Hot air smd rework station. SMD removal??? Defective Machines???

When I was developing my homebrew hotair station (

) I found that a few simple tests helped while setting temperature and airflow. These tests are not intended to be quantitative, merely to indicate basic functionality.

1. Basic temp test is to direct the airflow at a piece of bond writing or typing paper at a distance of about one-half inch (13mm); it should blacken a spot in one second at your chosen air velocity.
2. Use a scrap board with SMT components to experiment with air velocity and temperature settings; airflow that doesn't blow away the smallest components when used with the minimum temperature that reflows the solder within a couple of seconds is ideal. Increase temperature and airflow for larger components as determined by experiment. Note the settings for production use.

It can't hurt to calibrate you settings with proper temperature probes and pressure gauges. My station has an accurate low-velocity air gauge but as of yet no temp. probes.

Michael

SAUHING LEE wrote in news:49e06cf0-a934-4b23-b83f- snipped-for-privacy@e67g2000hsa.googlegroups.com:

There is no reason to go about 225 C (437 F), for any normal solder, and, if you remember the book title by Robert Henlein, paper doesn't burn till you get to 451.

[quote] C F 183 361.4 63/37 It has the lowest melting point (183 °C or 361.4 °F) of all the tin/lead alloys; and 220 428 SnAg3.0Cu0.5, tin with 3% silver and 0.5% copper, has a melting point of 217 to 220 °C 218 424.4 SnAg3.5Cu0.7 is another commonly used alloy, with melting point of 217-218 °C. 217 422.6 SnAg3.5Cu0.9, with melting point of 217 °C, is determined by NIST to be truly eutectic. 218 424.4 SnAg3.8Cu0.7, with melting point 217-218 °C, is preferred by the European IDEALS consortium for reflow soldering. 223 433.4 SnAg3.8Cu0.7Sb0.25 is preferred by the European IDEALS consortium for wave soldering. 32 SnAg3.9Cu0.6, with melting point 217-223 °C, is recommended by the US NEMI consortium for reflow soldering. 32 227 440.6 SnCu0.7, with melting point of 227 °C, is a cheap alternative for wave soldering, recommended by the US NEMI consortium. 199 390.2 SnZn9, with melting point of 199 °C, is a cheaper alloy but is prone to corrosion and oxidation. 198 388.4 SnZn8Bi3, with melting point of 191-198 °C, is also prone to corrosion and oxidation due to its zinc content. 240 464 SnSb5, tin with 5% of antimony, is the US plumbing industry standard. Its melting point is 232-240 °C. It displays good resistance to thermal fatigue and good shear strength. 225 437 SnAg2.5Cu0.8Sb0.5 melts at 217-225 °C and is patented by AIM alliance. 208 406.4 SnIn8.0Ag3.5Bi0.5 melts at 197 to 208 °C and is patented by Matsushita/Panasonic. 139 282.2 SnBi57Ag1 melts at 137-139 °C and is patented by Motorola. 138 280.4 SnBi58 melts at 138 °C. 118 244.4 SnIn52 melts at 118 °C and is suitable for the cases where low-temperature soldering is needed. [unquote]

It looks like your 420 may be a bit too low. 433.4, 437 or 440.6 might be necessary.

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

Can you beg or borrow a thermocouple to independently confirm the settings ? Airflow rates you can check with element off or very low and part filling a rubbish sack in a certain time.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I disagree. In order to get the solder to acquire melting temperature in any reasonable length of time, the heat source has to be far above the temps you mention. It is tedious to hand solder with a tip that is less than 800 F, and desoldering requires similar temps. The problem lies in heat transfer and loss.

Smitty Two wrote in news: snipped-for-privacy@news.phx.highwinds-media.com:

You mistake the transfer of heat (calories) for temperature. This is a common mistake. The rate of heat transfer depends on TWO things: the difference in temperature and the heat conductivity. In soldering there are other important factors that often comes into play, the heat capacity of the soldering iron and the power of the heating element.

For soldering irons, there are two philosophies.

1) use a hot enough iron to rapidly transfer heat to the target and remove the iron before the temperature gets high enough to damage the parts. Often the iron does NOT have enough power to raise a large object to the iron temperature. This can make for both over heated components and cold soldered joints. 2) use a temperature controlled iron that has enough heat capacity and conductivity to rapidly heat the target to the soldering temperature. In the second case, it is NOT important to remove the iron quickly because it will NOT overheat the components.

Given good heat conductivity (clean and tight joints in the iron) and sufficient heat capacity (plenty of watts), a temperature controlled iron is much better. The iron temperature should be set slightly higher than the melting temperature of the solder. There is no need for dozens or hundreds of degrees in excess of the melting temperature.

