What an FAE Actually Does for a Passive BOM, and When You Do Not Need One

An FAE reads your BOM against the manufacturer’s real data and tells you where each line
fails. For a passive BOM that means four checks: does the part number pin the dimensions, is
the rated power right for the case size, is the qualification status what the application
needs, and can the part be bought in the form you specified.

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What is an FAE, and what does one actually do with a BOM?

FAE stands for field application engineer. On a passive BOM the work is unglamorous and
specific: take each line, resolve it against the manufacturer’s parametric data and the
datasheet, and report the mismatches field by field. Not a verdict on whether the part is
good. A list of where it differs from what you asked for.

Venatech Limited, an authorised Walsin distributor in Hong Kong, holds parametric data for
359,974 Walsin Technology (Walsin) part numbers across eight product lines. That is what the
check is run against. The support scope is part selection, cross-reference, qualification,
FAE assistance, AEC-Q200 support and mass production. Everything below is inside it.

The reason the role exists is that a passive part number looks complete and often is not. The
three sections that follow are the three ways that happens in Walsin’s own published data.

Why does a correct part number not fully specify the part?

Because some part numbers carry more than one body height. 358 Walsin MLCC part numbers in
our extract appear with two or more different nominal thickness figures. The two rows are
separated only by a thickness Symbol code that does not appear anywhere in the part number
itself. The part number alone does not pin the height.

This is the single most useful thing an FAE catches on a passive BOM, because a height
mismatch is invisible until the board is populated. The affected part numbers concentrate in
the middle-to-high voltage, general purpose and SH soft termination families, and in the
1206, 0805 and 1210 case sizes. 363 of the 742 rows involved carry a remark asking the
customer to contact a Walsin representative, so Walsin knows the ambiguity is there.

Unique to Venatech’s data. 358 Walsin MLCC part numbers return two or more different
nominal thicknesses in Walsin’s own parametric export, distinguished only by a Symbol
column that is absent from the part number. 333 of the affected rows are 1206, 231 are
0805 and 77 are 1210. This count is not published anywhere; it comes from grouping
Walsin’s export by part number and measuring the spread of the thickness field.

How does an FAE catch a height mismatch before it reaches the line?

By asking for the height ceiling and then checking every candidate against both rows, not
one. Give a ceiling and the ambiguous part numbers sort themselves immediately: one variant
clears it, the other does not, and the FAE tells you which Symbol code to specify on the
order. Without a ceiling there is nothing to check against.

Four real examples, all verbatim from Walsin’s MLCC export, tested against a 1.00 mm height
ceiling.

Part number	Case size	Specification	Thickness, variant A	Thickness, variant B	Match / mismatch against a 1.00 mm ceiling
TT31X106K100	1206	X5R, 10 µF, 10 Vdc	1.15 mm (Symbol J)	0.85 mm (Symbol T)	Mismatch on A — 1.15 mm exceeds the ceiling. Match on B — 0.85 mm clears it. Same part number
0805B104K500	0805	X7R, 100 nF, 50 Vdc	0.80 mm (Symbol B)	1.25 mm (Symbol D)	Match on A, mismatch on B. Variant B is marked as a customised product in Walsin’s export
0805B102K201	0805	X7R, 1000 pF, 200 Vdc	1.25 mm (Symbol D)	0.80 mm (Symbol B)	Mismatch on A, match on B
SH32B225K101	1210	X7R, 2200 nF, 100 Vdc	2.50 mm (Symbol M)	2.00 mm (Symbol K)	Mismatch on both — neither variant clears a 1.00 mm ceiling
MT21B104K500	0805	X7R, 100 nF, 50 Vdc, AEC-Q200	0.80 mm (Symbol B)	1.25 mm (Symbol D)	Match on A, mismatch on B. The automotive series is affected too

TT31X106K100 is the one to remember. It sits in Walsin’s Low Profile TT series, which is
chosen precisely because height is constrained, and it still returns two heights under one
part number with a 0.30 mm spread between them. A designer who specified it for a 1.00 mm
gap on the strength of the series name alone has a coin toss, not a specification.

What happens when the case size is right but the rated power is wrong?

The board passes assembly and fails in service. Case size does not set rated power in a
Walsin chip resistor; the series does. 26 Walsin parts are 0805, 10,000 Ω, ±1% — and their
rated power spans 1/8 W, 1/4 W, 1/3 W and 1/2 W. A fourfold difference in the same
footprint, same value, same tolerance.

