Electronic boards: what does High Reliability Soldering mean?

Written by ME Electronics

March 12, 2020

The reliability of the soldering joints is the central theme, one could even say the compass, that has guided the development of our Company over the years. The passion for the processes that are the basis of the work we do was in fact the driving force that led the corporate Governance to make very specific choices to reach levels of excellence in our core business: electronic micro-soldering

In the field of electronic board production, reliability is crucial every time the product is designed to ensure constant performance over time even in critical situations.

There are many areas in which reliability proves to be an added value. In fact, if it is obviously vital, for example, for products that fly, which manage safety equipment in transport or for prototypes destined for large science experiments, reliability can be a great factor of competitiveness even in industrial applications. where it becomes strategic to limit the costs of returns and assistance, for the satisfaction of the products marketed and therefore for customer loyalty in times in which reliability is an important discriminant between competitors.

Furthermore, a partner who knows how to produce reliable soldered equipment tends to have a systematic and efficient approach in all his business processes because he is used to “dealing with special processes”.

In the field of electronic board production, reliability is crucial every time the product is designed to ensure constant performance over time even in critical situations.

A special process

The soldering process – not only in electronics but in all areas – is defined as “special” because it is one of those industrial processes whose results cannot be fully ascertained through non-destructive visual checks. This means, in practice, that in order to be sure that each soldered joint has been made perfectly, it should be sectioned and analyzed under a scanning microscope each time.

It is clear that this approach is not compatible with production as we would have to destroy every piece produced to verify its conformity.

For this reason, in the presence of special processes, it is necessary to apply a method.

First you need to make sure that the planned production process is truly optimal through a validation that effectively involves the destructive analysis of one or more pieces made as samples.

Once the process that allows the best results has been validated, a control system must therefore be established that guarantees the repeatability of the results achieved on all the pieces subsequently produced with that same production process.

In order to establish an effective control system, there is a lot of work to be done. The first thing is to identify all the fundamental elements to be monitored and then the first step begins with the knowledge of the bases on which the process that you want to control is built.

A metallurgical matter

Soldering in our field is a commonly used term but in reality the metallurgical process used for the realization of electronically soldered joints is technically called “soft soldering”.

Soldering differs from welding because the union between the two metals does not occur by fusion between the two but is achieved by adding a filler metal.

The technique used in our sector is called “soft” because it occurs at temperatures below 450 °C and the filler metal is typically an alloy formed by Tin (Sn) together with one or more different metals such as: Lead (Pb ); Copper (Cu); Silver (Ag) and, more rarely, others.

The two metal parts to be bonded with the filler metal are on the one hand the terminals of the electronic components and on the other their landings in the printed circuit board commonly called “pads” and each of these parts is in turn made with different metals.

The finishing of the electronic components

Electronic components are now built in many types, footprints and sizes to meet the most varied needs but each of them has metal terminals suitable for soldering on the printed circuit board in surface mount (SMD) or through metalized holes (THD).

Depending on their shape and size, the electronic components have metal leads built from a good conductor such as copper which, however, has the problem of oxidation which would make its surface non solderable in a short time and is therefore covered with a thin layer of another metal that preserves its solderability such as tin, silver or gold that are deposited there with various techniques: electrolysis, immersion and others. A particular type of SMD components are the BGA (Ball Grid Array) and CGA (Column Grid Array) which instead of having external leads have spheres or columns under their metal or ceramic body that are entirely made of solder alloy.

These spheres or columns literally merge with the solder paste deposited on the landings and form a unique soldered joint that runs from the bottom surface of the component to the circuit board seamlessly.

The “landings” of the printed circuit board

On the side of the printed circuit board or PCB, the area immediately affected by the solder joint is the part of the conductor designed to house the metal lead of the component which is therefore made in a suitable form and left free from the protective cover of the solder mask.

These spaces are commonly called “landings” and are made of copper which, in order not to suffer from the oxidation by the oxygen present in the air, are covered with a thin layer of other metals that maintain good solderability such as Nickel / Gold , Silver or the same Tin alloy asof the filler metal which can be with Lead or without in case the process is LeadFree (RoHS compliant).

The alloys superimposed on the copper to keep it solderable are deposited there through various chemical processes such as: immersion, redox, galvanic deposition etc … and each of these processes brings with it potential technological criticalities and chemical impurities, therefore knowing the construction processes of a printed circuit board is fundamental but the discussion on the PCB would be very broad and deserves to be deepened in a dedicated article because the constructive implications go far beyond the soldering pads alone: ​​every part of the circuit, in fact, starting from the raw materials and through the countless processes that make it possible to make it, is crucial to obtain reliable electronic boards over time.

The circuits in fact are the basis on which all the work of assembly is built and knowing how to evaluate and treat them is of fundamental importance to have constant and reliable soldering processes.

Electronic components are now built in many types, footprints and sizes to meet the most varied needs but each of them has metal terminals suitable for soldering on the printed circuit board in surface mount (SMD) or through metalized holes (THD)

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:

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.


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.

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