0402 Resistor Drift in a Hot Enclosure, Italy: 20,109 of 23,923 Walsin 0402 Part Numbers Can Drift Past Their Tolerance by 155 C, and All 55 in Hong Kong Stock Can

A 0402 resistor with a 100 ppm per degree TCR drifts up to 1.0 percent by 125 C and 1.3 percent by 155 C, measured from 25 C. We counted Walsin’s 0402 range: 20,109 of 23,923 part numbers can drift past their stated tolerance by 155 C, 9,263 by 105 C, and all 55 in Hong Kong stock can by 155 C.

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Venatech Limited, an authorised Walsin distributor in Hong Kong, holds the Walsin Technology (Walsin) chip resistor exports, 137 files, and the Walsin WR and MR resistor datasheets. This post answers the question an engineer in Milan or Turin asks when a 1 percent 0402 resistor in a sealed cabinet reads 1.3 percent off after a summer: how much of that is the part, and which 0402 lines would have stayed in tolerance? Every part number is real, and every count was made by us from the export.

How far can a 0402 resistor drift in a hot enclosure?

By the TCR limit times the temperature rise above room temperature. A part limited to 100 ppm per degree can move 0.60 percent at 85 C, 0.80 percent at 105 C, 1.00 percent at 125 C and 1.30 percent at 155 C, taking 25 C as the reference. Walsin defines TCR as the resistance change between a reference near 20 C and the test point of minus 55 or plus 155 C, divided by the reference and the temperature step (WR sheet ASC_WR_V24, SEP 2016, clause 4.8, JIS C5201-1). The general form is set out in a Vishay resistor application note, fetched 20 October 2026.

Two cautions come with that number. A TCR is a limit, not a typical value, so the real drift is usually smaller. And the drift adds to the tolerance, because a 1 percent part that starts 1 percent high can end 2 percent high at 125 C. The 5 K gap between a 20 C and a 25 C reference moves 100 ppm parts by 0.05 percent, which does not change any count below.

What does Walsin’s 0402 TCR column say, class by class?

It names 11 limits, and the tolerance beside each decides how early the drift eats it. We parsed every 0402 row in the chip resistor export that carries a percentage tolerance and a TCR, excluding arrays, networks and attenuators. Rows sit in 18 files, from Thick_Film_General_Purpose_1_1_16W.xlsx (5,593 part numbers) to Metal_Low_Ohm_Current_Sense_Chip_Resistor__1of1.xlsx (5), and total 23,923 distinct part numbers. For each we computed the temperature at which TCR drift equals tolerance: 25 C plus tolerance divided by TCR.

Tolerance	TCR limit (ppm per C)	Part numbers	Drift equals tolerance at	Example	Match / mismatch with a 125 C enclosure
plus or minus 0.1%	50	914	45 C	WF04K1000BTL	Mismatch — gone by 45 C
plus or minus 0.1%	100	1,712	35 C	SF04H10R2BTL	Mismatch — gone by 35 C
plus or minus 0.5%	100	2,453	75 C	SF04H10R2DTL	Mismatch — gone by 75 C
plus or minus 0.5%	50	1,371	125 C	WK04K1000DTL	Match exactly at 125 C, none to spare
plus or minus 1%	minus 200 to plus 400	1,149	50 C	MR04W1R00FTL	Mismatch — 4.0% at 125 C
plus or minus 1%	100	9,086	125 C	WR04X1003FTL	Match exactly at 125 C, none to spare
plus or minus 1%	50	1,371	225 C	WK04K1000FTL	Match — 0.65% at 155 C

We have not seen this counted anywhere: the 9,086 plus or minus 1 percent, 100 ppm parts, 38 percent of the range, reach their whole tolerance at exactly 125 C. 125 C is also where Walsin’s X7R capacitors end their range (catalogue page 5 of 00_MLCC_Full_Catalogue.pdf), so a board that runs those at their limit runs these resistors on the line. Only the 50 ppm class, 1,371 parts at plus or minus 1 percent, ends the range inside the tolerance.

How many Walsin 0402 parts can drift past their tolerance, and at what temperature?

At 105 C, 9,263 of 23,923, and at 155 C, 20,109. The count barely moves between 85 C and 125 C, 9,230 to 9,377, then jumps by 10,732 between 125 C and 155 C. We multiplied each part’s TCR limit by the step above 25 C and compared it with the tolerance, at four film temperatures.

Film temperature	Step above 25 C	Parts whose TCR drift exceeds tolerance	Share of 23,923	Match / mismatch
85 C	60 K	9,230	38.6%	Mismatch — the 0.1% and 0.5%, 100 ppm classes
105 C	80 K	9,263	38.7%	Mismatch — 33 more parts than at 85 C
125 C	100 K	9,377	39.2%	Mismatch — 9,086 more reach the tolerance without exceeding it
155 C	130 K	20,109	84.1%	Mismatch — the 100 ppm class crosses at 1.30%

The 9,086 parts at plus or minus 1 percent with a 100 ppm limit sit exactly on the line at 125 C. They pass a strict test at 125 C and fail it at the 155 C top of Walsin’s own operating range. A design that holds the film at 125 C or lower sees the tolerance used up, not exceeded.

How hot is a 0402 film in an enclosure at 85, 105 or 125 C?

