PCB Manufacturer’s Challenges -- In Building Of Lead Free SMT Assembly

- Aug 22, 2017-

SMT Assemblies are getting more and more complicated. While SMT Assembly manufacturers strive for 100% efforts, the fact is that achieving it is really difficult. While a majority of electronics today use SMT Components however the reduced component sizes make putting them onto PCBs very difficult. Other than this, there are many other defects that SMT Assembly has to overcome as follows:

Poor Solder Paste Release
Solder Paste Release is determined by aspect ratio and surface area ratio. Aspect ratio compares the smallest dimension of the stencil aperture to the stencil foil thickness. An aspect ratio of lower than 1.5 is not accepted. Surface Area Ratio compares the surface area of the stencil aperture to the surface area of the stencil aperture walls. The lowest surface area ratio is 0.60. While the aspect ratio and surface area ratio help indicate solder paste release, what is important is the adhesion strength of the solder paste to the SMT pad, which is determined by the size of the SMT Pad. Difference in surface finishes can influence the SMT Pad sizes. To be able to exactly calculate solder paste release a modified surface area ratio principle must be considered which takes into account changes in SMT Pad sizes on account of copper weights and surface treatment. This is getting more and more importance as smaller components become more and more popular. Especially the bottom of the SMT Pad matches the size in the electronic PCB files while the top is smaller. It is this smaller size top that needs to be planed in calculating the stencil surface area ratio as the smaller size top has less surface area.

Bridging at Print
Besides solder paste release, copper weights and surface treatments also effect bridging. Heavy copper weights or non-flat surface finishes lower the seal between the PCB and the stencil. This can in turn allow solder paste to squeeze out during printing and also cause bridging at print. The seal is dependent on the size of the SMT Pad and stencil aperture. Stencil apertures larger than the SMT Pads can cause solder paste to squeeze out between the PCB and the stencil.

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In order to solve this problem, a width reduction when it comes to stencil apertures is requested. This is especially true of heavy copper weights and non-flat PCB surface treatments. This assures that the chances of the solder paste squeezing out between the PCB & stencil is minimized.

Insufficient Solder Volume at SMT reflow
While it is a common defect, it is caught only at the end of the SMT Process during visual or automated optical inspection. A DFM review sometimes can also catch the insufficient volume before production. To overcome this problem, the volume increase is based on the size difference of the leadless termination and PCB Land pad. Also the additional solder paste volume needs to be printed to the toe side on case of leadless components. Also increasing the stencil aperture width needs to be avoided. What is also important to note is the stencil foil thickness. In cases where the foil thickness needs to be adjusted to accommodate SMT components, the stencil aperture volume also needs to be increased.

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Bridging at SMT Reflow
SMT Reflow is caused because solder paste squeezing out between the PCB & stencil at print, at others it is on account of PCB manufacturing issues, placement pressure, reflow over settings etc. Bridging at SMT Reflow can also happen on account of packages as they have component lead exposed to the heating. Leadless packages, on the other hand have uniform heating. Gull wing packages also have limited amount of surface area to wet the solder. In case of too much solder, the excess can spill off on to the PCB Pad. The reduction in solder paste volume, however should always be centered on the gull wing foot and not the PCB Pad. While for most assemblies the volume reductions will reduce .While some SMT defects are limited to a assembly line or location, others such as solder paste release, bridging print, bridging SMT Reflow, insufficient solder volume at SMT Reflow and more mentioned are popular and are not limited to a set of variables.

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