Component procurement tips and fails

Abstract

This article is related to my general procurement article, which gives a brief overview about distributors. Here I share my individual experience procuring parts from different vendors, compare parts to each other, rate quality and share my opinion.

Heat sinks comparison

As pointed out in my suppliers overview, my first address for heat sinks is Fischer Elektronik and Bürklin retails a lot of them. Fischer Elektronik offers excellent quality, but is a bit pricey and so I was looking for cheaper alternatives. I was searching on AliExpress and after a while, found parts that could fit. What complicates looking for parts is that the Chinese vendors post many images, but usually no technical drawings. One of the vendors provided a technical drawing on request that looked right so I bought several heat sinks from him. Below are images of both parts for comparison. On the left with the gold colored solder pins is the Fischer SK 437 25 STS 2 and on the right, a generic Chinese equivalent. I picked random parts for the comparison, the first of each box.

The view on the fins shows that the two heat sinks have the same height, but the Chinese heat sink has the mounting hole placed lower. Maybe this is just due to wide manufacturing tolerances, but it could be intentional. To be fair, the Chinese model is advertised to suit TO-220 packages, which it certainly does, while Fischer Elektronik claims their design to be also compatible with TO-247 packages.

The bottom view shows that the soldering pins have the same size and location. The Chinese model has a bit different profile. I believe the profile from Fischer Elektronik is slightly better for heat transfer inside the heat sink. For a heat sink this size, it shouldn't matter that much. The Chinese model shows excessive burrs at the mounting hole on fin side. While this doesn't matter at all, it indicates that the drill might not be sharp enough anymore.

The view on the semiconductor mounting surface shows that both heat sinks have their mounting hole drilled reasonably central. Parallax effects on the other images may show the hole off center. The surface is also even and smooth for both parts. In general, the Fischer Elektronik parts have better surface finish. It is unknown how the Chinese parts looked like prior to shipping.

China heat sink with diode

Putting the Chinese heat sink on a PCB together with a diode in TO-247 package clearly shows that the mounting hole location has impact on fit of the device to be cooled. Just a millimeter makes a big difference here. This is no issue with TO-220 or smaller packages. I find the design by Fischer Elektronik better in this aspect again because it is more versatile.

Fischer Elektronik China generic
Cost for 50 heat sinks 115.00 € 10.20 €
Price per piece excl. shipping 2.30 € 0.20 €
Shipping cost 7.70 € 3.30 €
Shipping time A few days 7 weeks
Total (incl. VAT) 122.70 € 13.50 €

Verdict is that this type of heat sink from Fischer Elektronik has better design, quality and arrives more quickly, but this comes at almost ten times the cost. Planning ahead can save a lot of money buying near identical parts from China. Problems to be aware of when buying from China is lack of clear product specification and possible quality issues. Nobody knows where the mounting hole will be drilled on the next batch and whether the parts fit the intended purpose.

Packaging fail

I prefer to buy from reputable distributors in any case. They sell genuine parts and know how to handle and ship them adequately. During many years of doing procurement professionally I can't remember receiving shipments that were not properly packed. Well, until I ordered a pair of audio signal transformers from RS Online. While those parts look rugged having a metal can, they are in fact very sensitive, especially regarding mechanical shock. This is an extract of what Jensen Transormers writes about handling:

Remember that there are very tiny wires inside an audio transformer. Its wire leads should never be used like a handle to pick it up. The internal bonds are strong, but one strong tug might result in an open winding.
Don't drop a transformer. It can distort the fit of the laminations in output transformers and affect their low-frequency response. Mechanical stress (as in denting) of the magnetic shield can of an input transformer will reduce its effectiveness as a shield. For the same reason, don’t over-tighten the clamp on transformers mounted with them.

I assume RS Online either isn't aware of how delicate those parts are, or they don't have any packaging instructions in place or the person packaging the parts didn't care or a combination thereof. However, this is how the parts arrived:

Note that the envelope is only an un-padded, thin paper bag. Also, the plastic containers didn't contain any padding material. Well, this is what I paid 8€ shipping cost for. After I straightened the bent pins of one part, at least I was able to check that it fits on my PCB. I returned the parts as defective, damaged by shipping. They refunded the parts eventually. I don't know how they handled the case internally. Any decent company should set up an 8D report in order to improve. I found this very helpful for solving issues permanently. I don't dare to order from RS Online again, but still want those transformers.