Electronic – Using oblong / rounded surface mounted pads for chip resistors, capacitors and inductors

Thick film resistors are screen printed; the alumina substrate is metalized then a resistive paste is deposited on top of the terminals. It is later trimmed, coated, metalized on the edges, and plated.

Thin film (or metal film) resistors have said film vacuum deposited, allowing for a much more uniform and controlled resistive element. They then undergo similar finishing steps to trim, coat, and metalize the edges.

As a result, thick film resistors are generally cheaper than their thin film counterparts, but the tolerance and temperature coefficients one can get out of thin film resistors are generally better. Depending on the materials used, there is plenty of overlap between the two, but all things equal, thin film offer better performance for a cost premium.

Vishay has a decent app-note (PDF) on this.

Some cap vendors make their own parts. Some buy caps from a smaller manufacture with the reels re-branded at the fab. Watch out. I got into a mlcc failure investigation in 2002 and started inspecting caps on a reel under a microscope. 3/10 came off the reel cracked. A crack will sooner or later lead to a short. Cracks are not obvious even under a microscope. They may present as a subtle colour shift if the crack is below the surface layer. Some cracks may be sufficient to present a short immediately. Not all. The vendor's manufacturer in this case eventually identified a hopper where the caps were getting cracked.

MLCC are very sensitive to mechanical stress. Especially bigger than 1210 sizes. I found a big power bypass cap adjacent to a heavy mechanical connector once. The nearest mounting hole was 2" away! They were cracking at a rate of 5/10 during install of the unit. A fraction of those were catching fire. The fire would continue to burn until it melted the copper, breaking the power connection.

Another effect of a crack is a reduction of the max working voltage of the cap. It might be spec'd for 200 V. But once cracked it may break down at 40 V. Cracked caps burst into flames in my lab when tested - even below their rated voltage.

Another way to warm up caps is exceeding their max ac current. It's easy to think of caps as zero power dissipation devices. Especially the higher Q mlcc. But they're not. Calculate the power dissipated in caps and do not exceed power/ac current limits. Shows up in power circuits and converters commonly.