Be careful with your bends: Why you shouldn’t take shortcuts by bending component leads

In the lab, we often use pliers to bend the pins of perforated power transistors so they can fit into circuit test boards. We promptly reattach our anti-static wrist straps, keep an eye on the angle of the pliers, and bend. If the odd staggered spacing after the bend prevents it from fitting, we straighten the pins and try again. This time we hold the pins with the pliers and bend them with our "calibrated" thumbs. This time it looks better, but then one pin falls off because metal is not a material that can achieve a zero bend radius, especially on the second bend. You might be wondering, aren't round-nose pliers made of metal too?

Old-timers know how to get viable results with prototypes, but in a production environment, we need better control over pin bending. Failure to do so can result in pin breakage, plating loss, or damage to the molded device, leading to moisture intrusion and ultimately failure in use.

But are all components now surface mount? If they are, that would be great, but TO-style pin packages are not going to disappear immediately, especially in high-power products. Regardless, engineers love to use screwdrivers, cutting tools and 100W soldering irons to modify expensive stress-bearing components. Why? Because those parts can't be used, maybe they are not the right shape. At this point, designers are faced with the "chicken or egg" problem.

Some designers hedge their bets by using a TO package and making the pins “gull-wing” or C-shaped to get the best of both worlds: a screw-mounted heat sink and surface-mount terminals that can both connect properly ( Figure 1 ). However, under normal operating conditions, the material will expand unevenly with temperature changes, which may pull the trace off the board.

弯曲针脚可以缓解应力，但有意将铜针脚设计成可以持续弯曲的样子却不太可行，而且消除多少应力才合适？即使是使用符合 JEDEC 标准的封装，针脚基料的厚度也会出现 +/-20% 的变化，而典型的镀锡材料厚度可在 300 至 1200 µin 之间，所以针脚刚度会有显著不同。无论如何，铜是一种硬金属，在用于器件针脚时，必须制成铜合金并回火至半硬状态。我在想，“半硬” 的容差是多少？

Then there are the issues of reflow temperatures and lack of moisture sensitivity level (MSL) ratings for pinned devices used as surface mount types. In practice, to address these issues, the pins may end up being hand soldered, but that kind of defeats the purpose.

By following a few guidelines, you can achieve pin bending without damaging the device itself. There are a few things you should not do. Pins should never be bent laterally ( Figure 2 ), and pins should always be clamped to avoid the bend being pressed against the component.

It is recommended that the fixture should not contact the plastic material of the package to avoid abrasion of the plating or exposure of bare copper, but in practice the surface of the bending tool will at least rub against the surface of the plating, which is unavoidable. Specify the distance between the bending part and the body of the device, taking into account all built-in rivet nut standoffs and the minimum bending radius, which is usually expressed as a multiple of the pin width or thickness. The bending tool and workplace should be kept clean and tidy, and appropriate ESD protection measures should be taken.

UnitedSiC sells wide bandgap semiconductor devices in a variety of pin styles: TO-220, TO-247 (3-pin and 4-pin), TO-264, etc., and comes with a useful application note, AN0021: "Bending Through-Hole Pins", which summarizes common mistakes and suggestions when bending device pins, so as to help you bend the corners reliably and avoid breaking the pins .