In the case of a hot air gun, when there is sufficient heating capacity and air flow, everything within the area of the air flow will be quickly brought to the set temperature.

caveat: Many components are rated for limited exposure to higher temperatures. I don't know of ANY transistors or ICs that are rated to withstand 800 F degrees for any length of time.

Final point: using the correct solder is important also. True eutectic solders are best because they melt and 'freeze' at a single temperature.

Non eutectic alloys, like 50/50 or 60/40, pass through a 'plastic stage' where crystals of one of the components start to form.

Any movement while the joint cools through the 'plastic stage' temperature range results in a 'cold solder joint' where there are actually separate crystals of the component metals and conductivity is unreliable.

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

I don't believe I'm mistaking the two, but I'm willing to be corrected if you'll point out my error more clearly.

I keep thinking that you speak from theory, not practice. It is virtually impossible to solder with a 500 F iron, yet many components supposedly can't stand even that for more than 5 seconds.

We disagree quite strongly on this point, and I wonder on what you base your perspective? Many, many years ago, the military presumed to insist on 600F, and that proved to be woefully inadequate for hand soldering. Now, a solder bath, having an immense thermal mass, as well as providing virtually total joint immersion, can solder well at 500. But hand soldering with any measure of expediency requires *at least* 700, and in my experience, 800 is far better.

Hot air guns are much, much hotter than the melting point of solder. Why would that be, if the very low temps you advocate are actually sufficient?

True. And they needn't be exposed very long. Through-hole ICs and transistors may need approx. 1 second per lead. With a typical 16 pin surface mount IC, three seconds is plenty to skate down one side and solder all 8 pins, at 800F.

And we had a discussion here not long ago, begun with the question of why anyone would use anything other than 63/37, assuming a leaded formulation. As I recall, no one offered any very plausible reason to use anything else.

And Fahrenheit 451 was written by Ray Bradbury, not Robert Heinlein.

Deke

"Deke" wrote in news:46bc7$47e5dde4$943f4036$ snipped-for-privacy@STARBAND.NET:

....

You are correct. I was wrong on the author. Both wrote good stories but I misattributed. My bad. Sorry.

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

Smitty Two wrote in news: snipped-for-privacy@news.phx.highwinds-media.com:

[correction, Ray Bradbury, not RH. Sorry!]

If the explanation that I gave was not clear, enough, I suggest the study of thermodynamics and heat transfer.

is a good place to start.

The raising of the temperature to the melting point of the solder requires the transfer of a sufficient number of calories to the component. This does NOT require a high temperature, it just requires efficient heat transfer.

Incorrect. I soldered my first connections in the late 50's. When I was 8 years old.

First connections were made with 50/50 solder. Well I remember trying to hold the lead perfectly stationary while my fingers burned.

I was first licensed as a ham in 1961 as WN5DQP at age 16. I built Heathkit TVs and Scopes in the early 60's. Using 60/40 solder.

I worked on a resistor/capacitor production line in the late 60's, early

70's.

First as a Process Technician then as a Process Engineer for Sprague Electric Co. I WROTE process specifications for Sprague's production line for soldering capacitors to their leads in the early 70's.

After the 'mini recession' in the early 70's I went into consumer electronic service. I owned a consumer electronics repair shop in the

70's. Got a bit of practical experience there.

In 74-76, I fixed radars and electronics on ships on the Mississippi. Got a bit of practical experience there also.

I did board repair on DEC and DG computers in the late 70's. Component level repairs on PCBs.

A couple of years ago, I built an Elecraft K2/100 ham transceiver.

Recently, I built several SoftRock RXTX SDR radios, using SMT components.

I think that qualifies as a bit of practice to go with a bit of theoretical knowledge, a BS in Chemistry, 1970.

I don't find that to be true. All one needs is a good clean iron properly tinned, good 63/37 solder, and a good flux pen, clean, pretinned leads on the components, a clean, pretinned PCB and proper technique.

Of course, ANY oxide (and solder does oxidize rapidly) on the tip of the iron and you have just added thermal resistance. The iron tip MUST be freshly cleaned and tinned.

Many components CAN'T.

As I said originally, there are two philosophies.

This is true, IF the iron is not clean or is underpowered. The point at which a temperature controlled iron measures the temperature is also important. The nearer the tip, the better.

Correct.

If I recall correctly, our 95/5 (tin/silver) solder pots ran at

495F(257C). The 62/36/2 pots ran considerably cooler but I don't remember the numbers. There was a layer of hot wax on top of the solder to 'preheat' the parts and protect the solder from oxidizing.

