Pin-in-Paste Reflow Calculations and Special Cases

The major concern of the pin-in-paste reflow process is the quality of the solder joint. It must meet IPC610 standard. The criteria for IPC610, class 2 are:

1. Both sides of the wire leads must obtain at least a 75% solder coverage

2. Air cavities should be less than 30%

3. The surface of the drilling as well as the component’s lead must have 100% wetting.

To meet the above requirements, the following steps need to be considered:

1. Solder alloy volume calculation

The volume between the hole and pin (Vsolder >= V hole + V fillet – V pin) should be filled up by the solder alloy with good fillet.

2. Minimum applied paste volume calculation

Solder paste contains about 50% of metal material and 50% additional material (flux) which oozes out during the reflow process. So the minimum paste volume requirement is:
V paste = 2 x Vsolder = 2 x (Vhole + V fillet – Vpin)

3. Stencil aperture oversize design

The area of the aperture is calculated as follows:
Apaste = Vpaste / Stencil (where the Stencil = the stencil thickness.)
The Stencil aperture size is calculated as follows:
If it is a circle, then the Dstencil = 2 x SQRT(Apaste / ? ) , where Dstencil is aperture diameter.
If it is square, then the Lstencil = SQRT(Apaste ), where the Lstencil is the aperture length.
If it is rectangle, then Apaste = Lstencil x Wstencil, where the Lstencil is the aperture length and Wstencil is its width.
In most of situations, the aperture size will be bigger than pad’s size. This means that the solder paste will cover or extend over the entire hole.

4. Double Stroke: Special Paste Printing

To get enough paste into the hole, double stroke is an acceptable process when the right pressure is applied. The squeegee print speed should be adjusted at a slower rate to let the paste fill the apertures more completely. This causes more solder paste to be deposited.

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