Part number	Case size	Resistance	Tolerance	Rated power	Series group	Match / mismatch against a 1/4 W requirement
WR08X1002FTL	0805	10,000 Ω	±1%	1/8 W	Thick Film General Purpose	Match on size, value, tolerance · mismatch on power — half of what is needed
WF08P1002FTL	0805	10,000 Ω	±1%	1/4 W	Thick Film High Power	Match on all four fields
MK08S1002FTL	0805	10,000 Ω	±1%	1/3 W	Thick Film Anti-Surge	Match with margin · different series, confirm the surge rating is wanted
MF08E1002FTL	0805	10,000 Ω	±1%	1/2 W	Thick Film Anti-Surge (triple power surge)	Match with margin · mismatch if board space assumed a general-purpose part
WF08A1002FTL	0805	10,000 Ω	±1%	1/2 W	Thick Film High Power	Match with margin · 4× the general-purpose figure at the same footprint

An FAE reads this the other way round from a designer. The designer starts from the
footprint on the layout and looks for a part that fits it. The FAE starts from the dissipation
the circuit demands and looks for a series that carries it, then checks the footprint last.
The two approaches land on different part numbers from the same 0805 pad. The general
principle is set out in
Chip resistor power rating by case size.

How does an FAE check whether a part is really AEC-Q200?

By reading the series, never the prefix. Walsin runs two automotive MLCC families whose
part numbers differ by one character, and only one of them is AEC-Q200 qualified. The MT
series is described in Walsin’s own data as qualified to AEC-Q200. The MG series is described
as automotive without AEC-Q200 certification.

Part number	Series as Walsin states it	Case size	Specification	Thickness	Match / mismatch against an AEC-Q200 requirement
MT21B104K500	Automotive Caps Qualified to AEC-Q200 (MT series)	0805	X7R, 100 nF, 50 Vdc	0.80 mm	Match — qualified, and the specification lines up
MG21B104K500	Automotive Caps Without AEC-Q200 Certification (MG series)	0805	X7R, 100 nF, 50 Vdc	0.80 mm	Mismatch on qualification only. Every other field is identical

Two part numbers, one character apart, identical on case size, dielectric, capacitance,
rated voltage and thickness, and opposite on the only field that decides whether the line
passes an automotive audit. This is why “same specification” is never the question worth
asking. The field-by-field split between the two families is covered in
AEC-Q200 MLCC: MT vs MG series.

The same care applies on the resistor side. Walsin appends “(Automotive)” to a number of
chip resistor series names, and an automotive series name is not by itself a statement of
AEC-Q200 qualification.

What does an FAE not do?

An FAE does not approve your design and does not certify a part for your circuit. The output
is a documented set of matches and mismatches against the manufacturer’s published data, plus
the datasheet references behind each one. Qualification in the application remains yours to
run and yours to sign.

Nor does an FAE tell you a part is equivalent. That word hides exactly the fields that break
a cross — thickness, termination, qualification status and packaging. The output is a table
with a mismatch column, and where the mismatch is material it says so rather than averaging
it away.

Where our own data is incomplete or contradicts itself, the answer is a flag and a question
to Walsin, not a guess. Several of the notes in this article are exactly that.

What should I send to get useful FAE input?

Part numbers, quantities, and the three constraints that most often break a passive line:
the height ceiling in millimetres, the dissipation the circuit actually demands, and whether
the programme requires AEC-Q200. Those three turn a list of part numbers into something that
can be checked rather than merely looked up.

Add the application if you can. A decoupling line, an RF matching network and a current
sense path pull toward entirely different Walsin series from the same case size, and the
series is where rated power, thickness tolerance and qualification status are actually set.
Certificate of Conformity is available on request on any line supplied.

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FAQ

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Next step

Send the BOM lines with the height ceiling, the dissipation and the AEC-Q200 requirement.
Venatech Limited, an authorised Walsin distributor in Hong Kong, will return each line
resolved against Walsin’s parametric data and datasheet, with a mismatch column, the
thickness variants where a part number carries more than one, and the orderable part number
including its packaging suffix.

Send your BOM to Venatech · Browse the Walsin MLCC range ·
Read AEC-Q200 MLCC: MT vs MG series 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.