Once you get the joint above the melting point of the solder, there is no need for higher temperature. Proper heat transfer is the key.

Depends on the settings of the gun. My 'CSI Hot air gun 2'

solders and desolders quite well at temperatures not much greater than the melting point of the solder.

I have soldered and removed 48 lead SMT ICs without damaging the surrounding SMT components. I have removed and later reused 8 lead SMT ICs. This was done without badly charing the paper dams around the parts. The paper WAS heated to a light brown so I believe the digital read out on the hot-air gun.

My hot air gun WILL go hotter, but after playing around with it for some time, salvaging parts from old PCBs, I found that I could work quite well with much lower temperatures than I had first tried. Nozzel size and air flow rate are very important. The nozzel must be large enough to heat the area and the flow rate must be sufficient to do so quickly.

Or even at 440 F, with 63/37 solder, and then to 'mop up' with clean, well fluxed solder braid.

One 48 lead component I worked with recently has an absolute max of

300C(572F) for 10 seconds.

Your 800F is 427C. There is a very good chance of damaging such an IC at that temperature.

A small percentage of silver helps prevent leaching of silver from some components and is vital for the mounting strips in some Tektronix scopes. A small percentage of copper helps prevent leaching of copper from PCB traces.

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

It isn't your explanation that lacks, only your suggestion that I'm confused on the topic.

I agree. Unfortunately, efficient heat transfer is difficult to impossible in many hand soldering applications. The excess temperature to a small contact area makes up for that.

Contact area is the holy grail of efficient heat transfer.

Credentials accepted.

How long does it take you to make one solder connection on a 1/4 watt through-hole resistor on a typical board two-sided board with plated through holes, at 500F? I ask because in production, 3 seconds is far beyond unacceptable.

Then why do you assert that with low temps, component damage is impossible?

And one of them works in practice!

See above.

We do it all day long every day. Less than 1/2 second per lead. No damage.

I'll accept your assertions. The original dialogue to which I refer concerned only tin/lead mixtures.

Smitty Two wrote in news: snipped-for-privacy@news.phx.highwinds-media.com:

....

Sorry. I didn't intend to imply that I could read your mind, just that many people confuse temperature and heat. Many of those that confuse the two think that a higher temperature is better for soldering.

A clean iron, well tinned, clean circuit board and component lead, fluxed. Clean the iron just before you bring it to the joint to be soldered. Clean it by plunging it into a tangle of stainless steel shavings (a pot scouring pad) and twisting as you remove it. The shavings do NOT cool the iron like a sponge would.

Use thin solder. Put the solder against the component lead and board. Touch the iron to the solder, board and component lead, all at the same time.

Feed enough solder to wet the surfaces and encourage good heat transfer. For leaded components, move the solder wire around to the other side of the joint and make sure all wets well and you get a good solder fillet.

1/2 second should be enough.

EXACTLY CORRECT! [for conductive heat transfer]. CLEAN contact area is vital. ....

In production, the board is preheated before it reaches the solder fountain and spends a couple of seconds in the fountain. It is then cooled at a controlled rate to avoid temperature shock.

Or the solder paste is silk screen printed onto the board, the parts are robot applied. The board is then preheated for several minutes, the temperature is quickly ramped up to the melting point of the solder and back down. The board is then cooled and cleaned. This is done as the board is carried by a belt through the different temperature zones.

I certainly didn't mean that component damage is impossible.

Let me restate my opinion: It is easier to solder without damaging components using a clean, high wattage, controlled temperature iron set at a temperature slightly above the melting point of the solder than it is when using a low wattage, high temperature iron.

With practice, either method can be used, but I have tried both and prefer high wattage, with a lower, controlled temperature.

BOTH work, in practice.

With proper [different] techniques, both can be used.

....

....

Yep. And I have seen what happens when the iron stays on the joint 1/2 second too long. I have had to change ICs that failed because of overheating.

I have also had to work with non temperature controlled irons, even battery powered ones.

I would ALWAYS prefer to have a high wattage (150W) temperature controlled iron set to 500 or so degrees rather than a 15 W 900 degree iron. You are, of course, welcome to use what you prefer. As I said, BOTH methods can be made to work.

Best regards. I suspect that we have exhausted the subject for now.

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

The discussion seems to avoid the point, that the parts to be desoldered sit on a base of polymerized organics.

I got the impression, that even at moderate temperatures, which allow a relative quick separation of parts from the boards, a notable depolymerization takes place, with the effect, that the monomers lounge in your clothing for several days, as far as you did not inhale them.

Regards, H.