Chemical and physical compatibility

In light of what has been said so far, it is therefore clear that many different metals from various processes can come into play in the soldering process and their compatibility is essential to create solder joints of excellent quality.

It is also preferable to make joints that are not made up of too many different metals, which is why it is very important to know how to recognize the raw materials with which you are working from time to time in order to be able to correctly associate them and identify the most correct processes to apply.

But even a perfectly balanced process can be nullified by defects or impurities present in the basic materials and it is therefore essential to keep the quality of the basic elements under constant control to obtain a successful soldering process.

The physical phenomena that allow the bond between metals are in fact capillary penetration, wetting and atomic diffusion of the alloy in the piece to be soldered. For this reason, even the conditions in which the surfaces to solder (components and PCB) are found are decisive for the success of the process.

To achieve a solid union between the metals that guarantees mechanical seal and electrical conductivity, not only the compatibility between the metals is needed but also the absence of impurities such as oxidation or pollution of the surfaces which, if present, would interfere between the metal bonds preventing correct formation of well soldered joints.

There would be a lot to write about on the types of defects that can be originated from incompatibility or pollution, but an in-depth study on soldering defects deserves a separate article.

Selection and control of the basic materials

So far we have understood that the three fundamental elements of electronic soldering are: the soldering alloy, the electronic components and the PCB. For these it is necessary to establish a system of selection and control so that the process can start from solid foundations.

The alloys and its activators, the fluxes, should be selected – after careful evaluation of the machines involved and their work cycles – by applying the Test methods suggested by the IPC standards: slump test, solder ball test, copper mirror test … based on the results obtained, it will be possible to identify compatible products that offer us the most suitable performances for the type of electronic boards and processes we deal with, giving us the confidence to move on a well-known ground made of ideal products for our work with  characteristics of which we are very familiar.

Once selected and validated, the chemical products must be constantly incoming checked according to a dedicated control plan so as to be able to count on a stable performance and avoid unexpected drifts.

PCBs and electronic components also need to be kept under control. Requirements must be established a priori and, when it is possible to intervene in the procurement process, we request tests and measures from our suppliers that demonstrate compliance with these requirements.

When we work on materials purchased or selected by others, we need to be able to evaluate the fundamental characteristics during the incoming checks and communicate with those who supplied them to resolve any doubts and limit the possibility of variables during the production process.

All the fundamental elements of the processing must therefore be treated in incoming with particular attention, for each there must be a dedicated control plan and the operators who deal with it must have specific training.

From materials to processes

Once we have made sure we have the base materials under control, it is time to focus on the soldering processes.

There are two different types:

Manual
made manually by an operator who positions and solderssolder one component at a time using a soldering iron and the soldering wire.

In this case, the realization of the optimal solder joint is determined by the compatibility between components, wire and fluxes and by the knowledge of the operator who must be able to activate the chemistry in the correct times, set the right temperatures for the type of soldering he is carrying out and maintain the correct times in the various steps.

This requires professional training of the operators and the presence of specific company procedures such as BPS for the various types of soldering to be carried out.

Automatic
these instead are performed with machinery and allow us to place and solder all the components mounted on the board and put even more boards together at the same time.

The automatic assemblies are then distinguished according to the type of components to be assembled: for the SMDs that are mounted resting on the surface of the printed circuit board, the alloy is in the form of solder paste and the process involves screen printing, Pick & Place assembly, reflow and control; for THDs that instead go through the PCB from one side to the other, the components are assembled by machinery or by operators on the line and the soldering takes place on the wave soldering machine loaded with solder bars that liquefy in the welding pot.

The thermal profile

Like all the metallurgical processes, soft soldering takes place during a thermal cycle governed by a curve that describes a temperature variation over a certain time interval and is called the thermal profile.

Its phases are: pre-heating, reflow and cooling and each of them is essential for the control of the process and therefore for the final result.

The pre-heating phase is very important to prepare the surfaces by heating them progressively avoiding subjecting the materials to excessive thermal stress that could cause mechanical damage such as delamination and breakage of PCBs and components, and it is also essential because it is the phase in which the fluxes are activated.

They are in fact inactive at room temperature but are activated in the first heating phase and prepare the surfaces by deoxidizing and activating them so that they can receive the filler alloy capillary.

The reflow phase is the crucial one in which all the parts must arrive together in the liquid state in order to merge and create a new bond.

The correct melting temperatures and the appropriate residence time in liquid allow for pure joints with a thin intermetallic thickness.

The smaller the intermetallic compound – that is the line in which the different alloys join to form a new bond – the more reliable the solder joint will be.

The cooling phase is also very important because a balanced curve allows the different metal elements to progressively return to the solid state, stabilizing the new bonds in a harmonious way and also serves to bring the entire product back to room temperature gradually, preventing related mechanical shocks. to thermal expansion.

Using a metaphor we can say that the welding profile is therefore the “magic recipe” for obtaining excellent soldering joints from carefully selected and controlled ingredients.

Setting out a profile: a question of harmony

Characterizing a profile requires the evaluation of different parameters: first of all the basic materials used since each of them has different melting temperatures, then the metallic masses of the components and the layout of the PCB which are heat absorption elements, finally the techniques of assembly used, the components layout on the board and other aspects that each time may affect the product or the process.

Everything contributes to the success of a very delicate process in which we often find ourselves working in non-optimal conditions because, even if the raw materials have been selected with the utmost care, an electronic board can have many variables such as uneven mass zones, large components. close to very small components with different thermal absorption and, perhaps, due to project needs, components with different finishes which therefore have reflowing times that are not always synchronized. For this reason the profile must take into account all these variables and harmonize them.

Furthermore, the profile varies according to the equipment and machinery used for the thermal process:either hot air or vapor phase oven for surface assembly (SMT), or wave soldering machine for through hole assembly (THT) and each of these needs different set parameters to create the desired thermal curve.

There are also profiles suitable for rework operations such as removing components or reballing. For each need it is a question of analyzing the components, materials and objectives and setting the optimal parameters in line with the technologies used.

A few words about the vapor phase oven

For years our Company has chosen to favor vapor phase technology for SMT assemblies because the vapor phase oven, if correctly integrated into the production cycle with the right choice of basic materials, equipment and set ups, allows for excellent reflow processes with more homogeneous and controlled heat transfers compared to air ovens, minimizing mechanical stress and overheating phenomena.

With the vacuum function, which can be activated in the liquid phase, it is also possible to eliminate the air residues trapped under the components during the assembly processes and therefore have exceptional results in terms of reducing voids especially in flat pack components such as QFN , D-pack, LCC e.c. who are typically afflicted with this problem due to their format.

In addition, the VP oven naturally works in an atmosphere completely devoid of oxygen providing the optimal conditions for obtaining reliable soldering joints, with thin intermetallic compounds and therefore pure and long-lasting.