Wetting is the ability of a liquid to displace the air or other liquid originally there and keep contact with a solid surface; how well solder takes is often judged by the contact angle: a contact angle below a right angle usually indicates good wetting and spreading, and above a right angle is unfavorable to wetting. On a joint, non-wetting is molten solder touching the surface but not adhering to the whole surface, leaving the base metal exposed; dewetting is molten solder not adhering firmly and pulling back, leaving irregularly shaped mounds of solder with just a thin layer of solder between them. Solderability can be measured with a wetting balance test (a research institute’s technical blog calls it the preferred method for evaluating solderability), looking at wetting force, time to start of wetting and how fast it rises; but a trade magazine article says that in the solderability standard this test has no acceptance criteria set, and recommends it for engineering evaluation. This article covers the principle of wetting, how to tell dewetting from non-wetting on joints, capillary action, and what solderability testing looks at; flux formulation, how to adjust the reflow profile and how to choose among surface finishes are out of scope. It is divided into 5 stages:
Stage ① | What wetting is: adhesion and cohesion, contact angle.
Stage ② | Surface and flux: three surface tensions, flux and temperature, cold joints.
Stage ③ | Reading the joint: good wetting, dewetting, non-wetting, wetting angle.
Stage ④ | Capillary action: flow in narrow gaps, capillary flow of solder and flux.
Stage ⑤ | Solderability testing: wetting balance, which figures to look at, the role of the test.
Each stage below covers: how to do it, what public sources say, and, where there is a match, what Jiahon can help with; sources, exceptions and this article’s own summaries are in the “Notes” at the end. This is the general case and still depends on the individual case.
Figure 1: The five stages of solder wetting (schematic). ① Wetting — two forces: adhesion against cohesion; contact angle: the smaller, the easier it spreads; boundary: a right angle. Result: able to read the contact angle. ② Surface and flux — surface tension: stops when the three balance; flux: cleans the surface; cold joint: not enough heat or not clean. Result: surfaces prepared first. ③ Reading the joint — wetting: smooth edges, small angle; dewetting: pulls back, leaving mounds; non-wetting: base metal still exposed. Result: tell dewetting from non-wetting. ④ Capillarity — definition: flows by itself in narrow gaps; cause: surface tension plus adhesion; in soldering: solder and flux flow. Result: know where the solder will flow. ⑤ Solderability testing — wetting balance: measures force during immersion; what to look at: force, delay, rate; role: for engineering evaluation. Result: know what the test looks at.
Stage ①: What wetting is
How to do it:
What wetting means: the ability of a liquid to displace the gas or another liquid originally on the surface and keep contact with a solid surface, arising from the difference in intermolecular forces between the liquid and the surface (source 1).
Two forces: adhesion makes a droplet spread on the solid surface, while the liquid’s own cohesion resists spreading; the degree of wetting depends on the balance of these two forces (source 1).
Judge by the contact angle: the contact angle is the angle where the liquid surface meets the solid surface; the more easily a droplet spreads, the smaller the contact angle, and the contact angle can serve as an inverse indicator of wettability (source 1).
Below or above a right angle: a contact angle below a right angle usually indicates good wetting, with the liquid spreading over a larger area; above a right angle generally indicates unfavorable wetting, with the liquid pulling into a tighter droplet (source 1).
The two extremes: a contact angle of zero is complete wetting; a contact angle of 180 degrees is complete non-wetting (source 1).
Figure 2: How to read the contact angle: below a right angle usually indicates good wetting, above a right angle is generally unfavorable to wetting (schematic, based on sources 1 and 5; shapes and proportions not to scale). Contact angle below a right angle: usually good wetting, the liquid spreads over a larger area. Contact angle equal to a right angle: the boundary between wetting and non-wetting. Contact angle above a right angle: generally unfavorable to wetting, the liquid pulls into a tighter droplet. The gold line shows the contact angle, measured where the droplet meets the surface.
What public sources say: an online encyclopedia entry on wetting says: wetting is the ability of a liquid to displace another substance and keep contact with a solid surface, and its degree depends on the balance of adhesion and cohesion; the wetting ability of a liquid and surface forces also determine capillary action; the smaller the contact angle, the easier the spreading, so it can serve as an inverse indicator of wettability; below a right angle usually indicates good wetting, and above a right angle is generally unfavorable to wetting (notes table 1).
