Trimmable Chip Resistors: Designing for Laser Trim

A trimmable chip resistor ships below its coded value with a one-sided tolerance, so a
laser cut in the resistive film brings it up to the value the circuit needs. Walsin
Technology (Walsin) lists 2,940 of them in three bands — 0/-10%, 0/-20% and 0/-30% —
across four case sizes. Every part number carries an embedded space at character nine.

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What is a trimmable chip resistor?

A trimmable chip resistor is a standard thick film chip built to a deliberately loose,
one-sided tolerance so that its value can be adjusted after assembly. A laser cuts the
resistive film, narrowing the conducting path and raising the resistance. The coded value
is the top of the range, not the centre of it.

Every thick film chip resistor is laser trimmed once, at the factory. Walsin’s general
purpose approval sheet describes it: the paste composition sets an approximate value, and
“the value is trimmed to within tolerance by laser cutting of this resistive layer”. A
trimmable part stops short of that step and leaves the adjustment for you.

The reason to want that is calibration. A gain-setting leg, a reference divider or a sensor
bridge can be tuned to the individual board rather than to the statistical worst case.

The cost is a second process step: a trim laser, a measurement loop, and clear line of sight
to the chip’s top face. Venatech Limited, an authorised Walsin distributor in Hong Kong,
holds 2,940 of these against 234,815 Walsin chip resistor part numbers in total.

Why is the tolerance written 0/-30% instead of plus or minus 30%?

Because the deviation only runs one way. A 0/-30% part is never above its coded value and
may be up to 30% below it. Laser trimming only raises resistance, so the coded value is
the ceiling you trim towards. A two-sided tolerance would make the part untrimmable in one
direction.

This is the most misread field on these part numbers. Read 10000Ω 0/-30% as “at most
10 kΩ, possibly as low as 7 kΩ”. Specify the coded value as the highest resistance the
circuit will ever sit at, then trim up to it.

Walsin splits the 2,940 parts three ways. The split is not even, and it is not what a
single-band assumption would predict.

Tolerance code	Tolerance as stored	Parts held	Prefixes it appears on
X	0/-30%	1,135	WF06M, WF08M, WF12M, WF20M, WK06M, WK08M, WK12M
Y	0/-20%	1,129	WF06M, WF08M, WF12M, WF20M, WK06M, WK08M, WK12M
Z	0/-10%	676	WF06M, WF08M, WF12M, WF20M only

Which sizes and powers does Walsin list trimmable parts in?

Four sizes: 0603 (1608), 0805 (2012), 1206 (3216) and 2010 (5025). Rated power runs
1/10 W to 1/2 W, but it is set by the part number prefix rather than by the case size
alone. At 0603 and 1206 two different ratings exist for the same footprint.

Prefix	Size	Rated power	Tolerance codes	TCR bands present	Resistance range	Parts held
WF06M	0603 (1608)	1/10 W	X, Y, Z	≦±200; -300～+500	1.0 Ω – 10 MΩ	507
WF08M	0805 (2012)	1/8 W	X, Y, Z	≦±200; -300～+500	1.0 Ω – 10 MΩ	507
WF12M	1206 (3216)	1/4 W	X, Y, Z	≦±200; -300～+500	1.0 Ω – 10 MΩ	507
WF20M	2010 (5025)	1/2 W	X, Y, Z	≦±200; -300～+500	1.0 Ω – 10 MΩ	507
WK06M	0603 (1608)	1/8 W	X, Y	≦±200	10 Ω – 4.7 MΩ	274
WK08M	0805 (2012)	1/8 W	X, Y	≦±200; -200～+500	1.0 Ω – 4.7 MΩ	322
WK12M	1206 (3216)	1/8 W	X, Y	≦±200; -200～+500	1.0 Ω – 4.7 MΩ	316

Two rows contradict the usual rule that a bigger chip means more power. WK06M is an 0603
rated 1/8 W while WF06M is an 0603 rated 1/10 W, and WK12M is a 1206 rated 1/8 W while
WF12M is a 1206 rated 1/4 W.

So the prefix decides three things at once: rated power, whether 0/-10% is available, and
where the resistance range stops. Pick the prefix first and the case size second.

How do I read a Walsin trimmable chip resistor part number?

Six fields, twelve characters, one of which is a space. WF12M103 XTL reads as: WF12 for
the prefix and size, M for the trimmable type code, 103 for the resistance, a space,
X for 0/-30%, T for 7-inch reeled taping, L for Sn base lead-free termination.

The M in position five identifies the series, and every one of the 2,940 trimmable parts
carries it.

The resistance code follows Walsin’s published chip resistor scheme. Three digits means two
significant figures plus the number of zeros, so 103 is 10 kΩ and 102 is 1 kΩ. 1R0 is
1.0 Ω, with R as the decimal point.

Part number	Size	Resistance	Tolerance	TCR	Rated power	What changed from the row above
WF12M103 ZTL	1206 (3216)	10 kΩ	0/-10%	≦±200	1/4 W	Reference part
WF12M103 YTL	1206 (3216)	10 kΩ	0/-20%	≦±200	1/4 W	Tolerance code Z to Y
WF12M103 XTL	1206 (3216)	10 kΩ	0/-30%	≦±200	1/4 W	Tolerance code Y to X
WF12M102 XTL	1206 (3216)	1 kΩ	0/-30%	≦±200	1/4 W	Resistance code 103 to 102
WK12M103 XTL	1206 (3216)	10 kΩ	0/-30%	≦±200	1/8 W	Prefix WF to WK, power halved
WF20M103 XTL	2010 (5025)	10 kΩ	0/-30%	≦±200	1/2 W	Size code 12 to 20
WF06M103 XTL	0603 (1608)	10 kΩ	0/-30%	≦±200	1/10 W	Size code 12 to 06

How much adjustment does each tolerance band actually buy?

