Monday, 8 September 2014

Tube Simulator - Practical Implementation - PCB layout, manufacturing, assembly (2/4)

PCB layout

The PCB has been designed in two layers, using a maximum of ground and power planes surface and numerous vias, specially in the central ground pads below ICs that dissipate more power: regulators and power audio amplifiers.

PCB width is mostly imposed by the front panel controls: switches, potentiometers and jacks.
With that in mind a proper aluminum box was chosen:
Hammond Manufacturing 1444-1372 13 x 7 x 2 inches (W x D x H) 

The L shape of the PCB leaves some space for AC power inlet, AC power switch, 1A AC fuse, AC/DC power supply, and the posibility of adding a PCB module for FX loop effects or digital reverb circuit (these modules have not been implemented yet)

Top layer

Bottom layer

PCB manufacturing

For PCB manufacturing I used Eurocircuits web site. Eagle CAD files can be uploaded and there is a very practical Price Calculator that allows having immediate quotes with options for standard pool or PCB prototypes for small PCBs . They also have the possibility to verify the PCB layout after file uploading.
I paid 140 euros (transport and taxes included) for one prototype on a 7 day working days delivery. Usually is recommended to manufacture at least two prototypes. PCBs are manufactured in Europe, they have a factory in Hungary.

Quality is very good with nice finish:

PCB assembly

The assembly drawing and the PCB ready for soldering components. SMD DC-DC converters with central pads where previously mounted using solder paste and hot soldering gun.
The solder paste that appears in the photo below was purchased in a generic DIY store and created lots of problems of oxidation. I recommend using a solder paste specific for electronic use with lowt melting temperature, usually RoHS (not leaded) solder pastes have a quite high melting point, but there are some RoHS compatible solder pastes like alloy 58 Bi / 42 Sn with a melting point as low as 138°C. The one I use is EDSYN - CR11 58/42 Bi/Sn from Farnell

It is very important to leave thermal reliefs on pads, specially in a design like this with large power and ground planes, otherwise solder iron heat will dissipate through the plane and soldering those pads will be a nightmare. I know because it happened to me.

The PCB with most of the components soldered. Rear connectors must be soldered with wires to allow PCB assembly in the chassis.

The AC/DC power supply module used was a Murata MVAD040-24 40W (23 euros) open switching power supply module with 120/230V AC@50/60Hz input and +24V DC@1.67A 40W. 

PCB mounted on the chassis aluminium base plate, with AC/DC power supply, 1A AC fuse and AC power switch: