Walsin Part Number Prefix: Four Characters Still Leave 160,161 of 359,135 Parts Ambiguous, and the Fifth Resolves Every One

A Walsin part number prefix needs five characters to name its family. We grouped all 359,135 distinct part numbers by their first one to fifteen characters: two characters leave 214,903 parts ambiguous, four still leave 160,161 (44.6%), and the fifth leaves none.

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Venatech Limited, an authorised Walsin distributor in Hong Kong, holds all 203 parametric export files published by Walsin Technology (Walsin). This post is written from the 114 files that hold part numbers, among them 00_ALL_MLCC_parts_combined.xlsx, Walsin_Inductor_ALL.xlsx, Walsin_MOV_ALL.xlsx, Walsin_Antenna_ALL.xlsx and the 64 chip resistor and 11 thin film files, read as raw strings with no trimming and no case folding. Two-character prefixes are already known to be shared. The question a BOM checker needs answered is how long the prefix must be before it is safe to trust. Every part number below is real.

How many characters of a Walsin part number do you need before the family is certain?

Five, and four is not enough. We parsed all 359,135 distinct part numbers in the eight families and, for every length from 1 to 15, counted the parts whose first characters are also used by a part in a different family. At one character that is 345,040 parts (96.1%). At two it is 214,903 (59.8%). At three it is 187,912 (52.3%). At four it is 160,161 (44.6%). At five it is zero, and it stays zero at every longer length. No part number appears in two families, so the full string is always a clean key. The question is only how much of it you must read first.

The curve is not smooth. Going from two characters to four removes only 54,742 parts from the ambiguous pool, 25.5% of what two characters left. One more character removes the last 160,161.

Prefix length	Distinct prefixes	Prefixes used by more than one family	Parts in an ambiguous prefix	Share of 359,135	Families still involved	Match / mismatch for a router that stops at this length
1	21	8	345,040	96.1%	all eight	Mismatch — 8 of 21 first characters are shared
2	65	8	214,903	59.8%	seven (not disc capacitor)	Mismatch — WF, MF, SF, SR, RF, MA, WT, RG
3	177	11	187,912	52.3%	five	Mismatch — WF0, WF1, WF2, SF1, SF2, SR1, SR2, MF1, RFA, RFC, RFD
4	302	12	160,161	44.6%	three: chip resistor, thin film, varistor	Mismatch — 12 prefixes, all resistor or varistor
5	687	0	0	0.0%	none	Match — every prefix names exactly one family

We have not seen this counted anywhere. The published discussion of Walsin prefixes stops at two characters. The three extra rows are where the answer to the practical question sits: a rule keyed on the first four characters is wrong for 44.6% of the catalogue, and a rule keyed on the first five is wrong for none of it.

How quickly does the ambiguity fall from one character to five?

Slowly, then all at once. We checked the first-character level by hand. Eight first characters are shared between families: A and B (inductor and RF), H (inductor and MLCC), M, S and W (several), R (MLCC, RF and antenna) and T (MLCC and RF). The other thirteen belong to one family each. Yet the eight shared ones cover 345,040 parts, because W, S and M carry the resistor catalogue.

The step from three to four characters is the cruel one. At three characters the RF-versus-antenna prefixes RFA, RFC and RFD are still shared, with 9 and 9, 6 and 15, and 13 and 93 parts respectively. A fourth character separates all of them. It is the only step that removes a whole group of families at once, and it takes the count of families still involved from five to three. After that the pool holds only chip resistors, thin film resistors and varistors, and one more character empties it. The one-character neighbours that cross the thick film and thin film boundary are counted in thick film or thin film: 6,836 part number pairs differ by one character.

Which families does a short prefix settle early, and which does it never settle?

Disc capacitors settle at two characters, inductors and MLCC at three, RF and antenna at four, and chip resistor, thin film and varistor at five. We counted, for each family, how many of its own parts sit in a prefix shared with another family at each length, reading Walsin_Filter.xlsx and its six RF siblings for RF and Walsin_DiscCap_ALL.xlsx for disc capacitors.

Family	Distinct parts	Ambiguous at 1	At 2	At 3	At 4	Characters needed	Match / mismatch for a two-character router
Disc capacitor	929	308	0	0	0	2	Match — settled at two
Inductor	5,410	5,240	143	0	0	3	Mismatch — 143 parts misrouted
MLCC	23,601	10,300	3,615	0	0	3	Mismatch — 3,615 parts, all in the RF series
RF	457	454	418	120	0	4	Mismatch — 418 parts
Antenna	44	44	44	28	0	4	Mismatch — all 44
Varistor	379	379	379	128	41	5	Mismatch — all 379
Chip resistor	234,401	234,401	116,390	93,722	86,522	5	Mismatch — 116,390 parts
Thin film	93,914	93,914	93,914	93,914	73,598	5	Mismatch — every part

Read the thin film row. All 93,914 thin film parts share a first-three-character prefix with a chip resistor, and 73,598 of them, 78.4%, still do at four. The reason is simple. In Thin_Film_Precision_Chip_Resistor_WF_Series_0402_100.xlsx and the chip resistor files alike, the stem and the case-size digits in positions three and four are the same. Only the fifth position differs. The varistor row, from Walsin_MOV_ALL.xlsx, is the smaller surprise: the 379 varistors share the SR stem with 12,871 chip resistors, and 41 of them still share four characters.

What is left in the twelve four-character prefixes that survive?

Twelve prefixes hold the entire 160,161-part residue: seven of the form WF plus a case-size pair, MF12, SF12, SR10, SR12 and SR20. We listed each one with the fifth characters that each family uses under it, taking the thin film side from Tantalum_nitride_Thin_Film_Chip_Resistor_MF_Series_1.xlsx and Anti_Sulfuration_Thin_Film_Chip_Resistor_SF_Series_1.xlsx among others. In every one of the twelve the two sets are disjoint, which is why a fifth character is enough.

Four-character prefix	Family A, parts and fifth characters	Family B, parts and fifth characters	A real pair	Match / mismatch
WF04	Chip resistor 8,552: AGHKP	Thin film 6,359: BFQRTUWZ	WF04P1024FTL / WF04B10R0TTL	Mismatch — disjoint letters
WF06	Chip resistor 15,424: AGHJKMNPSV	Thin film 7,562: BFQRTUWZ	WF06K10R0BTL / WF06B4R70TTL	Mismatch — disjoint letters
WF08	Chip resistor 15,135: AGHJKMNPSV	Thin film 7,985: BFQRTUWZ	WF08K10R0BTL / WF08B4R70TTL	Mismatch — disjoint letters
WF10	Chip resistor 4,602: AHP	Thin film 7,216: FQRTUWZ	WF10H10R0BTL / WF10F10R0TTL	Mismatch — disjoint letters
WF12	Chip resistor 16,350: AGHJKLMNPSV	Thin film 8,427: BFQRTUWZ	WF12K10R0BTL / WF12B4R70TTL	Mismatch — disjoint letters
WF20	Chip resistor 8,083: AHMNPSV	Thin film 7,436: FQRTUWZ	WF20H1R00BTL / WF20F10R0TTL	Mismatch — disjoint letters
WF25	Chip resistor 8,408: AHLNPSV	Thin film 7,823: FQRTUWZ	WF25H1R00BTL / WF25F10R0TTL	Mismatch — disjoint letters
MF12	Chip resistor 2,460: EHP	Thin film 20,520: FQRW	MF12H1R00DTL / MF12Q10R0ATL	Mismatch — disjoint letters
SF12	Chip resistor 1,972: HP	Thin film 270: QRTUW	SF12H10R0BTL / SF12T1R00ATL	Mismatch — disjoint letters
SR10	Chip resistor 1,799: WX	Varistor 18: 12	SR10W1R00FTL / SR101K05D	Mismatch — letters against digits
SR12	Chip resistor 1,799: WX	Varistor 6: 12	SR12W1R00FTL / SR121K05D	Mismatch — letters against digits
SR20	Chip resistor 1,938: WX	Varistor 17: 1	SR20W1R00FTL / SR201K05D	Mismatch — letters against digits

The sides sum to 86,522 chip resistor, 73,598 thin film and 41 varistor parts: 160,161 in all. Take the WF10 pair. WF10H10R0BTL and WF10F10R0TTL are both 1210, both 10.0 ohm, both on 7-inch reels with a lead-free tin termination. The first is a high-precision thick film part at plus or minus 0.1 percent, 100 ppm per degree Celsius or better and 1/2 W. The second is a thin film part at plus or minus 0.01 percent, 15 ppm per degree Celsius or better and 2/5 W. Four characters cannot tell them apart, and the data says they differ on family, tolerance, TCR and power rating.

Is the fifth character a rule or a coincidence?

A rule, with a boundary on it. Under the three stems that thick film and thin film share, WF, MF and SF, we read the fifth character of all 195,949 resistor parts. Chip resistors use exactly twelve letters there, A E G H J K L M N P S V, thin film uses exactly eight, B F Q R T U W Z, and the two sets do not overlap. Twenty letters decide 195,949 parts.

The boundary is that the letter alone is not a global rule. W is a thin film letter under WF, MF and SF, and it is a chip resistor letter under SR, where the high-grade anti-sulfuration series writes W or X in the same position. That is why the table above is keyed on four characters plus the fifth, never on the fifth alone. What each thin film letter means for temperature coefficient, including the two that change meaning at one case size, is covered in the thin film part number: position five and 10 codes that disagree. We make no claim here about what the twelve chip resistor letters encode, because we hold no datasheet for the thin film or varistor exports and we do not guess.

