The PCB Designers Guide to Solder Defects
During the inspection process you notice that some passive components aren’t sitting quite right. It seems as though they have pivoted up to stand at attention on one of the two pads that they should be connected to. Unfortunately, a test of the circuit will result in an open.
The resistor stands there like a sentinel, or, aptly, a tombstone to mark the spot of the dead connection. I use the term “resistor” but capacitors and other parts are also candidates. Larger components have the weight to overcome the capillary action of the solder in its liquid state.
Every solder joint goes through its own moment of truth. That moment starts when solder paste reaches a temperature where it acts like a liquid and ends when it freezes into solid metal.
Equal Geometry for Equilibrium in the Oven
Our footprints are typically created with perfect symmetry. The pads are identical and set evenly around the component leads. The pick and place machine, or the human, does a good job of putting the pieces exactly where they go.
Even if the part is not placed exactly right, a well-controlled solder process will have the effect of centering the part between the pads while the solder is in the liquid state. Each pad exerts a capillary force on the lead or ball that it contacts so even if we’re a little off on the placement, our components will self-center themselves under the right conditions. This same capillary action will be the thing that causes the open circuit by lifting the component from one of its pads when the conditions are unfavorable.
That’s where the panic sets in at the assembly house. The assembler has to create the right conditions for 100% of the solder joints going through the SMT line that day and every other day. The PCB design can help or hinder based on a few parameters.
- Solder volume - How much paste is deposited on the board determined by the stencil design.
- PCB Thermal Load - Pad geometry and whether copper is flooded or has thermal relief.
- Solder Paste Composition - This isn’t a board design thing but we are often expected to provide some detailed notes on the assembly drawing.
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Frontier Rigid-Flex Printed Circuit Board - Rigid-FPC CMO&CT
6 年Quality is designed.
Engineering Manager at Nortec Solutions Ltd.
6 年Great little article to outline a few of the DFM sins I see daily. Involving your CEM (Manufacturer) at an early stage of design is beneficial all around. Although, I dare say the majority of this can still be eradicated with process management rather than re-design of artwork.?
General Manager at Oritech Pty Ltd
6 年Thanks for that! It's a very important topic that needs more attention.