[SOLVED] Negative length of solder fillets in IPC-7351?

Hi,

In IPC-7351 there are three solder fillets defined: toe, heel and side.



I naturally tend to believe the width of solder fillets must be positive, since the physical quantity “length” is a scalar rather than a vector. I also find in an illustration in Kraig Mitzner’s Complete PCB Design Using OrCAD Capture and PCB Editor, page 86, that all solder fillets are drawn positive.

However, what I do see in IPC-7351 table 3-2 to table 3-14 is that there are many negative values for fillets. For example, all heel solder fillets for capacitors and resistors < 0603 are negative; in table 3-9, side fillets can be either positive or negative.

I am really puzzled on the negative values found here: how can solder fillets, a concrete physical object which is formed of cooled solder, be of negative dimensions?


Matt

I think it is not really the size of the solder fillet but the difference between the pad and component leg dimension. So a negative value means the pad is smaller than the component leg, at least in one dimension.

Keith

Protrusion

This is the amount by which the Land Pattern extends beyond the Terminal (the terminal being the contact area between the component lead and land pattern).

Protrusions are categorized as Toe, Heel and Side.

• Toe Fillet refers to the amount of land between the end of a terminal and the end of its land pattern. The Toe is usually away from the body of the part.

• Heel Fillet refers to the amount of land between the start of a terminal and the other end of its land pattern. The Heel is usually close to the body of a part.

• Side Fillet refers to land between the terminal and pad on the remaining two sides.

From the IPC-7351 glossary, as Keith said it is the maths of subtracting one from the other.

keith and marce,

I think keith's explanation makes sense. This is solved.

Matt