Introduction

Through-hole power transistors have to be lead-formed to fit on a breadboard, and you end up reaching for pliers. I remember connecting the ESD strap, checking the angle and then bending. I even straightened the lead and tried again because the pitch didn't match.

About lead forming

In the old days, prototypes were used to evaluate for viable results in real production, but in real production, lead forming needs to be better controlled. Without proper controls, lead cracking, plating delamination, or device molding damage can occur, allowing moisture to enter and ultimately rendering them unusable.

These days everything is more surface mount as well, but the use of leaded packages for existing TO packages isn't going away anytime soon, especially in high power products. Our users love the idea of being able to replace expensive parts with a screwdriver, a cutter and a 100W soldering iron. why do i need to replace it? This is because when the device breaks, it needs to be replaced.

Some designers can take the risk and use a TO-package with a "gullwing" or C-shaped lead to get the benefits of both a screwed heatsink and surface-mount terminals. (Fig. 1). Under normal operation, the differential expansion of materials with temperature changes can rip the traces from the board.

Lead bends provide strain relief, but the idea of intentionally designing continuous flexing of copper leads is not appealing, how much strain relief is enough? Even with a JEDEC compliant package, the thickness of the lead-based material can vary ±20% and typical tin plating thickness can be 300-1200 µin, so the stiffness is significantly reduced. to Copper is a hard metal anyway, and must be alloyed and tempered to a semi-hard state if it is to be used for device leads.

Then there is the problem of reflow temperature and the moisture resistance level (MSL) of surface mount type devices with leads. In practice, you might end up manually soldering the leads to get around these problems, but this makes little sense.

At least there are guidelines describing how to do lead forming without damaging the device itself. The lead should never be bent sideways (Figure 2). Also, always clamp the bend so that it does not hit the component body directly.

It is recommended that the clamping equipment not touch the plastic material of the package to avoid abrading the plating or exposing the copper, but the face of the bending tool should at least impart a "shiny" appearance to the plated surface. The distance of the bend from the device body is specified to account for any built-in standoffs, with the minimum bend radius quoted as a multiple of the lead width and/or thickness. Tools and work area should be clean and appropriately ESD protected.

ON Semiconductor sells wide bandgap semiconductors in a variety of leaded formats, including TO-220, TO-247 (3-lead and 4-lead), and TO-264.

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