Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Mark D.

#13538

Wavesolder PCB Temp. Settings | 11 November, 1998

What are the factors that go into determining PCB Board temperature requirements for the Wavesolder process? Most of the recipes I've seen call for preheating the PCB to around 230F - 250F. This temp is about 210F - 230F below the typical solder temps in the pot. This seems to diverge from the reflow process where the board and solder are brought through the same temperatures during preheating, soaking, reflow and cooldown. Is the preheating of the board for wavesoldering done to primarily activate the flux and prevent thermal shock to the components or is there something about the temperature range that primes the substrate of the board for optimal solderabilty or both (or neither)? Any help in understanding this would be greatly appreciated.

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Earl Moon

#13539

Re: Wavesolder PCB Temp. Settings | 12 November, 1998

| What are the factors that go into determining PCB Board temperature requirements for the Wavesolder process? Most of the recipes I've seen call for preheating the PCB to around 230F - 250F. This temp is about 210F - 230F below the typical solder temps in the pot. This seems to diverge from the reflow process where the board and solder are brought through the same temperatures during preheating, soaking, reflow and cooldown. Is the preheating of the board for wavesoldering done to primarily activate the flux | and prevent thermal shock to the components or is there something about the temperature range that primes the substrate of the board for optimal solderabilty or both (or neither)? Any help in understanding this would be greatly appreciated. | You've answered all your own questions. Just remove the question marks and you are right about all you said.

In the through hole only era, thermal issues were dealt with more easily and the PCB substrate was of primary concern. Thermal shock is more of an issue concerning SMT. This especially true for bottom side chip devices as ceramic types that will crack under severe stress or shock. Also, don't forget the board. It too may suffer damage with high delta T's.

Many people are starting to crank up the preheat temperature while lowering the wave temperature hoping to minimize adverse affects. There are practical limits to this practice as solder temperatures must be maintained to effect solderability just as flux activation temperatures must not be too far out or negative effects also will be caused.

As with everything PCB, compromise always must be faced. Now that DoD no longer specifies process parameters for solder temperatures (500 F +/- 10 degrees) it may be possible to experiment with them to the extent acceptable solder joints are made. The solder joint is number one, but product also must be acceptable without damaged components or boards.

Earl Moon

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J.A.B.

#13540

Re: Wavesolder PCB Temp. Settings | 13 November, 1998

Also the preheating of the board is done to help evaporate any water/alcohol that is in the flux - water is a big problem in No-clean processes. From our experience, a temp range around 200f - no more than 220f is ideal for the bigger boards, but temp needs to decreas as the board size gets smaller. If the the temps are TOO hot, it will evaporate the flux and cause no solders on your smt parts. The substrate problems are pretty much a thing of the past with smt spun boards. Trial and Error is still the only proven way to go when dealing with these types of issues.

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Earl Moon

#13541

Re: Wavesolder PCB Temp. Settings | 14 November, 1998

| Also the preheating of the board is done to help evaporate any water/alcohol that is in the flux - water is a big problem in No-clean processes. From our experience, a temp range around 200f - no more than 220f is ideal for the bigger boards, but temp needs to decreas as the board size gets smaller. If the the temps are TOO hot, it will evaporate the flux and cause no solders on your smt parts. | The substrate problems are pretty much a thing of the past with smt spun boards. Trial and Error is still the only proven way to go when dealing with these types of issues. | You're sure right about "overtemping" the flux. However, there are still and always issues concerning doing it to PCB substrates - especially pure SMT types.

In pure through hole, component holes were supported by definition. In those SMT, none are. Therefore, when a board gets too much thermal stress or shock, it may delaminate or hole walls may come apart.

Z axis expansion is the primary culprit in hole wall failure followed by delamination through holes thus providing electrical opens. The opens may close during bed of nails testing causing suspicion elsewhere.

Copper plating ductility is an ongoing serious issue with respect to about 50 ppm/degree C expansion rates in the Z axis for most board materials. Poor designs, poor supplier evaluation (concerning copper plating quality as tensile and elongation characteristics), poor material selections, poor MLB constructions and process management, and poor thermal management during assembly processes all provide paths to substrate destruction and hole wall failures. This is very true for SMT today and tomorrow when faced with expansive thermal excursions found during wave soldering operations.

There's so much more, but for now enjoy,

Earl Moon

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