Of course, additive composition depends on its needs.

Solder paste has a certain stickiness, so it can adhere to electronics components at a given position, while temperature rises, electronic components, and PCB will be soldered and form a permanent connection as the volatilization of solvents and some additives.

It can be said solder paste like is glue, its function is to glue two separate objects together. So, solder paste is a key point at PCBA manufacture.

We must depend on SMT Solder Paste Screen Printer Machine if have a big-volume order. That’s a needs condition to ensure quality and product sufficient.

If you have chosen an unsuitable material, not only increases manufacturing workloads but also effect the product’s quality. That’s why solder paste is so important reason.

And then, Let’s introduce different solder paste classification methods, that are according to material, additive, or practice manufacturing needs.

First, solder paste has three categories according to post-processing: normal rosin cleaning, free cleaning, and water-soluble pastes. Normal rosin cleaning is divided into two categories: rosin activated and rosin mildly activated.

In the soldering process, this type of paste has performance better “tinning speed” and granted good “soldering effect”. After work is finished, PCB’s surface has more rosin residues, the worker can clean it with detergent and PCB will be shining without any residues, which guarantees PCB has good insulation resistance and various electrons properties technical test will be passed.

Free clean type

It was by soldering, PCB’s surface is relatively smooth and has less residue and doesn’t need recleaning, and can pass various electrics properties technical tests. Grantee soldering quality, at the same time, shorted the manufacturing process and boosted speed.

Water soluble pastes


Due to technical reasons at early manufacture, PCB surface’s remains is too much, badly influence product quality and electronic properties weren’t ideal too. At then, manufacturing mostly cleaning processes with CFC, but it did not friendly to the environment, many countries have already be prohibited it. For the needs, the water-soluble paste is coming and it could be cleaned when soldering work is done, which reduces product cost and got the environmental request.

Second, solder paste has various categories according to function, product, or manufacture, which could be complicated. So, you can choose it, according to alloy composition, mesh number, and viscosity.

Alloy compostion:


In general, you can choose Sn63 or Pb37 solder alloy composition, it can match the soldering requests. To Ag or Pd’s plating product, you can choose Sn62 or Pb36 or Ag2’s solder paste. For a no-thermal shocking product, you can choose Bi-containing solder powder.

Mesh:
Also known as particle size, it refers to the number of meshes per square inch of the screen; in the actual tin powder production process, most of the tin powders are collected with several layers of sieves with different meshes, because each layer of sieve The mesh size of the mesh is different, so the particle size of the tin powder passing through each layer of mesh is also different. The particle size of the finally collected tin powder particles is also a regional value.

Therefore, when the mesh index of the solder paste is larger, the particle diameter of the tin powder in the solder paste is smaller; and when the mesh number is smaller, it means that the particles of the tin powder in the solder paste are larger.

That is, when the manufacturer of the solder paste selects the solder paste according to its mesh index of its, it should be determined according to the distance between the solder joints with the smallest distance on the PCB: if there is a larger distance, the solder paste with a smaller mesh size can be selected. On the contrary, when the distance between the solder joints is small, the solder paste with a larger mesh should be selected; generally, the particle size diameter is about 1/5 of the opening of the SMT stencil.

Viscosity:


In the SMT workflow, there is a process of moving, placing or handling the PCB in the middle from the laser stencil printing (or spotting) of the solder paste and attaching the components to the reflow heating process; in this process In order to ensure that the printed (or dotted) solder paste is not deformed and the components attached to the PCB solder paste are not displaced, it is required that the solder paste should have good viscosity and retention time before the PCB enters the reflow soldering heating.

Among them, the solder paste of 200-600Pa·S is more suitable for the production process equipment of the needle type or high degree of automation; the printing process requires a relatively high viscosity of the solder paste, so the viscosity of the solder paste used for the printing process is generally Around 600-1200 Pa·S, suitable for manual or mechanical patch stencil printing;

High-viscosity solder paste has the characteristics of good solder joint piling effect, and is more suitable for fine-pitch printing; while low-viscosity solder paste has the characteristics of faster falling, no tool cleaning, and time saving during printing;

Tips: the viscosity of the solder paste will change with the stirring of the solder paste, and the viscosity will decrease when stirring; when the stirring is stopped, the viscosity will return to its original state; this is important for how to choose different viscosities. The solder paste plays a very important role. In addition, the viscosity of solder paste has a great relationship with temperature. Under normal conditions, its viscosity will gradually decrease with the increase of temperature.

Finally, in practical manufacturing, we must consider other factors that could affect quality besides just mentioned factors above. Four factors must be considered below.

Of course, including Supply stability, Storage stability, Printing stability, and soldering effect.

Different batch solder paste quality has become instability, derive from supplier manufacture no-control factors had often change.Of course, the instability of viscosity is a relatively common problem. It is unrealistic to require suppliers to be completely indistinguishable. It is more realistic to require suppliers to control this parameter within a certain range.

Storage stability

We need solder paste to have stable quality, but in real-world solder paste stability will be changed since from purchase to take warehouse and keep the store a period time.

Printing stability


In the actual production process, the performance of reflow is also very important, which has a great impact on quality.

soldering finally effect

It mainly includes the following four aspects: better wetting, BGA less poor fusion, preheat collapse, and print stop recovery ability.

Solder wetting is the part of the process in which metal in the solder bonds with metal on the printed circuit board (PCB) or component. In the comparison to different solder pastes, you can find A is more wetting than B.

In the BGA process, due to conditions, limited solder paste fusion capability has more important.

The higher the preheat collapse level, the less incidence of poor bridging. you can find no collapse in the left pic which is 0.3mm/0.2mm, but the right pic has a collapse which is 0.4mm/0.3mm.

Excellent stop recovery can improve printing production efficiency to a certain extent. From this pic, you can see the difference curve after stopping for an hour.

In summary, we can know that choosing the right solder paste should not only consider the characteristics of the solder paste itself and its impact on quality and production but also consider various factors in mass production.

Therefore, it is necessary for us to continuously record and summarize the impact of various suppliers’ solder pastes on products in actual production.

Our company pays special attention to the selection of solder paste. Our quality, production, engineering and other departments regularly hold relevant meetings. The quality department will record and track the actual use of solder paste from various suppliers and monitor the use of solder paste. This is where our internal quality management system comes into play. For problems that arise, we will strictly implement improvements according to the process, and ultimately ensure the quality of our customers’ products.

Because we know that providing customers with high-quality products is our mission, and it is also related to the company’s long-term development.

Leadsintec Technology Co., Ltd is an electronics manufacturing service (EMS) provider located in Shenzhen, China, and has been operating since 2004. As an ISO 9001 certified electronics contract manufacturer, we offer a full range of services including, Printed Circuit Board Assembly (PCBA), PCB Component Procurement, Box-Building, Testing, as well as Cable and Accessory services.