Sometimes You Keep the Original. We Will Tell You When.

The clearest guide to when not to substitute a component is the reason the original was
chosen: a part-specific safety certification, a qualification held against a series, a
termination built for board flex, or a loss figure the parametric fields do not carry. In
those cases, keep the original and put the sourcing effort somewhere else.

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Venatech Limited, an authorised Walsin distributor in Hong Kong, publishes this from Walsin’s own parametric data and the datasheets it holds on file. Every part number below is real.

When should you not substitute a component at all?

When the property that made the part correct is not one of the fields a cross-reference
matches on. Case size, dielectric, capacitance, tolerance and rated voltage are the five
fields any cross compares. A certification, a bend rating, a loss figure and an existing
qualification record are none of them.

Venatech Limited is an authorised Walsin Technology (Walsin) distributor in Hong Kong, and Walsin Technology
(Walsin) is our principal. Recommending a Walsin part is our commercial interest. This post
is the opposite list: five places where the honest answer is that the incumbent stays, and
where a specification match should be declined rather than quoted.

Each case below is grounded in real Walsin series data, so the point is checkable rather
than rhetorical.

Why does a safety-certified capacitor have to stay the certified part?

Because the certification is written against the specific part, at a specific AC voltage and
a specific class, and capacitance has nothing to do with it. Two parts of the same
capacitance and the same tolerance can hold entirely different mains ratings. The value
matches; the approval does not travel.

Walsin’s safety-certified disc capacitors make this unusually easy to demonstrate. Filtering
our 929-row disc capacitor extract for 100 pF, ±10%, YP(Y5P) dielectric with an X-class
rating returns 14 parts carrying 8 distinct safety ratings. Set against an incumbent
certified at X1-760Vac/Y1-500Vac:

Walsin part number	Series	Capacitance and tolerance	Published safety rating	Match / mismatch vs X1-760Vac/Y1-500Vac
YP0AS101K070	Safety Certificated X1/Y1 (AS series)	100 pF ±10%	X1-760Vac/Y1-500Vac	Match
YP0AH101K060	Safety Certificated X1/Y1 (AH series)	100 pF ±10%	X1-400Vac/Y1-250Vac	Mismatch — lower X1 and Y1 rating
YP0AH101K050	Safety Certificated X1/Y1 (AH series) small size	100 pF ±10%	X1-400Vac/Y1-250Vac	Mismatch — lower X1 and Y1 rating
YP1AH101K060	Safety Certificated X1/Y1 (AH series)	100 pF ±10%	X1-400Vac/Y1-400Vac	Mismatch — lower rating, Y1 class
YP5AH101K060	Safety Certificated X1/Y1 (AH series) 500V	100 pF ±10%	X1-500Vac/Y1-500Vac 1500Vdc	Mismatch — Y1 matches, X1 lower
YP0AC101K060	Safety Certificated X1/Y2 (AC series)	100 pF ±10%	X1-440Vac/Y2-250Vac	Mismatch — Y2 class, not Y1
YP1AC101K060	Safety Certificated X1/Y2 (AC series)	100 pF ±10%	X1-440Vac/Y2-300Vac	Mismatch — Y2 class, not Y1
YP5AC101K060	Safety Certificated X1/Y2 (AC series) 500V	100 pF ±10%	X1-500Vac/Y2-500Vac 1500Vdc	Mismatch — Y2 class, not Y1
YP7AC101K060	Safety Certificated X1/Y2 (AC series) Auto	100 pF ±10%	X1-440Vac/Y2-300Vac 1500Vdc	Mismatch — Y2 class, not Y1

Unique to Venatech’s data. Fourteen Walsin disc capacitor part numbers share 100 pF,
±10% and the YP(Y5P) dielectric while publishing eight different X/Y safety ratings between
them. A cross that matches on capacitance, tolerance and dielectric matches on none of the
fields a safety agency cares about. The rating is in the UR column and in the series
name, not in the value.

A Y1 part bridging a mains barrier is there because an approval body assessed that part.
Replacing it with a Y2 part of the same capacitance is a safety change, and no distributor can
authorise it. If the certification is referenced in your product’s own approval file, keep the
incumbent.

The same caution applies to Walsin’s MLCC safety line. The S2 series holds 173 parts and is
described as Safety Certificated X1/Y2, but its parametric rows publish a DC withstand figure
of 5000 Vdc or 6000 Vdc rather than the AC safety class, which lives in the series name and
the datasheet. Read the datasheet.

Does AEC-Q200 qualification move with the specification?

No. AEC-Q200 is held against a series, and two Walsin series can share all five headline
fields while only one of them carries the qualification. This is the cleanest available proof
that a field-by-field match is not a grade match.

MT18B104K500 and MG18B104K500 are both 0603, X7R, 100 nF, 50 Vdc, both 1.60 × 0.80 ×
0.80 mm on a +0.15/−0.10 mm band. MT is “Automotive Caps Qualified to AEC-Q200”; MG is
“Automotive Caps Without AEC-Q200 Certification”, and our extract holds 788 MG parts. Walsin
names the distinction in the series title, which is about as unambiguous as a catalogue gets.

