The importance of temperature profiling is widely understood in the mass production PCB assembly environment. The slow ramp and preheating phase help to activate flux, prevent thermal shock and improves solder joint quality. However, when it comes to reworking, prototyping or one-off DIY projects, it is easy to forget the significance of the preheating stage which can result in far inferior if not damaged device. So, for such an important step, why is it so often forgotten for the board on the bench? And what are the consequences of omitting this stage?
What is PCB preheating?
PCB reflow ovens are what come to mind when technicians and enthusiasts hear the words temperature profiling or temperature curves. It is easy to visualize the 4 main temperature-controlled zones along the huge length of the oven that ultimately result in perfectly soldered boards, hopefully. Each phase is carefully controlled, refined via technician experience and trial and error and each phase has its role in contributing to solder joint quality and in reducing defects. But other industrial soldering machines may not have such fine temperature control. One thing they have in common though is the preheating phase.
The role of the preheating stage is to steadily ramp up the temperature of the entire assembly from room temperature to a soak temperature that is below the melting point of the solder paste, around 150 C°. The temperature change is regulated to maintain a constant ramp of a few degrees a second. Directly after the preheating phase is the soak phase, which maintains this temperature for a period of time to ensure that the board is heated uniformly. This is then followed by the reflow phase which initiates solder joint formation. During the preheat and soaking phases, volatile solvents in the solder paste are burned off and the solder flux is activated.
Benefits of PCB preheating
Allows outgassing of volatile solvents
One of the primary benefits of preheating is to allow volatile substances in the solder paste to gradually evaporate, or ‘outgas’. If the temperature change is too sudden, the rate of outgassing may cause splattering of the solder paste, causing solder balls to form and reducing the solder volume on the joints. Solder pastes with more solvents may require more time to completely expel these gases, which may validate a longer preheat and soak times.
As anyone who has handled a soldering iron knows, flux is what burns off when the iron is applied and is a crucial component of solder. Flux removes oxides contaminating the surface of metal contacts and effective removal of oxides results in clean strong adhesion and good pad wetting. If the preheat temperature is too low, then this may not effectively activate the flux and result in cold joints and incomplete reflow.
Prevents thermal shock
Possibly the most important purpose of preheating is to increase the temperature of the entire assembly linearly and consistently to the soak temperature. Steady heating reduces thermal stresses in both the actual boards and the components which is crucial for certain packages such as BGAs, and other ICs but SMD chip resistors and capacitors can also be susceptible to thermal shock and may crack as a result. The presence of thermally sensitive components has the biggest influence in determining the preheating rate.
Lowers the reflow temperature
Related to the previous point, preheating and soaking can actually decrease the required reflow temperature and duration after soaking since the entire assembly is at a higher temperature. This reduces the temperature gradient when moving to the reflow temperature as localized temperature differences are minimized. Compare this to soldering a through-hole lead that is connected to a ground plane with a soldering iron. The temperature that has to be applied at that point is much higher to compensate for the heat transfer to the copper plane, and heating a point to higher temperatures for excessive amounts of time risks pad lifting, delamination and burning.
Benchtop preheating methods
On the rework bench and hobbyist’s stations, the luxury of such refined preheating control is often not available or limited. But this can give rise to many problems and defects that are not visible until later in the device’s life and may actually cause more damage. Here we will look at various benchtop PCB rework methods and their disadvantages.
Hot air guns
It is not so uncommon to see technicians using hot air guns to perform localized reflow for jobs such as replacing chips like BGAs. BGA rework is a complex science in itself, but even simpler chips, particularly those with pads under the package body are also highly susceptible to error. Hot air guns apply heat from the top-down, meaning the body of the package is the first to heat up, rather than the board or contacts. The method also relies heavily on the technician’s skill to ramp up the temperature gradually without causing too much thermal stress. Overly cautious technicians may not achieve adequate temperatures for reflow and too much could cause cracking or internal damage to the part.
Those with access to a preheating oven benefit from the uniform temperature across the top and bottom sides of the board however, the temperature begins to fall once it is removed. The technician is therefore on a time limit and these pieces of apparatus are often not affordable or viable for the average enthusiast.
IR preheaters, such as the one found in the selective soldering machine pictured earlier, use an array of coils to radiate heat towards the boards from below. Benchtop versions also exist, however these tend to be very large and expensive and temperature control is often inconsistent; forcing operators to rely on guesswork and experience. Technicians also have to work under the constant glare of the IR coils, causing eye strain and fatigue.
Hot plate heaters
Hot plate heaters offer efficient heating capabilities by conducting heat uniformly to the bottom of the PCB board, which favors heating of the pins rather than the body of the part. However, this is only effective if the board assembly is single-sided. Nowadays purely single-sided boards are increasingly rare and these large devices are also not convenient for the hobbyist. Since the large plate also stores heat even when turned off, delayed cooling may also affect solder joint quality.
As we have seen, the lack of preheating can be catastrophic for a board being manually soldered or desoldered and traditional methods have their disadvantages. Luckily preheating solutions are becoming increasingly more innovative and accessible. One such product to hit the market in recent times is the Mini Hot Plate Preheater – MHP30 now available on Seeed Bazaar.
This mini hot plate is designed for spot soldering and desoldering tasks in a pocket-sized device. The MHP30 consists of a tiny 3 x 3cm nano-ceramic coated brass plate but the small size packs a punch, capable of heating up to 300C° in 150 seconds. Unlike larger full board plate heaters, the smaller size allows for localized work where the boards do not have to be completely single-sided and the localized heating minimizes cooling issues. In addition, the energy consumption and cost are a fraction of that of other preheaters.
The MHP30 also packs a variety of modern features including digital temperature control, OLED display, LED temperature indicators, auto-sleep mode, topple protection and quick swap-in replaceable plates, not to mention the stylish aluminum shell design. At just $69.90, it is a welcome addition to the bench of professionals and enthusiasts alike.
Leave PCB Assembly to the professionals
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