Second-Source the Part Without Touching the Board: Two Fields Decide It

You can second source passive components without touching the layout only when the
candidate matches the footprint, the body height, the termination and the qualification
grade — not just the case size and the value. Walsin Technology (Walsin) publishes
thickness as a separate ordering attribute, so two rows sharing one part number can differ
by 0.45 mm.

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What does a second source actually have to match?

Five electrical fields and four mechanical ones. Case size, dielectric, capacitance,
tolerance and rated voltage decide whether the circuit still works. Length, width,
thickness and termination type decide whether the board still builds. A candidate that
passes the first five and fails any of the second four is not a drop-in.

Most cross-reference work stops at the electrical five because that is what a parametric
field can hold. The mechanical four are where a second source quietly turns into a layout
change. Venatech Limited, an authorised Walsin distributor in Hong Kong, holds an extract of
24,026 Walsin MLCC part numbers and 234,815 chip resistor part numbers, which is enough to
measure how often the two halves disagree.

Walsin’s own cross-reference tool is direct about this. Step four of its instructions tells
the user to double-check the thickness and tolerance of the suggested part. That is a
manufacturer telling engineers the truth about their own catalogue, and it is the single
best reason a cross needs a human pass afterwards. The rest of this post is that check,
written out.

Does the same case size mean the same footprint?

Usually, and not always. A case size code fixes the land pattern for discrete two-terminal
parts of that size. It does not tell you how many elements are inside the body, how tall
the body is, or whether the terminations are on the long edges or the short ones.

Two measured cases from the Walsin data make the point. First, arrays. Filtering the chip
resistor extract on a size field reading 0603(1608) returns 42,374 parts, and 5,076 of
them are resistor arrays — 0603*2, 0603*4, 0602*8, convex and concave. The element is
0603; the body is not. Those parts sit in the same size bucket as a discrete 0603 and will
not land on a two-pad footprint.

Unique to Venatech’s data. 5,076 of the 42,374 Walsin chip resistor part numbers whose
size field reads 0603(1608) are multi-element arrays rather than discrete 0603 chips.
A size-only filter returns them alongside single resistors. The array geometry is in the
series name, not in the size field, so the only safe read is the series column.

Second, reverse geometry. The 0306 MLCC parts are 0.8 mm long and 1.6 mm wide, with the
terminations on the long sides. They share no footprint with an 0603 despite looking
adjacent in a size table. Three exist in our extract.

Why can one Walsin part number carry two different thicknesses?

Because thickness is ordered separately from the electrical specification, and Walsin
carries it in a thickness symbol column rather than inside the part number stem. The
electrical fields are identical between the two rows. The body height is not, which makes the
symbol the field to read before a height-critical position is committed.

The clean example is 0805B104K500: an 0805, X7R, 100 nF, ±10%, 50 Vdc part, 2.00 ± 0.15
mm long and 1.25 ± 0.10 mm wide in both rows. One row publishes 0.80 ± 0.10 mm thick with
thickness symbol B. The other publishes 1.25 ± 0.10 mm thick with thickness symbol D. That
is 0.45 mm of difference behind one part number.

Field	0805B104K500, symbol B	0805B104K500, symbol D	Match / mismatch
Case size	0805	0805	Match
Dielectric	X7R	X7R	Match
Capacitance	100 nF	100 nF	Match
Tolerance	±10%	±10%	Match
Rated voltage	50 Vdc	50 Vdc	Match
Length	2.00 ± 0.15 mm	2.00 ± 0.15 mm	Match
Width	1.25 ± 0.10 mm	1.25 ± 0.10 mm	Match
Thickness	0.80 ± 0.10 mm	1.25 ± 0.10 mm	Mismatch — 0.45 mm apart
Thickness symbol	B	D	Mismatch — the field that resolves it

Unique to Venatech’s data. 399 of the 23,601 distinct Walsin MLCC part numbers in our
extract appear on more than one row, and 385 of those carry more than one published
thickness. 378 of the 385 are separated by the thickness symbol column. The part number
stem alone does not fix the body height; the stem plus the thickness symbol does. Most of
the affected parts sit at 1206 (333 rows) and 0805 (255 rows).

How many Walsin parts fit a single 0603 X7R 100 nF 50 V line?

Thirteen, across six Walsin series. The five ticked electrical fields narrow the choice to
thirteen candidates, not to one. What remains is the decision no cross tool can make: the
qualification grade, the termination construction and the dimensional tolerance band.

