Sense and risks regarding topside preheating of PCBs during (Selective) wave- or dip soldering.
Introduction
If a PCB design shows lack of thermal solderability, often the question is raised if topside preheating will help to improve solder hole filling. For the solder process as such this seems a good solution, but for the PCB the practical realization may create a bigger quality problem than the not completely filled holes. Aspects related to topside preheating will be elucidated next.
Topside preheating using quartz-elements
A preheating system for the topside of the PCB should be a fast reacting flexible system. For such a system IR-heaters seems to be the best solution, because they are fast and easy to control.
This system has however also big disadvantages. The temperature and energy that is transferred is higher if we are closer to the elements. This means that high components on a PCB get more heat energy transferred than the PCB surface that is at a greater distance. Fast reacting elements are short-wave elements. The energy transfer for these elements is very much depending on the color of the objects which are heated and their material constitution.
Materials that are transparent for the wavelengths of < 1.5 micrometer, such as the epoxy FR4 will not absorb all the emitted energy and are therefore heated not as fast as the standard components on the PCB surface, which are not transparent. Shiny materials will reflect this radiation and so will absorb less heat. Small components will heat-up relatively fast, without a real indication of their final temperature. Although this is an undesirable situation, it is part of this topside preheating system when short wave IR-heating is used.
Last but not least; if topside preheating is applied to a PCB using IR-radiation, most of the heat will often be absorbed by the components. This is due to the effect that these components are shielding the PCB part that should be preheated to promote solder filling into the holes underneath these components, like electrolytic capacitors. Topside preheating is actually meant to improve solder hole filling.
Note: The energy transfer by radiation is related to the fourth power of the element- temperature in Kelvin. This means that a small increase in power setting can have a large effect on the final component temperature The final component temperature is a mathematical addition of the preheat temperature and the thermal load during the contact with the molten solder.
Topside preheating using forced air
Preheating with forced air is in general slower than preheating with quartz elements. Another technical drawback of the mechanical construction of such a system isthat the system has often much more mass and volume than a preheater with quartz-elements does. This makes it impossible to implement such a preheating system in a gripper, such as used for Selective Soldering. An advantage of a preheater with forced convection is that the maximum temperature is known and limited. During the preheating cycle no component can get to a higher temperature than the gas temperature. With a quartz preheater, a temperature limit for components can never be guaranteed.
Specific points related to the use of topside preheating in Selective wave soldering
During drag soldering at the SelectWave the parts of the PCB that are not in Contact with the solderwave will cool down during the handling of the PCB. To keep the PCB at a constant preheat temperature to compensate for this cooling, a kind of temperature regulation has to be part of that preheating system, so that the PCB temperature can be controlled and kept as constant as possible. This is of course a legitimate wish, but the practical realization is not so simple. This we may conclude from the previous statements. Therefore such a system can not been worked out for a general application.
For soldering PCBs with a good thermal design this additional preheating is not necessary. As long as the flux has been pre-dried to its correct temperature, there is for such boards no special need to keep the preheat temperature on the PCB at a constant level before soldering. This assuming that the temperature drop will not be so large that it will affect the soldering quality.
Component temperature during soldering conventional leaded components
For most conventional components it is important to reduce the thermal load as much as possible, to prevent them from thermal damage. This means that one always must check if all components on the board can withstand the soldering treatment, without being damaged or reduced in reliability.
A standard tests for checking the components for their resistance to soldering heat, is the IEC 68-2-20 test Tb. This test does however not take into account the use of topside preheating. For this the customer should interpret the relevant component data.
Sometimes heat sensitive components can be mounted on a spacer so that the distance between component body and solder joints is increased. This will give a lower inside temperature in the component body during the soldering process. Care must be taken for such components if topside preheating is demanded. We believe that, if components can withstand it, this may give a solution for boards that have no optimal thermal solderability, although bottom-side preheating is often preferred.
However, if a board is well designed and has a good solderability, reliable solder joints will be formed using a dwell time of approx. 3 s at 250°C, using SnPb37 solder. If the design does not make this possible than, as an alternative, it could bea good solution to use a higher preheating setting if the components allow for that. An option might be to increase the solder temperature, or to enlarge the dwell time in the solder wave, if putting more heat into the board by using topside preheating may damage the board.
The reasons are:
1. Any degree of heat that is already in the board before soldering needs not to be added by the wave.
2. Higher solder temperatures gives a faster heat transfer, which helps often better than prolonged dwell times.
3. Conventional components can withstand a resistance to soldering heat for 5 or10 seconds at 260°C if the heat is transferred via the leads and not directly generated to the component body, as topside preheating will do.
Also if these solutions are chosen one has to be sure that the components can withstand such a process. The answer to the question if components will withstand the soldering process without any degradation has to be found in the product documentation from the component supplier.
In such documentation mostly several soldering process profiles, including preheating, are given. However one has to take into account, that often this preheating data are related to the general soldering process for the PCB and not to the component itself.
Conclusions
• The use of topside preheating will sometimes not add so much to the temperature increase at the PCB on the critical solder joints, but may give a substantial increase in temperature of the topside mounted components, often shadowing these spots. This is in most cases an undesirable side effect that has to be taken into account when one wants to use topside preheating.
• One must always control if the critical components are not heated too much or for a too long time, since such a treatment may reduce the lifetime expectations for these components considerably.
• In most cases poor solder joints are formed due to lack of (thermal)solderability of the component leads and / or the board. The solder process cannot improve solder failures that occur due to lack of solderability.