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Soldermask thickness



Soldermask thickness | 1 October, 2003

We have a 1.6 mm thick automotive PCB board of size 35x70mm. One of our vendor has given us a batch with lesser soldermask thickness 5~8 microns. Will this affect the circuit board on a long term. What should be standard for the soldermask thickness? We do reflow soldering & assemble the components.

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Soldermask thickness | 1 October, 2003

Tough to say. It depends on the type of solder mask, the producer of the solder mask, and your board fabricators' process.

As one angle on this, solder mask type affects thickness. Minimum clearance: * Liquid photoimageable: 2 thou * Screened liquid: 6 thou * Dry film: 3 thou

On another tact, the thickness of solder mask varies between suppliers. The thickness of several popular dry film solder masks is: * Dupont VACREL 8100 available in 3 thou and 4 thou * Shipley (Dynachem) DynaMASK 5000 available in 3 thou and 4 thou * Shipley ConforMASK 2500 available in 1.5 thou and 2.3 thou

Finally, processing affects thickness. Typical solder mask consumption [10mm coverage over 65 - 70mm tracks][Tibbals, Electra Polymers & Chemicals] Aplication method dry||Thickness um||Coverage m^2/kg Screen print||~25-30||~17 Curtain coat||~40-45||~12 Electrostatic spray||~30-35||~14 Air spray||~30-35||~16

IPC-SM-840 "Qualification & Performance Of Permanent Solder Mask" may be a good starting point in understanding the acceptability of the material on your boards.

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Soldermask thickness | 2 October, 2003

Out of curiousity, Isn't soldermask only used as a solder processing aid and to cover the copper?


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Soldermask thickness | 2 October, 2003

Yes, Russ

IPC-6012A w A#1 - Qualification & Performance For Rigid Printed Boards talks to solder mask requirements in 3.8. One interpretation of what it says: * Solder resist [solder mask] must meet the qualification / conformance requirements of SM-840 - Qualification & Performance Of Permanent Solder Mask. * Solder resist coverage manufacturing variations are subject to restrictions that are detailed in 3.8.1. * Solder resist must be cured and adhere well. * Solder resist thickness is not measured unless specified in the purchase documents.

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Soldermask thickness | 3 October, 2003

the puspose of solder mask is to prevent exposure of cu tracks to atmosphere. the lesser thickness can effect the performance of board but it only depends on its application. if you are making telecom board then its not good to use the board.

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Soldermask thickness | 3 October, 2003

Mask Musings.

Soldermask (SM) and its functions have changed and evolved over the years. As an ex-solder mask applicator, I can remember screening Colonial High Gloss Epoxy soldermask on boards back in the early 80's, ending up with a thickness that could at times seem as thick as the board itself, especially before we found out about reducers which were used to think it before screening!!!!

As stated earlier, the main function of SM is to protect the copper traces and any other feature you cover up with it. These days however, SM is being used to do a lot of other functions like shaping pad or providing barriers to keep liquid solder from migrating to areas where you don't want it during assembly. There are even terms like SMD-LLP, Solder Mask Defined Leadless Leadframe Package, or, NSMD-LLP, Non Soldermask Defined Leadless Leadframe Packages.

Liquid solder mask thickness is something that is hard to predict from vendor to vendor as different application methods produce different results. Dry film mask is the same thickness no matter who applies it but liquid, as Dave stated, can be screened, curtain coated, spray coated etc. Even different operators can cause variances in the thickness as squeegee pressure and speed produce different results. Bobby the body builder may push harder on the squeegee than Barbie the ballerina. Both cover the boards but the extra pressure that is exerted by one will deposit thinner solder mask than less pressure from the other.

Our experience is that most vendors when asked about thickness will specify a window that gives them maximum manufacturing tolerances such as, .0008 - .0012 finished thickness as the standard for liquid mask. Because it is liquid, it is very difficult to guarantee even thickness across the whole surface no matter what they tell you. Edges typically are different than the center etc.

Dry film is great for even thickness but it is becoming harder and harder to find shops that still apply it. Also, when you get into the fine features of today's technology, even the vendors that apply it may balk at using it below say .025 (.5mm) pitch sizes due to adhesion challenges etc. Liquid photo imagable (LPI) is better for fine pitch but is definitely thinner when applied per manufacturers recommendations.

Our company uses soldermask to shape solid solder deposit pads. We specify soldermask thickness as .002 + .002, -.000 and leave it up to the vendor to figure out how to achive it. Most of our vendors have developed methods to supply this thickness however, there was some grumbling from a few at first.

So, in conclusion, soldermask is not just for breakfast any more. It is an important part of a succerssful assembly and can be applied thickler if needed. You just have to twist some arms now and then to get what you want.


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Soldermask thickness | 6 October, 2003

It was a nice discussion. What I did here was a thermal shock test 1 hour@ +90 deg.c and immediately 1 hour @ -40 deg.c. The second test I did was a high humudity test for a week. After each test, I carried out a peel test with scotch tape.

Those boards passed the test and the electrical parameters were the same before & after.

I checked with some leading PCB manufacturers such as AT&S, and I found they were even doing 4 microns in few cases. So, it is the process control capability that plays a major role.

Another PCB mfr told me that they do 0.20mm thick sodlermask for shaping up the pads to take more SOLDER (HALS).

Although my tests gave me a satisfactory result, by looking at the process capability of the vendor, I have requested him to maintain minimum 15 microns thick solder mask.

(Kindly note, I have used metric units and referred LPISM while describing the above)


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