2- Electronic assembly technologies

The point of PIP soldering technology (in other name "intrusive reflow") is that the assembly of through hole components is carried out by reflow technology, together with surface mount components. Thus the assembly steps of PIP technology are the same as Surface Mount Technology. The first step is the paste deposition, which can be done by dispensing or stencil printing (Fig. 1-1). The second step is the component placement, the leads are inserted into the plated holes and the SMD components onto the pads (Fig. 1-2), and finally the third step is the reflow process of the solder paste, in a reflow oven (Fig. 1- 3). During the reflow oven profiling, make sure that the temperature of solder on the bottom side exceeds the melting temperature in the peak zone as well as on the top side, in order to form good solder fillets on the bottom side as well. In PIP technology the through hole components must meet the following requirements: where shrinkage factor of solder paste – usually S=0.5, therefore 1/S=2. While calculating the volume of the plated hole, it should be taken into account that the diameter of the drilled hole decreases by about 100 μm after plating.

For calculating the volume of the solder meniscus, suppose that the solder wets the lead of the component to the same height as the distance between the end of the pad and the lead. Thus the area of the meniscus cross section:

The distance of the meniscus gravitational center from the center of the lead:

where a: the radius of the component lead. Assuming that the meniscus is a body of revolution, its circumference is:

Therefore the volume of the meniscus:

Thus the required volume of solder paste can be determined:

If the solder paste is deposited to the board by stencil printing, the volume of the deposited paste is nearly the same as the volume of the stencil aperture for through hole components, which can be calculated by equation (7), neglecting that the cross section of the aperture is a trapezoid in case of lasercut and electroform stencil manufacturing technologies. where is the width of the aperture, is the length of the aperture, and is the thickness of the stencil foil.

For PIP technology, large volume of deposited paste is required to form solder joints of good quality. To be able to deposit a large volume of solder paste, increasing the size of aperture or the foil thickness is necessary. On the other hand the increase of the foil thickness can lead to solder bridging between small pitch leads of surface mount components. Usually, enough volume of solder paste can be deposited by increasing only the size of the aperture (Fig. 3.) if the pitch of the component is large, in other words, there is large space between the pads. Obviously, the possible largest space should be left between apertures (increase as less as possible) in order to prevent bridging failures between the leads of the through hole component. Recommended hole- and lead diameters, and aperture sizes can be found in table below