Stage ②: Surface and flux
How to do it:
Three surface tensions: a trade magazine article (June 2005) explains with the Young–Dupré equation: molten solder spreads on the substrate until the three surface tensions — substrate/gas (or flux), substrate/molten solder and molten solder/gas (or flux) — are in balance; the driving force of spreading is the imbalance of these three surface tensions (source 5).
Flux affects the surface: the same article says that using different fluxes affects the surface tension between the substrate and gas (or flux) — that is, how clean the flux gets the surface; the other two surface tensions are constant at a fixed temperature and composition (source 5).
Temperature matters too: the test temperature affects the surface tension and contact angle of the solder; the author writes that this can be used to find the temperature conditions under which a given solder and flux combination reaches acceptable wetting (source 5).
Cold joints: a NASA surface mount workmanship standard (August 1999 edition) defines a cold joint as a joint with poor wetting and a gray, porous appearance, caused by insufficient heat, insufficient cleaning before soldering, or too many impurities in the solder (source 3).
Contaminants: the same standard divides contaminants into ionic and non-ionic; ionic contamination is usually process residue, such as flux activators, fingerprints, and etching or plating salts (source 3).
What public sources say: the trade magazine article mentioned above says: molten solder spreads on the substrate until the three surface tensions balance, and the driving force of spreading is the imbalance of surface tensions; flux affects the surface tension between the substrate and gas (or flux), that is, how clean the surface is; the test temperature affects surface tension and contact angle. The NASA workmanship standard mentioned above says: a cold joint is a joint with poor wetting and a gray, porous appearance, caused by insufficient heat, insufficient cleaning before soldering or too many impurities in the solder; contaminants are ionic or non-ionic, and ionic contamination is mostly process residue such as flux activators, fingerprints, and etching or plating salts (notes table 2).
Stage ③: Reading the joint
How to do it:
Good wetting: the same workmanship standard defines wetting as the flow and adhesion of a liquid on a solid surface, characterized by smooth, even edges and a small angle (source 3).
Non-wetting: the surface has contacted molten solder, but the solder has not adhered to the whole surface, and the base metal is still exposed (source 3).
Dewetting: the molten solder has not adhered firmly and has pulled back; there is a clear boundary between the solder and the conductor (or termination), leaving irregularly shaped mounds of solder with just a thin layer of solder covering the area between them (source 3).
Wetting angle: measured from the vertical, a solder fillet forming a positive angle is called positive wetting and a negative angle negative wetting (source 3); this angle is the fillet measured from the vertical, and source 3 does not say it equals the contact angle of Stage ①.
Acceptance and rejection: the general acceptance conditions of the same standard include the solder wetting every part of the connection, forming a fillet all around the periphery, the lead outline being visible, and complete wetting; the rejection conditions list poor wetting, dewetting, non-wetting, cold joints and others (source 3).
What Jiahon Can Help With:
- When you need to magnify the edges of a joint, look for dewetting or exposed base metal, and photograph it for the record, you can use a 7-inch digital microscope (the product page says the JH-G1200 has 1 to 1,200 times continuous zoom (the JH-G1600 1 to 1,600 times), an adjustable lens angle to avoid glare, built-in photo and video functions, and instant saving as a basis for quality records and customer reports; the product page says the memory card for saving is included in the advanced accessory kit).
What public sources say: the NASA workmanship standard mentioned above says: wetting is the flow and adhesion of a liquid on a solid surface, characterized by smooth, even edges and a small angle; non-wetting is the surface contacting molten solder without the solder adhering to the whole surface, with the base metal exposed; dewetting is molten solder pulling back with a clear boundary, leaving irregular mounds and a thin solder film; positive and negative wetting are distinguished by the angle of the fillet measured from the vertical; the general acceptance conditions require the solder to wet all parts with complete wetting, and the rejection conditions include poor wetting, dewetting and non-wetting (notes table 3).
Stage ④: Capillary action
How to do it:
What capillary action is: the flow of a liquid in a narrow space without the help of external forces such as gravity — for example water being drawn into a thin tube or paper (source 2).
Where it comes from: intermolecular forces between the liquid and the surrounding solid surfaces; when the tube is narrow enough, the liquid’s own surface tension and the adhesion between the liquid and the tube wall drive the liquid together (source 2).