If your PPAP, your customer’s approved parts list or your own qualification file references
the AEC-Q200 series, a move to the uncertified series is a qualification change with a paper
trail attached. The specification match is real. The certification is not transferable, and
the honest recommendation is to keep the qualified part.

If the incumbent is a soft-termination part, can you cross out of it?

Not without replacing the mechanical margin it was chosen for. Soft termination exists to
survive board flex: a conductive polymer layer between the ceramic and the outer termination
absorbs strain that would otherwise crack the chip. Nothing in the five electrical fields
records that.

Walsin publishes four soft-termination MLCC lines, and the SH, SG and ST datasheets are drawn
against a 5 mm board bend. SH is Ag-poly across 0402 to 2225. SG is Cu-poly across 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 visible consequence is dimensional. In the 0603 X7R 100 nF 50 V group, ST18B104K500,
SG18B104K500 and SH18B104K500 publish a ±0.30 mm band, while 0603B104K500,
MT18B104K500 and MG18B104K500 publish +0.15/−0.10 mm. A part whose band is 0.15 mm
narrower at worst case is a different mechanical object, and it also lacks the polymer layer.

If the original designer specified soft termination, they were solving a board-level problem:
a panel edge, a press-fit connector, a mounting boss, a depanel operation. Removing the
soft-termination part removes the solution. Keep it.

Why is a five-field match uninformative for a high-Q RF capacitor?

Because Q and equivalent series resistance are the reasons you chose the part, and neither is
one of the five fields. In a matching network or a filter arm, ESR appears in series with the
signal, so it becomes insertion loss directly. A cross on case size, dielectric, capacitance,
tolerance and voltage cannot see it.

Here is that gap measured. One specification — 0805, NP0, 56 pF, ±5%, 50 Vdc — returns four
Walsin parts across four different series:

Walsin part number	Series	Published thickness	What the series is built for	Match / mismatch on the five fields
0805N560J500	General Purpose Caps	0.80 ± 0.10 mm and 0.60 ± 0.10 mm	General purpose decoupling and timing	Match on five fields
HH21N560J500	High Q & Low ESR Caps (HH series)	0.80 ± 0.10 mm	High-Q, low-ESR RF positions	Match on five fields
RF21N560J500	Microwave Caps (RF series)	0.85 ± 0.10 mm	Microwave coverage across voltages	Match on five fields, wider L and W band
MT21N560J500	Automotive Caps Qualified to AEC-Q200 (MT series)	0.60 ± 0.10 mm	Certified automotive positions	Match on five fields

Four parts, four intents, three published thicknesses, and every one of them ticks all five
fields. Walsin publishes separate HH, RF and UF datasheets precisely because the distinguishing
property is a curve rather than a field, and the curve is where the answer is. If a part was
chosen from an HH or RF datasheet, a general-purpose part with the same five fields is not a
candidate for that position.

Note also the heights: 0805N560J500 is published at two thicknesses, the RF part at 0.85 mm
and the automotive part at 0.60 mm. In a shielded module that spread can matter as much as
loss.

What about a part that has already passed customer qualification?

Leave it alone unless the line genuinely cannot be filled. A part that has been through
qualification carries evidence that took engineering time, a build slot, a test sequence and
a change-control cycle to produce. Recreating that evidence for a different part number
consumes all of it again.

The arithmetic is about effort, not about the part. A single BOM line change on a qualified
product can require a first-article inspection, a profile verification, a board-flex
sequence, an environmental stress run and a customer notification. If the incumbent line can
still be filled, none of that is worth spending.

One honest exception. If the position is a genuine single-source risk, qualify a second
candidate deliberately, on a calm schedule, with its own build. That is a resilience project,
not a cross.

What should you do instead of crossing these lines?

Fill the incumbent line, and spend the sourcing effort on the lines where a cross is
appropriate. Most BOMs contain plenty of those: general-purpose decoupling, pull-ups,
non-certified bulk capacitance, positions with no flex constraint and no loss requirement.

A short decision rule:

1. Is the part named in a safety approval file? Keep the original.
2. Is a certification referenced in a PPAP or approved parts list? Keep the original.
3. Was soft termination or anti-sulfuration specified deliberately? Keep the original.
4. Was the part selected from a Q, ESR or drift curve? Keep the original.
5. Is the part already qualified and the line fillable? Keep the original.
6. None of the above? Cross it, then qualify the candidate before production use.

Every cross result, ours included, is a specification match offered as a candidate for your
evaluation. It is not an approved substitution, and we cannot assess how a part behaves in
your circuit. On the five cases above, the useful thing a distributor can do is say so.

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FAQ

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Next step

If you are unsure which category a line falls into, send it. Venatech Limited, an authorised
Walsin distributor in Hong Kong, will tell you which of the five cases applies, and will say
plainly when the answer is to keep the incumbent. Where a cross is appropriate, you get the
candidate Walsin part numbers with matches and mismatches set out field by field.

Ask Venatech about a BOM line · Browse the
Walsin disc and safety capacitor range · Read
X1/Y1 vs X1/Y2 safety capacitors and
AEC-Q200 MLCC: MT vs MG series.

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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.