Why do some TH passive componets have such long leads?

[george graves]

Totally random question - thought someone might know the answer.

Why do some TH passive componets have such long leads?  Seems like a waste.  It is for machine placement?

[poodyp]

Because circuits used to be built like this.

[free_electron]

@poodyp : nope. has nothing to do with it.

circuits in volume are assembled by machine and parts are typically deliverd either in rails ( like resistors : one strip of pper on either side ) or in ammo-pack : a cardboard holder tht traps the part pins. the cardboard holder is then folded and deposed in a box much like ammunition for a machine gun is stored ( those belts that hold cartridges ) hence the name ammo pack.

now, this requires an  amount of copper to securely hold the part down.

Second the pick and place robo grabs the part and snips it off the rail. it is NOT pulled out , it is cut off.
third : the legs need forming. for resistors they get bent downward 90 degrees and there is a stub between the actual 'pin' and the body' so you need more material there.
for radial capacitors the pins are typically bent 45 degrees out and then down to fit a 100 mil pitch.  ( if the cap pitch itself is less than 100 mils ) so you need more material again there.

then the part is inserted in the pcb. the protruding legs at the bottom are slightly bent outward so the parts don;t fall or vibrate out of the board as it continues along the assembly line.

after wave soldering the excess is snipped off.

the pick and plce macine themselves always grab a part by the leads as they do not know about the physical size of the part. so you need a bit of material to hold on to it there.

add all those 'bits' up and you end up with a leg that is between an inch and 1-1/2 long. simple becasue of automated handling requirements.

[mariush]

Here's a lead forming machine (albeit the very simple forming, 90 degrees):



and here's a machine that inserts axial and radial leaded devices:


Radial 8XT UIC ?? ? ?

And there's loads of tools that you can use to manually bend the leads in different shapes, see here: http://www.olamefusa.com/e_products.htm

An example for why long leads are needed... in some cases, you want the resistors or diodes to stay further away from the PCB so you put a bit of bend on the leads which keeps the resistor or diode far away from PCB ... so you can use the 2nd or 3rd type of lead bending in the link above for such purpose.