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Double sided PCB Reflow Soldering

Double-sided PCBA (Printed Circuit Board Assembly) are becoming more and more popular and getting complex day by day. Due to demand for compact size and lite weight of electronic product like camera, smartphone, tablets, and laptops it forced to the electronic product Designers to designers to dense and double side mountable PCB Assembly.  The main advantages of double-sided PCB Assembly are PCB space savings and lower product costs as compared to the single side PCB assembly.

PCB Assembly Process Flow:-

In every Printed circuit board Assembly certain set of instruction and process are followed. Normally the fabrication of electronic component was done through robot bases fully automatics machines. The cost of one line set up is may up to 10 cores bases of machine selections. The whole Printed Circuit board assembly process can be divided into below steps. When single side assembly completed the same process was followed for the secondary side fabrication process and all steps are replicated to produce double side assembled the board. Now come to the point, during the double-sided assembly process the board was passed through the reflow oven on the almost same set of temperature settings that were used in first-time reflow soldering process. 

Even after during second-time reflow soldering temperature excedding melting point of solder paste, the components that are hanging opposite to PCB Surface remain stick-on PCB Surface and do not falling off in reflow oven.

                     Assembled Printed Circuit Board

The reason of that can be described as during double-sided assemblies, the solder joints on the top side of the PCB board are inverted and reflowed again. During the second reflow time, the temperature inside the oven reaches more than the melting points of the solder paste. But still, components are held in place and do not fall off due to the surface tension, that helps to prevent the components falling off due to the gravitational force.

In other simple words, we can say that the flex was already evaporated due to high temperature during the first time reflow soldering process. Even getting the higher temperature more than the melting point of solder paste the components still having the sufficient strength and the holding force to hold and prevent them falling off into the reflow oven.

In some cases during reflowing the secondary  side of a double-sided printed circuit board assembly, it was found  that once the solder is molten, the component may fall off from the weakest of below three locations on PCB Assembly:

  • The point of contact between the molten solder Joint and the pad on the PCB board side.
  • The joint place or point of contact between the molten solder paste and the pad on the component pad side.
  • Within the molten solder paste particles.

Now we can understand  that, if the molten solder Paste should wet the pad, the adhesive force at the liquid-solid interface is strong than the cohesive force (within the molten solder paste), so the molten solder should be the weakest location and the component may fall off within the molten solder during the second reflow passing through.

Below actions are recommended can be used to overcome components failing off problem during double-side reflow soldering 

  • Apply some adhesive glue on required particular specific components location.
  • Re Fine tune the reflow profile temperature settings and if possible try to reduce the temperature of second reflow profile setting within the window recommended by solder paste supplier.
  • request your design team to Redesign the PCB pads or modify stencil apertures of affected slots to get better results.
  • Re-Check the component quality of view of designing parameters and pads for an oxidation related issue and if the possible component of the different batch or manufactures.

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