Its relation to wetting: a liquid’s wetting ability and surface forces are also the causes of capillary action (source 1).
Capillary flow of solder and flux: the NASA workmanship standard defines “wicking” as the flow of molten solder, flux or cleaning solution by capillary action (source 3).
What public sources say: an online encyclopedia entry on capillary action says: capillary action is the process of a liquid flowing in a narrow space without external forces such as gravity, for example between the bristles of a paint brush, in a straw, in paper and in plaster; it arises from intermolecular forces between the liquid and surrounding solid surfaces, and when the tube is narrow enough, surface tension and adhesion drive the liquid together. The workmanship standard mentioned above says: wicking is the flow of molten solder, flux or cleaning solution by capillary action (notes table 4).
Stage ⑤: Solderability testing
How to do it:
There are standard tests: a German research institute’s technical blog (February 2023) says the wettability and solderability of circuit boards and component metallizations can be tested with standard tests, such as J-STD-002 and several IEC test methods (source 4).
How a wetting balance measures: dip the specimen into a solder bath or solder globule, and a sensor measures the force during immersion; at first the surface tension of the solder and buoyancy push the specimen up, and once the specimen starts to wet, a force pulling it into the solder arises, plotted as a curve of wetting force against time (source 4).
Which figures to look at: the basis of evaluation is the wetting force, the delay time and how fast wetting rises (source 4); another article says one commonly used figure is the time at which the force crosses zero (the moment of changing from non-wetting to wetting, when the contact angle equals a right angle), and the maximum wetting force per unit area is also measured (source 5).
The role of the test: a trade magazine article says that in J-STD-002 the wetting balance test is classified as a test without acceptance criteria, recommended for engineering evaluation and not as a pass/fail judgment on the production line; conditions differ between studies, so results are not easy to compare directly (source 5).
Other approaches: the same research institute lists other tests that can be done: in-situ observation of melting behavior, wetting efficiency tests with a solder crucible, measuring solder spread, measuring contact angle in cross-sections or with a laser scanning microscope, and solder ball tests on non-wetting substrates (source 4).
When to do it: the same blog says wetting and solderability tests are often needed when the soldering process shows anomalies or joint quality cannot be achieved (source 4).
Figure 3: The wetting balance curve: at first buoyancy, the zero crossing marks the change from non-wetting to wetting, then it rises to the maximum wetting force (schematic, based on sources 4 and 5; the curve shape is not to scale). Axes: force against time, with zero marked; upward push (buoyancy) below zero, pull into the solder above zero; zero crossing: from non-wetting to wetting; rate of rise; maximum wetting force. What to look at: wetting force, rate of rise; also the zero-crossing time and maximum wetting force.
What public sources say: the German research institute’s technical blog mentioned above says: wettability and solderability can be tested with J-STD-002 and IEC standard tests; the wetting balance is the preferred method for evaluating the solderability of components and circuit boards, measuring the force during immersion in solder and plotting it as a wetting force curve, looking at wetting force, delay time and rate of rise, and a sharp contact angle also indicates good wetting; other tests include wetting efficiency, spread, contact angle and solder ball tests; these tests are often needed when the process shows anomalies. The trade magazine article mentioned above says: the wetting balance is also called the meniscograph test, measuring the force exerted by molten solder on the specimen over time; the force is negative at first (buoyancy), and the zero crossing represents the change from non-wetting to wetting; in J-STD-002 it is a test without acceptance criteria, recommended just for engineering evaluation (notes table 5).
Notes: sources and exceptions
The following is for readers who need to check; skipping it does not affect the approach above. Where your product specifications, the customer’s quality requirements, or the data of your component and solder makers differ from this article, follow them.
