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July 10, 2019

How Do You Calculate Solder Paste Volume?

I have a connector with 6 rectangular mounting tabs. The tabs are 1.5 mm x 0.6 mm.

How can I calculate the recommended volume of solder paste for these mounting tabs?

F.G.

Expert Panel Responses

The issue that makes this complicated is the coplanarity variations between the tabs on the connector and the PCB itself. If three tabs are touching the board the other three tabs will not in general be touching the PCB pads uniformly. This tilt makes it difficult to determine the volume of solder that will end up under each tab making the calculation challenging and specific to each connector tab-PCB pad pair.

The reality is that the fillets that will be formed between the pads of the board and the tabs will be the reservoir of solder that will take up the excess when the pad and tab match perfectly and will be consumed if the tab is hovering above the pad.

I have two recommendations:

1) If the component was standard or close to some standard component dimensions, you could look up the recommended solder volumes in the IPC standards as they provide standard solder paste volumes for most components. Since this is a connector, he might be able to find a QFP with similar pad geometry.

2) The approach is to calculate the fillet volume and dispense enough solder paste to create the maximum allowable fillet without bridging and let as much of it disappear under the tab when the component is reflowed. You’ll need to model the volume of the desired fillet and multiply the solder paste volume by 2 for screen printing or 2.5 for dispensable paste. Most solder paste is close to 40% solids by volume for dispensable paste and 50% volume for screen print paste. This means that during reflow, the volume of the paste will drop by a factor of 2 or 2.5.

Since this is a connector, it might need additional joint strength. There is little downside in putting too much paste on the pad as long as there is no bridging between the pads. Unless there is little design margin in the connector strength, most surface mount solder paste applications are insensitive to volume variation and have large margin on electrical connectivity and mechanical strength.

An additional recommended resource is to contact Nordson ASYMTEK who have extensive dispensing experience with non-coplanar devices.

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Carlos Bouras
General Manager
Nordson SELECT
Carlos Bouras is the General Manager of Nordson SELECT and has over 30 years of experience in the electronics manufacturing industry. Carlos's expertise is in process engineering, product development and manufacturing operations. For the past 15 years Carlos has focused specifically on automated assembly issues and is the holder of several US patents for non-contact dispensing and precision dispensing of adhesives for the packaging of microprocessor devices.