What should a BOM checker do with a prefix?

Never decide a family from fewer than five characters, and never decide it from a prefix at all when the whole string is available. We have built the free cross-reference tool to index the complete code, which is why a part number pasted in full is not misrouted. Four rules follow from the counts.

1. A two-character rule misroutes 214,903 parts. That is 59.8% of the catalogue. Any lookup that stops at WF, MF, SF, SR, RF, MA, WT or RG is wrong more often than it is right for the resistor families.
2. A four-character rule misroutes 160,161. The twelve prefixes in the table need a fifth-character branch, and the branch must be keyed on the prefix as well as the letter.
3. A five-character rule misroutes none, on the data we hold. It is safe as a first pass, not a substitute for reading the part number in full.
4. Do not normalise before you prefix. The catalogue is upper case throughout and 39,162 chip resistor codes carry an embedded space.

We hold 28 Walsin datasheets, 451 pages, covering 26 MLCC and 2 chip resistor series, and the datasheet is the document to qualify against. We hold none for thin film, varistor, inductor, disc capacitor, RF or antenna, so on those lines this post describes the export and nothing more. The structural reason none of the columns join blind is in only 2 of 90 columns join across all eight Walsin families, and the string lengths are in 10 lengths, zero of 12 positions agree.

What is in Hong Kong is on ready stock: 1,125 part numbers as at 23 September 2026, a dated snapshot and not a live feed. The resistor range sits on the chip resistor page.

The short version

Five characters name a Walsin family and four do not: 345,040 of 359,135 parts sit in a shared prefix at one character, 214,903 at two, 187,912 at three, 160,161 at four and none at five. Twelve four-character prefixes hold the residue. Under the three stems that thick film and thin film share, chip resistors use twelve fifth-character letters and thin film eight, with no overlap, and W flips family between WF and SR. A pasted part number is safe to route on five characters and safest on all of them. We will not tell you two parts are the same because four characters agree. We will show you the fields that differ. Qualification stays with you.

Send us the part numbers your BOM tool grouped by prefix, and we will tell you which groups it merged that it should not have. And a question back: if your checker keys on four characters, which of your resistors has it been calling by the wrong family?

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Frequently asked questions

<!– wp:rank-math/faq-block {"questions":[{"id":"faq-question-01","title":"How many characters of a Walsin part number identify its product family?","content":"Five. We grouped all 359,135 distinct part numbers by their first one to fifteen characters. At four characters 160,161 parts, 44.6%, still sit in a prefix used by more than one family. At five characters none do, and the count stays zero at every longer length up to fifteen.","visible":true},{"id":"faq-question-02","title":"Why is a two-character prefix not enough to route a Walsin part?","content":"Because eight two-character prefixes are shared between families and they cover 214,903 parts, 59.8% of the catalogue. WF alone is used by chip resistors, thin film resistors and inductors. A router that stops at two characters will misroute most resistors and every RF series capacitor.”,”visible”:true},{“id”:”faq-question-03″,”title”:”Which Walsin families are identified by the first two or three characters?”,”content”:”Disc capacitors are settled at two characters. Inductors and MLCC are settled at three. RF and antenna parts need four, and chip resistors, thin film resistors and varistors need five. The slowest is thin film, where all 93,914 parts share their first three characters with a chip resistor.”,”visible”:true},{“id”:”faq-question-04″,”title”:”What does the fifth character do under WF, MF and SF?”,”content”:”It separates thick film from thin film. We read the fifth character of all 195,949 resistor parts under those stems: chip resistors use twelve letters, A E G H J K L M N P S V, and thin film uses eight, B F Q R T U W Z. The two sets do not overlap, so the letter alone decides the film type under those stems.”,”visible”:true},{“id”:”faq-question-05″,”title”:”Can I use the fifth character on its own to tell resistor from varistor?”,”content”:”No. The letter W is a thin film letter under WF, MF and SF, and a chip resistor letter under SR, where the varistor uses a digit. The fifth character has to be read together with the four before it, which is why the table of twelve prefixes lists them as pairs.”,”visible”:true},{“id”:”faq-question-06″,”title”:”Does a five-character prefix prove a part exists or is orderable?”,”content”:”No. A five-character prefix only names the family on the data we hold. It says nothing about whether the full code is a real part, whether it is in stock or whether it matches your incumbent. Check the full string against the export or the free cross-reference tool, then confirm against the datasheet.”,”visible”:true}],”listStyle”:””,”titleWrapper”:”h3″,”titleCssClasses”:””,”contentCssClasses”:””,”sizeSlug”:””} –>

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