PCB pre-heaters for SMD rework

[Fraser]

 I'm delving into the world of SMD rework using a hot air station and fine tipped temperature controlled soldering iron. The station that I have just purchased is the WEP 852D+ which is from the same factory as the KADA 852 etc. I must say the unit looks better finished than previous 852's that I have seen. It was certainly good value for GBP79.99 delivered from a UK warehouse.

Having read about SMD rework I have discovered that PCB pre-heaters are deemed beneficial to SMD rework. What I have not established is whether this applies to small components and IC's as well as the big BGA chips that I know need a pre-heater. I have also to establish whether the AOYUE 853 is a decent design of preheater, it is certainly more affordable than most other models. Has anyone managed decent reworking of SMD components without pre-heating ?

Any insight into what is required to do SMD rework on modern PCB's and their SMD components would be much appreciated. I will not be dealing with BGA's just SOT23, SOP, PLCC, QFP etc.

Thanks in advance.

[DaveW]

It's mainly useful if your soldering iron is having to fight large ground or power planes. If you've got chips with exposed pads then it's extremely useful. Similarly if you're reworking pins that are connected via close vias to the ground plane or similar then it reduce the chance of having a pin bend during removal.

The WEP 852D+ is a pretty good unit, had one as a hot air unit and backup iron for a couple of years, careful with putting the air too low as the heater will burn out easily

[tinhead]

For soldering - by default - you don't need any pre-heaters, even small BGAs can be soldered without. Where pre-heater make sense are PCBs with big ground/power planes. For sure a good soldering station can heat them up, but this is unnecessary stress for the station, components and PCB.

For big BGA chips you will use (semi-pro or hobby) anyway different equipment, probably IR reflow oven
or IR rework station - both will have pre-heaters installed on the buttom side of the PCB - if not then they
crap (well, you can install DIY pre-heater by yourself).

For rework - by default - it is easier to remove components it PCB got pre-heated, however you will never need more than 150°,
everything above that is already soldering (and more or less creating unnecessary stress for PCB and the other components on PCB).

A DIY pre-heater can be easily build, all you need are 2 or 4 of "Ceramic Far IR Radiator", like these
http://www.alibaba.com/product-gs/216402733/Ceramic_infrared_radiator_flat_shape_4.html

If you wish to buy check first what will be the max size of PCB, a too small pre-heater for large PCBs is creating additional
stress for PCB, the Aoyue 853 looks useless (unless you have only really small PCBs with Power MOSFets to be replaced ...).

Especially PCB from cheaper PCB fabs are not supposed to be heated over and over again (whcih too small pre-heater is doing).

[neoone]

I agree with others. It is very possible to solder packages like QFN without pre-heater just by means of hot-air station. Once my friend and me we soldered over 100 ICs in QFN using similar stations - AOYEU 852A++ to be exact and other one which I don't remember but it was also AOYEU or some other badge like "PT" (I believe they are all the same). If You want to solder some serious BGAs than I recomend using some oven - about 2 years ago a company where I used to work bought a cheap Chinese oven for about 1000-1500PLN = 350-500$ if I remember correctly. It worked fine.

[Fraser]

Thanks for the replies guys

You have told me exactly what I needed to know.

With regard to the Aoyue pre-heater, I have a suspicion it is really aimed at the mobile phone repair/modding market as that seems to be the PCB type shown in some adverts. I had a suspicion it would not be that great for larger PCB's and you have confirmed it for me. I now understand the reason for pre-heating better and shall look into a simple DIY solution for my humble needs 

Most of the work I will initially be doing will be practice on scrap PCB's in order to better understand how to use a hot air station. I will be able to judge any requirement for a pre-heater better then. Most of the repairs or builds that I will use the hot air station on will be small PCB's but the comment regarding the heat sinking effects of ground planes is well noted. Thank you.

The WEP 852D+ looked a good deal for a UK based supplier and seemed suited to my quite light requirements. I was torn between the 852D+ with its pump and the other models that have appeared with a fan in the hot air wand handle. I ended up with the technology that I had seen most often in more expensive systems, i.e. the pump type. The comment regarding overheating of the element is very well noted and thanks for the warning. I purchased a couple of spare elements for the hot air wand at a cost of only GBP3 each delivered from China. I have also bought five spare elements for the soldering iron and a bulk pack of the various iron tips..... All in all, I think I now have a pretty good chance against those dreaded little SMD chips that cause me so much frustration in modern kit 

Thanks again guys.