Walsin part number	Series	Tolerance	Thickness, L × W × T tolerance band	Match / mismatch vs a +0.15/−0.10 mm incumbent
0603B104J500	General Purpose	±5%	+0.15/−0.10 mm	Match on band, mismatch on tolerance
0603B104K500	General Purpose	±10%	+0.15/−0.10 mm	Match
0603B104M500	General Purpose	±20%	+0.15/−0.10 mm	Match on band, mismatch on tolerance
MT18B104J500	Automotive MT, AEC-Q200	±5%	+0.15/−0.10 mm	Match on band, mismatch on tolerance
MT18B104K500	Automotive MT, AEC-Q200	±10%	+0.15/−0.10 mm	Match on band and tolerance
MT18B104M500	Automotive MT, AEC-Q200	±20%	+0.15/−0.10 mm	Match on band, mismatch on tolerance
MG18B104J500	Automotive MG, no AEC-Q200	written -0.05	+0.15/−0.10 mm	Match on band, tolerance field needs reading
MG18B104K500	Automotive MG, no AEC-Q200	written -0.1	+0.15/−0.10 mm	Match on band, tolerance field needs reading
ST18B104K500	Automotive soft term ST, AEC-Q200	±10%	±0.30 mm	Mismatch — wider band
ST18B104M500	Automotive soft term ST, AEC-Q200	±20%	±0.30 mm	Mismatch — wider band and tolerance
SG18B104K500	Soft termination SG, Cu-poly	±10%	±0.30 mm	Mismatch — wider band
SH18B104J500	Soft termination SH, Ag-poly	±5%	±0.30 mm	Mismatch — wider band and tolerance
SH18B104K500	Soft termination SH, Ag-poly	±10%	±0.30 mm	Mismatch — wider band

Eight of the thirteen carry a +0.15/−0.10 mm dimensional band. Five carry ±0.30 mm. At
worst case that is a 1.10 mm thick part in one group against 0.95 mm in the other, from the
same 0.80 mm nominal. If the position sits under a heatsink, a shield can or a connector
overhang, that 0.15 mm is the whole decision.

Note also the tolerance field on the two MG parts. It is written -0.05 and -0.1 rather
than as a percentage, so a numeric tolerance filter will not return them. They are real
parts; they are just hard to reach with a filter.

Does a soft-termination part drop into the same land pattern?

The land pattern is the same for the case size. The dimensional tolerance band, the
termination construction and the reason for choosing the part are not. Soft termination
exists to survive board flex, and the conductive-polymer layer is why the published
dimensional band is wider.

Walsin covers this across four series. SH is Ag-poly and spans 0402 to 2225. SG is Cu-poly
and spans 0603 to 1206. ST is the AEC-Q200 automotive soft-termination line at 0603 to
0805. VT adds VW80808 on top of AEC-Q200 at 0603. The SH, SG and ST datasheets are drawn
against a 5 mm board bend.

Crossing in the direction of soft termination adds mechanical margin and widens the
dimensional band. Crossing out of soft termination removes the flex protection the original
designer specified. Those are not symmetrical moves, and only one of them is reversible on
paper.

What changes on the line rather than on the board?

Packaging, orderable part number and incoming inspection. Walsin’s MLCC parametric export
omits the CT packaging suffix, so an exported part number is not orderable as written.
The order code for the general-purpose 0603 above is 0603B104K500CT, and those two extra
characters are not something purchasing will guess from the catalogue key.

Process settings deserve one pass too. Walsin publishes a single MLCC reflow profile across
the range — 3 °C/s maximum ramp up, a 150–200 °C soak of 60–120 seconds, 60–150 seconds
above 217 °C, at least 30 seconds above 245 °C, a 260 °C peak and 6 °C/s maximum ramp down,
inside 480 seconds to peak. A second source in the same family does not change those
numbers. A second source that moves case size can change the preheat limit: ΔT is held to
190 °C at 1206 or less and 130 °C at 1210 or more.

Incoming inspection is where the thickness symbol matters again. If the drawing calls a
maximum height, measure the first reel rather than trusting the stem.

When does a second source stop being a drop-in?

When any mechanical field, any certification or any process limit moves. A wider
dimensional band, a different termination construction, a qualification that does not
transfer, or a case size that crosses the 1206 / 1210 preheat boundary all turn a paper
match into a change that needs a build.

A specification match is a candidate for your evaluation, not an approved substitution.
Check both datasheets, confirm the mechanical fields against your own drawing, and qualify
the part in your application before production use.

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FAQ

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Next step

Send the BOM lines you want a second source for. Venatech Limited, an authorised Walsin
distributor in Hong Kong, will return the candidate Walsin part numbers with the case size,
the published thickness and thickness symbol, the dimensional tolerance band, the
termination construction and the qualification grade set out field by field, so you can see
what matches and what does not before anyone touches the layout.

Send your BOM to Venatech · Browse the
Walsin MLCC range · Read
Thickness and tolerance: the two fields every cross tool asks you to check
and Soft termination MLCC: SH, SG, ST and VT.

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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.