Choosing the right selective soldering process for my end product?

It depends on many different factors:

Components used ( SMD/THT?), number of assemblies and solder joints, cycle times, PCB design and number of changes of soldering profile and finally the budget. These parameters are looked at in an evaluation, which takes place in the EUTECT technical centre. All wave and nozzle soldering processes as well as the laser, thermode, induction, piston soldering and laser knife processes can be used at this facility. In accordance with the customer’s specifications and economic requirements, we develop solutions which are refined in further steps with all necessary pre- and post-processing. Starting with the machine cell, kinematics, fluxing and quality control, highly efficient systems are created. The modules that are optimal for the task in terms of process technology and economy are selected and combined to form proven stand-alone, rotary transfer or inline production concepts.

Do you have too little flux on the solder joint but too much on the circuit board?

Always check, even when fluxing

Often the devil is in the details when it comes to precision fluxing. If you can adjust the flux application that applies the flux to the board, you can control the result quite accurately, even if the flux is applied quite quickly. New smartphones, in combination with a flashlight, are able to record videos at 60fps (frames per second). When a flashlight is used to illuminate the flux stream, you can detect it quite easily and see where it is applied.

In order to keep the assembly clean, you can also hold a thermal paper or sandwich paper in between. This paper clearly shows the direction of the flux stream. A small glass panel can also be used.

The positions can then be optimized and the performance parameters of the dispenser should be adjusted. This is done via the software of the machines. The ratio of speed, time and power of the dispenser is important. The simplest way to control the flux is the automatic module for flux stream control, which under certain circumstances can also be retrofitted in an Eutect systems.

How can voids be avoided when soldering THT components?

There are several reasons why voids can occur when soldering THT components.

One reason could be that the solder pin including the solder pin casings seal the hole where the pin is placed tightly from above and thus prevent the air from escaping upwards from the solder joint. In this case it is possible that the hole diameter is too narrow for the pin and that the pin has to be adjusted during a product review.

Another reason may be a steam cushion created by the flux. This cushion creates an increased vapor pressure which vents through the solder joint. This creates cavities and crater-shaped defects in the surrounding solder-meniscus.

Vapor pressure can also be generated in the circuit board if the moisture in the circuit board forms a vapor cushion during the soldering process and expands in the form of a void at the solder joint. For this reason, printed circuit boards should be dried for about 1 to 2 hours at 80 to 100°C after being manufactured.

A further cause of thread voids is the different thermal masses of the components involved in the area of the solder solidification zones during cooling.

Depending on the size of the defect, the volume of the solder joint and the location where it occurs, it has different effects on the service life.

In principle, however, it can be said that a few measures should always be kept in mind:

  • The components and the PCB environment should be thoroughly cleaned.
  • Hence, the cleanliness of the PCB and the components must be checked before the manufacturing process.
  • The same applies to wettability.
  • Furthermore, general cleanliness should be ensured, for example by wearing gloves when handling PCBs and components.

Do you have solder ball formations on the wire or the solder joint when laser soldering?

For a clean, reproducible laser soldering process, a perfect solder pattern is essential. Solder ball formation during the soldering process must be avoided at all costs, as they can cause short circuits on the assembly or the amount of solder applied is not reproducible. Particularly in the case of tight packaging densities, it is essential to ensure that the soldering process is as precise as possible. If solder balls occur nevertheless, they must be eliminated. Please consider the following points:

Have you chosen the correct angle between solder wire and the angle of incidence of the laser?

If not, readjust the angle.

Can unwanted reflections be caused by product or laser head positioning?

Ich bin ein Textblock. Klicken Sie auf den Bearbeiten Button um diesen Text zu ändern. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.

Are your positioning and the conveying the lengths of the wire correctly? Do you use the correct wire speeds when feeding, during the process and during wire withdrawal?

If not, adjust the speed to avoid the formation of balls. Wire withdrawal is particularly important here, as it guarantees a reproducible process. Read more about the SWF from EUTECT.

Is preheating of assemblies unnecessary stress for components?

Every preheating process has to be physical-chemical tested in coordination with the adjacent components, soldering surfaces, heat sinks, hole fill and quality requirements for the process, since every heating process can trigger an additional aging process for the components. Therefore, it is as important to pay attention to this process as to the actual soldering process. For this reason, preheating is always part of our evaluation for customer projects.

Depending on the duration and level of the preheating, three-dimensional mechanical stress levels, oxidation surfaces and thus possible damage to the processing area may occur in the assembly. Preheating prepares the area to be soldered for the actual soldering process and thus for the temperature increase from room temperature to 120 °C and for the soldering process temperature of, for example, 300 °C.

Does too high preheating temperature affect the flux?

In order to protect the components, the typical preheating temperature gradient of approx. 3°C /sec must be maintained. Furthermore, the peaks of the preheating temperature should not destroy the flux characteristics. Only the balanced interaction of processing know-how, flux preheating-soldering processing with the component heat characteristic reduces or prevents unnecessary stress factors on the entire component.

What are the effects of a too long preheating period?

An excessively long preheating time reduces the response of the solids in the flux and thus reduces the overall effect of the flux process. Increased stress due to preheating can also occur via thermal preprocesses such as reflow soldering.

If you have further questions about preheating your assembly, please contact us and we will help you to optimize your soldering process.

Does your laser soldering cause burns on the assembly?

Many factors and parameters have to be considered with regard to burns during laser soldering, ranging from the positioning, the selection of the correct focus diameter, the optimal solder wire, the process surfaces of the joints to other parameters.

Some basic settings, however, prevent burns on the assembly surface:

Is the laser’s impact angle selected correctly? 

If not, it should be adjusted to prevent burns from reflections. Please note the old physical principle of angle of incidence = angle of reflection.

