Round two:

5mm LED manufacturers provide severe warnings about the dangers of
stressing the LED body during installation or in service.
The industry standard arrangement is to have two separate mechanical
parts one per lead, with alignment and location being carried out by
the epoxy body prior to soldering and by the soldered leads plus body
post soldering. Forces on the leads relative to the body pre-soldering
must be resisted by the epoxy. Post-soldering the PCB/leads/body
alignment must not place excessive stress on the body. During
soldering and post-soldering when the epoxy body or the leads entering
the body are at elevated temperature stress tolerance is even lower.
LEDs may have "stoppers" - small protrusions on the leads designed to
stand-ff the LED from the PCB by a millimetre or few or may be
stopperless. Best practise and manufacturers' assumption are that
stopperless LEDs will be spaced off the PCB by some other means BUT it
is near universal practise in low cost and / or  low  quality
equipment to use stopperless LEDs and to push them home against the
PCB surface as closely as possible prior to assembly.

______

Given:  LEDs are mounted either

A) With "stoppers" so that a mm or few of lead exists between PCB and
base of LED.

B) Without stoppers and pushed home so base of LED is almost flush with PCB=
..
("Almost" because leads have epoxy protrusion at lead to body join -
in practice this may be small to absent.)

_____

If group "A" LEDs have "enough"  force applied at right angles to body
and at 90 degrees to the plane that the two leads are in, the LED will
bend on its leads, pivoting mainly around a bend near the PCB surface.
Medium finger pressure sufficeth. Dropping a small PCB with say 10
LEDs on it from 600 mm / 2 feet onto a wooden surface may cause
noticeable bending. Casual contact and "pushing around a work bench"
is not likely to cause noticeable bending.

Empirical tests, sample of few, brand "N" 5mm LED, 0.5mm square leads,
suggests that 2 Newton force (200 gram, 7 ounce) tends not to misalign
LED overly noticeably whereas 300 gram almost invariably does.

Applying force along the plane the leads are in will only bend the LED
if a lead buckles - a very large force and liable to cause immediate
and obvious damage to the LED body.

_____

Group "B" LEDs will not be visibly bent by any forces liable to be
experienced in reasonable handling. The LED body resting on the PCB
and/or the much greater stiffness of the very short leads (<< 1 mm)
will resist such forces. Where the LED body sits on the board a moment
arm is formed between  the two leads to resist the moment arm formed
by the LED body. A force multiplier results with force applied to body
being multiplied by about the ratio of

     LED_length / interlead_spacing.

Maybe 5:1 to 10: depending on various factors.

So a 3 Newton / 300 gram force may result in a say 1.5 to 3 kg pull on
one LED lead.

Where the LED body does not sit on the PCB and when force is along the
plane of the two leads one lead acts in compression and the other in
tension to resist rotation. Similar forces to the above result.

Impulse forces from dropping may be higher.

___________

In real world situations, LEDs mounted on stoppers may be bent to and
from with reasonable abandon without immediate evidence of damage.

LEDs mounted on the PCB surface without stoppers may have forces well
in excess of that required to bend stoppered LEDs (as above) with no
apparent signs of damage.

In past equipment, all or mainly hand soldered,  I see a very small
percentage of LED failures with no apparent correlation with soldering
methods.

__

Questions:

How significant a real-world issue is LED mechanical installation
damage caused by rough handling.?

If a stoppered LED is bent how much risk is there in rebending it?

How bad is on-surface mounting compared to using stoppers in real
world installations.

Are the manufacturers "gilding the Lily" (or leadening it) and is the
problems smaller than they suggest?

How much effect do elevated post soldering stresses have?

Any real world feedback?


     R
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