Where each stage comes from
| Source | Where it is used, and the source |
|---|---|
| Source 1 | Stages ①, ④: definition of wetting, adhesion and cohesion, contact angle, relation to capillary action; an online encyclopedia entry on wetting |
| Source 2 | Stage ④: definition and causes of capillary action; an online encyclopedia entry on capillary action |
| Source 3 | Stages ②, ③, ④: cold joints, contaminants, wetting, non-wetting, dewetting, positive and negative wetting, acceptance and rejection conditions, definition of capillary flow; NASA’s surface mount workmanship standard (August 1999 edition) |
| Source 4 | Stage ⑤: standard solderability tests, how a wetting balance measures, basis of evaluation, other tests, when to do them; a German research institute’s technical blog (February 2023; this institute offers solderability testing services) |
| Source 5 | Stages ②, ⑤: the Young–Dupré equation, effects of flux and temperature, zero-crossing time and maximum wetting force, the role of the wetting balance test in the solderability standard; a trade magazine article (June 2005) |
| Jiahon data | “What Jiahon Can Help With”; the digital microscope product page |
Table 1 | Contact angle and wetting
| Contact angle | What public sources say |
|---|---|
| Zero | Complete wetting (source 1) |
| Below a right angle | Usually good wetting; the liquid spreads (source 1) |
| Above a right angle | Generally unfavorable to wetting; the liquid pulls into a tighter droplet (source 1) |
| 180 degrees | Complete non-wetting (source 1) |
Source: 1.
Table 2 | Surface, flux and temperature
| Item | What public sources say |
|---|---|
| Three surface tensions | Molten solder spreads until the three balance; the imbalance is the driving force of spreading (source 5) |
| Flux | Affects the surface tension between substrate and gas (or flux), that is, how clean the surface is (source 5) |
| Temperature | Affects surface tension and contact angle (source 5) |
| Cold joint | Poor wetting caused by insufficient heat, insufficient cleaning before soldering or too many impurities (source 3) |
| Contaminants | Ionic and non-ionic; ionic is mostly process residue (source 3) |
Sources: see each row.
Table 3 | Wetting states on the joint
| State | What public sources say |
|---|---|
| Wetting | Smooth, even edges, small angle (source 3) |
| Non-wetting | Contacted molten solder but did not adhere to the whole surface; base metal exposed (source 3) |
| Dewetting | Molten solder pulled back, clear boundary, leaving irregular mounds and a thin solder film (source 3) |
| Positive and negative wetting | Measured from the vertical, the fillet forms a positive or negative angle (source 3) |
Source: 3 (the 1999 edition; before use, confirm the current standard you apply).
Table 4 | Capillary action
| Item | What public sources say |
|---|---|
| Definition | A liquid flowing in a narrow space without external forces (source 2) |
| Cause | Surface tension and adhesion drive the liquid together (source 2) |
| In soldering | Molten solder, flux or cleaning solution flowing by capillary action (source 3) |
Sources: see each row.
Table 5 | Wetting balance test
| Item | What public sources say |
|---|---|
| How it measures | The specimen is dipped into solder and the force is measured over time (sources 4, 5) |
| Curve | Buoyancy at first (negative); the zero crossing marks the change from non-wetting to wetting; then up to the maximum wetting force (source 5) |
| What to look at | Wetting force, delay time, rate of rise (source 4); zero-crossing time, maximum wetting force (source 5) |
| Role | In J-STD-002, a test without acceptance criteria, recommended for engineering evaluation (source 5) |
Sources: see each row.
Exceptions and limits
What was seen: this article read 5 public sources: two online encyclopedia entries (sources 1, 2), a NASA workmanship standard (source 3), a research institute’s technical blog (source 4) and a trade magazine article (source 5); the clauses of J-STD-002 or solder joint acceptance standards themselves were not read, and the role of the wetting balance test in J-STD-002 is quoted via source 5.
Dates: source 3 is the 1999 edition and source 5 a 2005 article; this article uses their definitions of terms, principles and explanations of test methods — before use, confirm the current standard you apply.
Sources with a position: the research institute of source 4 offers solderability testing and process consulting services.
Scope: flux formulation, how to adjust the reflow profile, how to choose among surface finishes, and specific defects such as wicking failures are out of scope for this article.
This article’s own summary: the scenario for the microscope point under “What Jiahon Can Help With”, and Figures 1 to 3 (Figure 2 based on sources 1 and 5, Figure 3 on sources 4 and 5) are this article’s own summary; the other sentences can be matched to statements in the sources or on Jiahon’s pages.
Equipment and products: in “What Jiahon Can Help With”, the functions and specifications in parentheses follow the product page text, and the specification is as on the product page; the link text uses the function name and does not give the product page’s model number, series code or brand.
This article is a summary of the general case, not a restatement of standard clauses, and not an endorsement of any research institute, solder maker or testing body, nor an assurance for your products and processes. Please follow your product specifications, the customer’s quality requirements, the relevant standards, and your own validation; this is the general case and still depends on the individual case.