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Wave soldering involves using waves of molten solder that act as glue to attach electrical components such as resistors, capacitors, and LED lights on the PCB. During this process, the electrical components are placed on the PCB, which is subsequently passed across a pumped wave of solder.
To prevent the solder from bridging the connections, a protective coating must be applied. The pouring solder wets the exposed metallic areas of the board and creates a reliable connection between the mechanical and electric components. While this process is expedient and efficient in producing high-quality products, it sometimes experiences glitches that can be prevented or corrected. Our posting explores some of the main issues and how to remedy them.
Pin and Blow Holes
These two types of holes occur when the gas bursts during the soldering process. They are either caused by a single blast or a continuous escape of gas. Blow and pin holes are basically the same, but some engineers use the two terms to distinguish the size of the problem. This issue is corrected by improving the quality of the board with a 25 µm of copper in the hole. It can also be eliminated through baking the board to remove excess moisture, but this solution does not address the root cause of the problem.
Solder Shorts on PCBs
Due to the ever-decreasing component pitches, the occurrence of solder shorts is on the rise. In the past years, the pitch between terminals used to be 0.050 inches; this size has since been reduced to 0.02 inches. This issue occurs when the solder fails to separate from two or more leads before it solidifies. Therefore, reducing the length and size of the pad will go a long way in decreasing the amount of solder that will be held on the board. While this seldom happens, solder short can also occur on the top side of the board. This occurs when too much pressure induces excessive solder penetration.
Poor or Inconsistent Hole Fill
This problem occurs when the solder wets the leads but fails to touch the surfaces of the through-hole. Poor flux application and low pre-heat temperature are the common causes of inconsistent hole fill. This issue is often a natural consequence of a manufacturer's action of changing from foam to spray fluxer, which causes poor flux penetration into the holes. Inconsistent hole fill can be avoided by maintaining the temperature of the printed board between 100–110°C. This is especially true for multilayer and double-sided PCBs. Single boards are processed at a much lower temperature as no solder penetration is required.
Inconsistent hole fill can be avoided by maintaining the temperature of the printed board between 100-110°C. This is especially true for multilayer and double-sided PCBs. Single boards are processed at a much lower temperature as no solder penetration is required.
Lifted components on PCBs occur due to a combination of physical and thermal issues. The heat produced during the soldering process can lower the adhesion of the copper, which makes the board weak. As such, any forceful handling of the board may cause a component lift. This problem can be avoided by using careful handling of the board when removing it from the soldering machine. You could also use epoxy with high thermal resistance and avoid exposing the PCB to excessive heat for a prolonged period.
Wave soldering is an effective method employed when assembling vital electrical components on printed circuit boards. However, sometimes a few problems such as pinholes, inconsistent hole fill, lifted components, and solder shorts might be experienced. By taking a little extra care and diligence, it's quite apparent that these issues can be effectively avoided or corrected.
Neil Blundell is the managing director of Blundell Production Equipment. For more information on Blundell Production Equipment, click here.