Arithmetic, not marketing. A 0/-30% part received at the bottom of its band sits at 70% of
the coded value, so trimming it up to the coded value is a 42.9% increase. A 0/-20% part
gives up to 25%. A 0/-10% part gives up to 11.1%. Those are the ceilings, not guarantees.

Tolerance band	As-received window	Maximum upward trim from the bottom of the band	Guaranteed adjustment available on any unit
0/-10%	90% – 100% of coded value	11.1%	None — a unit may arrive at the coded value
0/-20%	80% – 100% of coded value	25.0%	None — a unit may arrive at the coded value
0/-30%	70% – 100% of coded value	42.9%	None — a unit may arrive at the coded value

The last column is the one that catches designs out. A one-sided tolerance guarantees a
ceiling, not a floor. Nothing stops a reel of 0/-30% parts arriving clustered at 99% of the
coded value, in which case almost no upward trim is available on any unit.

Worked through on WF12M103 XTL: coded 10 kΩ, 0/-30%, so the population sits between
7.00 kΩ and 10.0 kΩ. Targets near 10.0 kΩ are reachable on nearly every unit. A target of
7.10 kΩ is reachable only on units that happen to arrive below it.

What resistance range can I trim, and where does it stop?

1.0 Ω to 10 MΩ on the four WF…M blocks. The WK…M blocks stop at 4.7 MΩ, and WK06M
also starts higher, at 10 Ω rather than 1.0 Ω. There is no trimmable part above 10 MΩ
anywhere in Walsin’s chip resistor catalogue.

The ceiling matters because high-value trimming is where the technique is most attractive
— a 10 MΩ divider is exactly the thing you cannot buy to 0.1%. Walsin’s answer stops at
10 MΩ, the same place the general purpose thick film range stops, and the parts above it
sit in a separate series group with conventional two-sided tolerances.

The WK12M block has one further gap. Its 0/-20% parts stop at 2.7 MΩ while its 0/-30%
parts continue to 4.7 MΩ — two ceilings under one prefix.

Unique to Venatech’s data. Only two of the fourteen chip resistor series groups
Venatech holds have an embedded space in 100% of their part numbers. The Trimmable group
is 2,940 of 2,940; the Chip Resistor Network, Convex Type group is 98 of 98. Across the
whole 234,815-part extract the figure is 39,576 — 16.9%. Trimmable is the largest series
group where the space is unavoidable, and the space always falls at character nine.

Does laser trim change the TCR or the power rating?

Walsin publishes the TCR band for the as-supplied part. In-circuit trimming narrows the
conducting path, which concentrates current density and heat in the cut region, so neither
the quoted TCR nor the quoted rated power should be assumed to survive a deep trim
unchanged.

The TCR bands are already wide, and asymmetric on some parts. WF…M parts are either
≦±200 ppm/°C or -300～+500 ppm/°C, and WK08M and WK12M add a -200～+500 ppm/°C band on 48
parts each. None of the 2,940 offers ≦±100 ppm.

That is the real trade. A trimmable part gets you an arbitrary value at build time and costs
you temperature stability against a ±1% fixed part such as WR12X1002FTL, the same size and
the same 10 kΩ at ≦±100 ppm/°C.

Part number	Series group	Size	Resistance	Tolerance	TCR	Rated power	Match / mismatch vs WF12M103 ZTL
WF12M103 ZTL	Trimmable	1206 (3216)	10 kΩ	0/-10%	≦±200	1/4 W	Reference part
WK12M103 XTL	Trimmable	1206 (3216)	10 kΩ	0/-30%	≦±200	1/8 W	Size, value, TCR match — tolerance and power mismatch
WF20M103 ZTL	Trimmable	2010 (5025)	10 kΩ	0/-10%	≦±200	1/2 W	Value, tolerance, TCR match — size and power mismatch
WF06M103 ZTL	Trimmable	0603 (1608)	10 kΩ	0/-10%	≦±200	1/10 W	Value, tolerance, TCR match — size and power mismatch
WR12X1002FTL	General purpose	1206 (3216)	10 kΩ	±1%	≦±100	1/4 W	Size, value, power match — tolerance two-sided, TCR mismatch, not trimmable
WR12X103 JTL	General purpose	1206 (3216)	10 kΩ	±5%	≦±100	1/4 W	Size, value, power match — tolerance two-sided, TCR mismatch, not trimmable

Why will my trimmable part number not paste into a BOM?

Because all 2,940 contain a real space. Walsin’s resistance code is two significant digits
then the number of zeros then a blank, so WF12M103 XTL has a space between 103 and
XTL. Strip it and the string is not a Walsin part number; keep it and many BOM tools and
search boxes reject it.

The space always falls at character nine of a twelve-character string, so a find-and-replace
at position nine repairs a whole stripped column.

Quote the part number with the space when you order. Venatech will recognise a stripped
string; a third-party search tool will not.

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FAQ

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Next step

Trimmable selection turns on the prefix and the tolerance letter, not the case size. Send
the BOM lines and Venatech Limited, an authorised Walsin distributor in Hong Kong, will
return the rated power that applies to that prefix, the tolerance band, the resistance
ceiling, and the part number written with its space in the right place.

Send your BOM to Venatech · Browse the
Walsin chip resistor range · Read
Thin film vs thick film chip resistors and
Chip resistor power rating by case size.

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Cross it yourself. The cross-reference tool takes a single part number or a whole BOM column and returns the Walsin candidates with every field that differs stated plainly — qualification stays with you. Ready stock shows what is held in Hong Kong today, with an RFQ against any line.