I'm attempting to develop paste in hole process so that I can reflow a few plated through hole components on a mixed tech board. Tried various pastes, profiles and stencil thicknesses. Would appreciate any insights and/or pointers anyone can share on the process.
The typical PIP process is incorporated into the surface mount process. This approach requires the following:
The stencil will be designed so that the correct amount of solder paste can be printed onto the through-holes to form an acceptable joint after reflow. The solder paste will be amenable to the PIP process. The stencil printing process should be optimized. Through-hole components will be selected so they can withstand reflow soldering temperatures. Leads should be rounded at the end and be the correct length to form a good solder joint. Through-hole components will be mounted correctly. The reflow profile will be acceptable.
The stencil must deliver the correct amount of solder paste to the through-hole during the stencil printing process. To achieve this goal, the amount of solder needed must be determined.
The volume of solder in the through-hole solder joint, VSJ equals the volume of the through-hole, Vh, plus the volume of the fillet, Vf, minus the volume of the pin, Vp. The Pappus-Guldin theorem for volumes of revolution determines the volume of the fillet. Therefore, the solder joint volume is:
Equation 1: VSJ = Vh + Vf - Vp
The amount of solder paste to be printed, VSP, is not the same as solder joint volume, because solder paste is only about 50 percent solder by volume. So the solder paste needed is:
Equation 2: VSP = VSJ / F. (F is the solder paste reduction factor, which is typically between 0.45 and 0.55.)
The stencil aperture must be designed so that it provides the needed solder paste volume. If the aperture is circular, the solder paste volume will be:
Equation 3: VSP = pr2 t
Where r is the aperture radius and t is the stencil thickness, the aperture radius and stencil thickness must be such that Equation 3 is satisfied.
If the aperture is rectangular, the equation for solder paste volume becomes:
Equation 4: VSP = l w t
Aperture length is l, and w is its width.
In some cases one stencil thickness cannot satisfy the needs for PIP and other components. It may be necessary to print twice with two separate stencils or use a step stencil. Type 3 solder powder is acceptable for PIP.For PIP to be successful, the stencil printing process requires a delicate balance between the needs for surface mount and through-hole components. The stencil printer should deposit a sufficient amount of solder paste on the through-hole pads without printing an excess amount on the fine-pitch surface mount pads. Although the printer parameters are process-dependent, some important variables include:
Pressure: As the squeegee blade pressure is increased, more solder paste is forced into the hole. The more solder paste that is deposited, the more robust the through-hole joint will be. If the pressure is increased too much, bridging will occur on fine pitch surface mount pads. In addition, higher pressure will require the bottom of the stencil to be wiped more frequently.
Print Speed: The squeegee print speed should be adjusted in conjunction with the squeegee pressure. A slower print speed will fill the apertures more completely, which results in more solder paste being deposited.
The right amount of solder paste must remain in the holes to form good solder joints. Therefore, the component pins must not be allowed to push the solder paste completely out of the through-hole. To avoid this problem, component leads should not protrude more than 60 mils from the bottom of the PCB. The leads should also be rounded. Rounded leads minimize solder paste "push through."