Hotter than the air, by an amount the datasheet does not give. Walsin’s WR derating line holds 100 percent of rated power to 70 C and falls to zero at 155 C, which leaves 58.8 percent at 105 C and 35.3 percent at 125 C. On a 1/16 W 0402 that is 36.8 mW at 105 C and 22.1 mW at 125 C. We read the line from page 5 of the WR sheet.

If the sheet is self-consistent, full power at 70 C lifts the film at most 85 K, so each fraction of rated power lifts it at most that fraction of 85 K. That is our bound, not a Walsin figure. KOA’s technical note on small chip components says temperature rise “depends on the applied power, wiring pattern of PCB, and layout of the component”, and its table gives 0402 the highest unique thermal resistance of six sizes, 24,390 against 3,125 for 1206 (KOA Speer note, fetched 20 October 2026).

Enclosure air	Power in the resistor	Film temperature, upper bound	Drift of a 50 ppm part	Drift of a 100 ppm part	Match / mismatch with plus or minus 1%
85 C	50% of rated, 31.2 mW	127.5 C	0.51%	1.02%	Mismatch for 100 ppm, Match for 50 ppm
105 C	25% of rated, 15.6 mW	126.2 C	0.51%	1.01%	Mismatch for 100 ppm, Match for 50 ppm
105 C	50% of rated, 31.2 mW	147.5 C	0.61%	1.23%	Mismatch for 100 ppm, Match for 50 ppm
125 C	25% of rated, 15.6 mW	146.2 C	0.61%	1.21%	Mismatch for 100 ppm, Match for 50 ppm
125 C	35% of rated, 21.9 mW	154.8 C	0.65%	1.30%	Mismatch for 100 ppm, Match for 50 ppm

Every row of an ordinary 100 ppm part ends beyond 1 percent once self-heating is counted, even at 85 C air. Our post on the chip resistor derating curve covers the line itself.

Which 0402 parts keep their drift inside the tolerance?

3,814 of 23,923, 15.9 percent, cannot exceed their tolerance even at 155 C. They are the 1,371 plus or minus 1 percent parts at 50 ppm and 2,443 plus or minus 5 percent parts with limits from 100 to 350 ppm. We filtered on TCR times 130 K at or below the tolerance.

Nothing else clears it. No 0402 row of plus or minus 0.1 percent, 0.25 percent or 0.5 percent keeps a full-range drift inside its own tolerance, because the tightest TCR in the 0402 range is 50 ppm and 0.65 percent is the smallest full-range drift. For a tight tolerance in a hot cabinet the honest answer is a budget, not a part: tolerance plus drift plus self-heating.

Parts with a larger TCR matter at the other end. The 1,851 plus or minus 1 percent parts with a limit of 300 ppm or more include the low-ohm current-sense lines. WW04ER050FTL, 0.05 ohm, carries a limit of 1,500 ppm, which is 19.5 percent at 155 C where a 50 ppm part gives 0.65 percent. Read the TCR column of any low-value line.

What does the Hong Kong shelf hold for a hot enclosure?

As at 23 September 2026 we hold 55 0402 chip resistor lines, and none is in the 50 ppm class. Of the 55, 52 are plus or minus 1 percent at 100 ppm, two are plus or minus 0.5 percent at 100 ppm and one is a 10 ohm part at minus 200 to plus 400 ppm. We joined the Hong Kong stock snapshot to the 0402 rows above.

Stocked part	Value, tolerance, TCR limit	Drift at 105 C film	Drift at 155 C film	Match / mismatch
WR04X1003FTL	100 kilohm, 1%, 100 ppm	0.80%	1.30%	Match at 105 C, mismatch at 155 C
WF04H1202DTL	12 kilohm, 0.5%, 100 ppm	0.80%	1.30%	Mismatch at both
WR04X10R0FTL	10 ohm, 1%, minus 200 to plus 400 ppm	3.20%	5.20%	Mismatch at both

All 55 can exceed tolerance at 155 C. At 105 C only 3 can: the two 0.5 percent lines and the 10 ohm part. Stock is a snapshot and moves. Check ready stock and ask for the 50 ppm class, for example WK04K1000FTL, through the enquiry form.

How should an Italian design budget the drift?

Add three terms: initial tolerance, TCR times the step to the real film temperature, and ageing, then compare with what the circuit tolerates. On our count a 1 percent part in a 125 C enclosure has no margin unless it is the 50 ppm class, which is 1,371 of 23,923 part numbers. Our post on what resistor TCR actually buys you shows the same logic from the thin-film side.

Ratio networks tolerate absolute drift if both resistors move together, so check the ratio TCR before you pay for a tight absolute one. Run your incumbent through our free cross-reference tool, which lists Walsin parts with the TCR and tolerance differences stated, and see the chip resistor range. Qualification stays with you: confirm against the Walsin datasheet and qualify in your own circuit. Drift after 1,000 hours of load is not in the export. It is on our request list.

The short version

A 100 ppm per degree 0402 resistor drifts 1.0 percent by 125 C and 1.3 percent by 155 C. In Walsin’s range 20,109 of 23,923 part numbers can drift past tolerance at 155 C, 9,263 at 105 C. Only 3,814 cannot, led by the 1,371 at plus or minus 1 percent and 50 ppm. None of the 55 stocked 0402 lines is in that class.

Send us the 0402 values on your hot-enclosure BOM, with tolerance, film temperature and drift budget, and we will return the Walsin parts that fit, with the TCR beside each. And a question back: is your tolerance budget written at 25 C or at the temperature the resistor really sees?

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Frequently asked questions

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