Is the diameter of your laser focus chosen for optimum results? 

It may be necessary to focus with greater precision or to defocus. Especially for strands, PCB or flex foils several approaches are available. Feel free to contact us.

Is the solder in the form of preforms, wires or paste in the right place at the right time and available? 

If this is not the case, readjustments should be made.

Is the axis, laser, solder, product or assembly positioning replicable?

Achieving this requires the integration of optical position correction.

Are the surface characteristics of the joints or the process area reproducible or possibly contaminated?

In this case you should contact your suppliers or ensure a cleane surface.

Have you selected the correct emission level in your pyrometer temperature control?

These parameters can be controlled in the machine settings.

Is your laser and pyrometer calibrated and or checked on a regular basis?

If you are not able to do this yourself, a service visit could be considered.

Do you work with the correct wire feed and pullback speeds and distances? Do you use the optimum solder wire diameter? Does your solder wire have the right flux?

These parameters are defined as part of our process evaluation. If these are no longer correct or if an evaluation has never been carried out (external systems), please contact the manufacturer’s service department. EUTECT also conducts this work on machines from other manufacturers, if this is requested by the customer.

For further, more detailed information or active support on-site, please contact us, we will help you to avoid burns on the assembly.

Do you desolder your SMD-components during selective soldering?

How to minimize the risk of desoldering SMD-components?

If this is the case, the heat input for the solder joints of the SMD-components positioned in the proximity of the THT component is usually too high. These SMD-components must be protected from excessive heat with a mask, unless a layout adjustment of the assembly is possible. Especially with high packing densities and increasing miniaturization, desoldering can be a challenge.

What to consider when using a solder mask?

However, the use of solder masks requires that some factors be taken into account already during the design of the PCB:

Ensure your THT solder joint is surrounded with at least 1.5 mm space and that high components are not positioned in the immediate vicinity. With the optimal solder mask, you can achieve a process window with maximum reproducibility. However, if you have unchangeable and narrow spaces, please do not hesitate to contact us, because we have a wealth of experience and special solutions for various problems.

Which solder mask types are available ?

There are many different types of solder masks for selective soldering. These can be manufactured in different dimensions and from different materials, additional these masks can also be used for large wave soldering. There are specific soldering masks for the selective soldering process only, that perform in combination with the right material and the appropriate soldering nozzle. In addition, masks can also be used for laser and induction soldering as well as piston soldering. In this case, they also protect the PCBA and its surroundings from reflections as well as solder and flux splashes.

For further information and your individual solder mask do not hesitate to contact us.

Lacking hole fill during THT-soldering?

What options support hole fill during your soldering process?

With THT-soldering it can happen that the hole fill does not form properly. In this case the solder does not completely perforate the hole and the counterminiscus is not formed 100%.

First check that the diameter of the hole in relation to the pin ratio and the joint geometry are correctly selected. If this is the case, you have the following options to optimize the process:

Mini wave or selective soldering:

  • Increase the temperature of the solder bath and check for the desired temperature at the process point.
  • Increase contact time between mini-wave and product
  • Increase preheating temperature, activation or preheating time
  • Check the optimal flux quantity
  • Verify that the correct flux is used in accordance to solids and other activators
  • More about the EUTECT IW1 & 2 module

Laser solderings

  • Use of a preheater to apply heat to the joints, thus facilitating hole fill
  • Position of the laser focus in the right angle and in relation to the product geometry, solder feed and overall product geometry
  • Test feeding speed of the solder wire
  • Additional energy input after fusing of wire
  • Test the optimal flux content and the correct alloy in the solder wire
  • More about EUTECT laser soldering


If you still have problems with hole fill, please contact us, we will help you to optimize your soldering process.

Wire bend? Problems with piston or laser soldering?

Are you busy troubleshooting the wiring clutter in your piston, induction or laser soldering automation daily?

Is your soldering process inaccurate due to wire feed, not reproducible or do you solder “antennas”?

Try to align your energy source, does not matter if soldering iron, induction outer circuit or laser focus, in such a way that the energy acts on the one hand in the joint partner with the highest heatsink and on the other hand in the solder wire. This allows a faster fusing of the solder wire and thus a thermal bridge between the connected partners and therefore allows for a better solder joint formation.

Please note the adjustable feed, process and withdrawal speeds of the solder wire, these three variables offer significant optimization possibilities.

If this does not help or if you want to optimize your takt time and intermetallic phase further, try pre-tinning the joints and then feed them into the soldering process with filler wire, paste or preform.

Would you like to know what your wire feed really did in the soldering process?

If you want to get data regarding your soldering process to know exactly what happens during the soldering process, we recommend you contact us regarding our SWF wire feed module – it’s worth it – it’s the world’s only “actio-reactio” based and power controlled wire feed in the world.

> More about the EUTECT SWF Module

As there are many different solder wire applications worldwide, there are also many different challenges for which we have developed our own solutions. Contact us and we will help you to optimize your soldering process.

Do you want to prevent solder beads?

Do solder beads form during your selective or wave soldering process?

Do you apply the right amount of flux?

Check the amount of flux applied. Place or glue litmus paper, simple fax paper or thermal paper in your product tray or on your product and see how much flux really arrives at the process point.

If possible, reduce the amount of flux to be applied.

Do you preheat your product to activate the flux?

If so, is it the right duration with the right temperature gradient? Implement trackable thermo elements and see what really happens with your product.

How quickly do you dip your product into and out of the selective wave?

Optimize the process depending on the geometry and energy requirements of your product.

Does your selective solder wave have a defined and rising bead preventing solder return as well as warm nitrogen gassing?

If all this does not help, we recommend our → brush module with adjustable speed, brush monitoring, ionization device, suction and much more.