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Desoldering Test (Quicktime Movie, ca. 1 MB)

How to test and compare desoldering braids

by Ernest Spirig

1 Introduction

DE-SOLDERING with a wick is actually SOLDERING the wick with the solder to be removed.
Solderability is therefore a reliable test to predict the desoldering "power" of a wick.
A desolder wick (or braid) is a copper mesh pre-coated with a flux of natural rosin or synthetic rosin based chemicals. Desolder wicks are available in various widths to fit the desoldering task area. The different wick width means different thermal masses or different heat flow needed to bring the wick to the melting temperature of the solder to be removed.

2 Desoldering Operation
The wick end is placed on top of the solder to be removed. A hot soldering iron tip is pressed from top onto the wick. The wick heats up and the highly heat conductive copper braid of the wick conducts the heat to the solder and melts the solder if heat supply is sufficient.

  • The heat source must heat up the copper mass AND the solder mass simultaneously.
  • The heat source must be able to quickly supply the needed thermal calories to the job.
  • The heat source must therefore correspond to the wick size.
  • The heat source is normally the heated tip of a soldering iron.

Quick and undelayed heat transfer stems from the heat stored in the thermal mass of the hot copper tip and only secondary from the available continuous heating power (wattage) of the iron.
A geometric too small tip size compared to the wick width will (even if set to an increased soldering temperature level) be immediately cooled below the melting temperature and this obviously means unsatisfactory desoldering action or no desoldering action at all.
Ideally, a soldering iron tip will have a chisel like shape and the tip width should be at least as wide as the wick width. This rule is a good fit for almost any desoldering task. The soldering iron should be a thermostatic or electronic temperature controlled unit. The temperature set on a electronic controlled iron should be, depending on solder alloy (lead-free) moved upwards to cater to the approx. 30°C higher melting point of lead-free against old solder alloys.

3 TESTING a Desoldering Braid

Testing a desoldering braid with a soldering iron gives a quick and rough idea of its function (good / no good) but will give no "measurable" results, or results which can be easily verified or repeated by the same test setup.
Spirig has used the Solderability definition since now nearly 40 years (1968) of wick manufacture for its internal quality control and audit instrument. Due to the unavailability of a commercial solderability tester in the late 60's, Spirig designed such a tester which later received as a solderability tester a US patent 4,227,415.
Today solderability testers use a similar or identical principle from leading vendors of solder or flux materials. The solderability test is based on the solder dip principle.

4 Solderability Test of Desolder Braid
Spirig US patent 4,227,415 describes such a system that could even be built by an in-house mechanic or electric shop.
The wick specimen is clamped vertically in a holder. A thermostatic controlled solder bath (pot) is located under the specimen.
The wick is slowly lowered down into the pot, enters the molten solder surface and starts to absorb (draw up) solder by capillary forces pulling the fluid solder into the wick mesh.
An electric circuit detects the mechanical contact of the wick to the solder surface. This STARTS a timing cycle for the dip/dwell time and calculates at given drive speed the time needed to reach the specified dip depth.
The picked up solder can be determined as the difference in weight before and after the wick sample test. Pick-up weight will vary with wick sizes (volume within braid available) and will be between a few mg (=1/1000 gram) for a very fine wick size and up to 100 mg for a very large wick size.
Weight must therefore be measured on a precision balance (resolution 1/10 mg = 1/10'000 gram), eg a Mettler® Delta precision electronic scale.
Tare the wick specimen holder with wick specimen attached to it before the test. The weight indicated after dip test will then indicate the net amount of solder absorbed into the wick.
The following test parameters have been used since 1968 to evaluate our wick and competitors wick solderability performance and have also been used as a basic rule for discussions with customers and competitors to avoid errors in advertising messages.

  • Lowering speed: 5 mm / second
  • Dip depth: 2 mm
  • Dwell time: 2 second
  • Bath temperature: 260°C (lead-free definition not yet fixed due to available alloy variations)

5 Visual Evaluation during Dip Test
Upon impact with the molten solder surface the wick will indent the solder surface (negative). After a short delay (time until wick is heated up enough from solder bath to start its own fluxing activity) the solder surface will turn into an upwards positive curve.
The capillary forces of the wick now draw the solder into the wick and even lift the solder surface on the wick sides upwards.
The flow of the solder into the wick can be easily seen. This gives a quick visual check and a good visual preview of the performance.

6 Numerical Evaluation of a Dip Test

A wick specimen is clamped into holder. Holder is put on precision balance. Auto-tare puts weight to <0>. Go through dip test and then put holder again on balance.
The weight displayed is the weight of solder absorbed.
Obviously only identical size wicks can be compared by the weight of the solder absorbed.

7 Potential Errors in Weight Results
The following factors can cause weight errors

(a) The wick test end touching the solder surface must be cut-off in a 90 degrees angle. Otherwise not the full cross section of the wick will be dipped. Dip depth still will be 2 mm, but the angle sized contact offers less squarage for the heat flow into the wick and end of wick offers less volume for solder "storage".
In case of a flattened tubular braid construction (almost 95% of makes available) the flattened tubular end may not be opened artificially as this creates extra absorption volume for solder in that end. In practice the solder tip pressure against the wick will keep the tubular construction flattened at end!

(b) A solder globule forms at the wick end by some irregularities, mostly caused from oxide layers not wiped off from the solder bath surface prior the dip test. A test wick specimen with a globule on end is not acceptable as regular test sample.
Before each dip test the molten solder surface must be wiped clean from oxides, eg using a piece of carton or a thin blade of stainless steel (not wetted by solder). Too thick a metal blade would absorb heat and temporarily cool the molten solder surface.

8 Comparison of Braids of Different Production

Solderability conservation of the fresh copper surfaces is an ongoing fight in the printed circuit board fabrication.
The desoldering braid is exposed exactly to the same problems. A braid starts to age from the moment it is coated.
Spirig's patented "vacuumisation" coat process minimizes influences on solderability during flux coating of the braids copper structure to an absolute minimum. Shelf life is accordingly high under regular storage conditions and extends well into years. We have 25 years old specimen from our qc control storage, samples still performing surprisingly well.
A comparison test between braids of different makes should take ageing into consideration. That means a braid stored under bad conditions (high humidity) compared in a dip test with a fresh coated braid will be an unfair comparison.
Comparison could be concentrated on fresh (up to a few months old) or on equally artificially aged (high humidity and elevated temperature storage) braids.
Wick with a quick ageing behavior might turn a valuable desolder braid stock with time into worthless copper scrap.

9 Wick Manufacturing Procedures

Anybody in urgent need for a desoldering braid can easily create one for himself. The braided shielding of a high-frequency (coaxial) cable is stripped from insulation. The tubular structure is flatened and then is dipped into liquid rosin flux, dried with hot air from a hot air gun and a piece of desoldering braid is born. How long it will operate, how costly it's "production" is, that is another issue. But in an emergency this makes a wick.
By the way this is not an inventive matter, it is an old plumbers trick. See also decisions in the US Court of Customs and Patent Appeals, Washington, "Spirig vs Solder Removal Company / US International Trade Commission".
A decision around 30 years ago was in favour of Spirig and Spirig's wick production technology did not infringe on Solder Removal US patents. The patent has in the meantime lapsed by age. So everybody could manufacture a vacuumized wick should their know-how sufficient. We have not seen yet a vacuumized wick made by a competitive source.
Now and then new vendors do and did get into the wick making business and most usually disappeared quickly. The product "wick" seems to be a simple technical product open for almost everybody with access to copper braid, a rosin flux bath and the intention to make a quick profit.
Practice then quickly shows the reality with consumer applications. To make a well absorbing wick the 'me too' manufacturer starts to increase the activator content in the rosin flux once his customers complain on poor performance. The higher the activator content, the more the need for a perfect raw copper material diminishes.
The increased activator content of residues left on the desoldered area can and will cause corrosion on pc boards and will start to create electric conductivity paths. A situation on which a user has no influence, except that he might clean his desoldered assembly very carefully to be sure to have removed any traces of flux residues and activator contents.
Sooner or later (or never) the user is faced with problems on assemblies now contaminated with such activator residues. He might look for all type of sources for his troubles but not identifying a highly activated wick as the source of trouble. He might never detect the reason if he is not a real professional. Professionals prefer to use a desolder braid from an established long term wick manufacturer. Spirig has now approximately 40 years experience in wick manufacture.

10 Standard Flux coating of a Wick
The bare copper braid is dipped or fed through a volatile solution of rosin (natural or synthetic) in a solvent. Then the wet braid is dried in a hot air stream and bobbined. The solvent is evaporated and discharged into the ambient. Solvents contain ozone depleting substances.
A solution of rosin at increased temperature has a rapidly increasing chemical activity (interaction) with the braids copper. Copper starts to form complex chemical combinations with the activator. Drying the flux solution with hot air heats the flux solvent.

11 Spirig's Patented Flux coating of a Wick

Spirig's patented vacuumisation production process dries the wet flux coated braid structure under vacuum and without heating. Therefore eliminating rise of chemical activity and interactions of the coating with the copper surface. The solvents in the vacuum pump exhaust are recycled.

  • Ozone depleting substances do not escape into nature.
  • Minimal loss of expensive solvents is one of the reason for economic wick production costs.

Spirig's patented powder flux deposit production process does not use solvents at all. Costly solvents and recycling is avoided completely. This is the ultimate economic approach to braid processing.
Ozone depleting substances do not exist at all in this patented flux coating process.

 

A listing of Spirig patents related to wick manufacture and desolder wick products: SPIRIG PRODUCTS ARE COVERED INTERNATIONALLY BY ONE OR MORE OF 100+ PATENTS GRANTED TO ERNEST SPIRIG. USA Re 32,086 4,206,029 4,336,122 4,113,601 3,957,618 5,217,507 4,263,584 4,087,814 4,086,586 4,241,338 4,072,936 4,416,408 4,459,046 4,354,105 4,280,244 4,194,705 4,323,631 4,164,606 4,078,714 4,081,575 4,133,291 4,227,415 GREAT BRITAIN 2,094,692 2,082,952 1,513,496 1,546,601 1,546,602 2,020,697 2,081,743 2,119,403 2,020,872 2,020,871 1,548,771 1,154,880 2,020,872B 2,052,731 2,030,857 1,469,667 1,554,880 CANADA 1,123,377 1,177,013 1,090,199 1,088,811 1,513,496 1,546,601 1,092,546 EUROPE 1,921 5,597 9,860 9,891 45,583 47,579 102,426 131,173 620,604 623,764 GERMANY 2,346,839 2,716,452 2,747,895 2,754,668 2,960,924 3,174,929 3,176,277 2,202,739 AUSTRIA 357,066 FRANCE 7,732,616 7,713,003 5,597 45,583 131,173 AUSTRALIA 552,949 518,349 502,482 525,573 543,866 557,997 506,887 495,837 INDIA 146,997 JAPAN 1,608,603 1,548,156 1,350,196 1,346,246 1,260,197 1,592,223 BRAZIL 7,807,303 7,707,234 1,100,933 7,902,965 8,104,869 7,902,965 7,905,553 7,406,904 7,708,155 ARGENTINA 217,092 ITALY 5,597 45,583 131,173 1,100,933 NETHERLANDS 5,597 45,583 SPAIN 474,830 480,475 428,292 SWEDEN 79,300,737 SWITZERLAND 620,604 624,149 KOREA 14,394 TAIWAN 11,899 19,342 SINGAPORE 24,583 HONG KONG 41,783.