Milkymist One SMT/DIP Process Flow

Here collected info is came from visiting smt factory during producing Milkymist One RC1 pcba.

• Auto/Semi-Auto PCB Carrier Machine

• Print Solder Paste

Printing machine is to print solder paste on the solder paste layer through stencil. Normally you don't need to specify a well-known brand solder paste to pcb maker, as long as request them to use a lead-free paste. But they all have famous brands' ones, for example;

• Produce Stencil

Stencil is a material for the process of printing soldering paste of smt stage. There are two methods to produce stencil: One is made by gerber file, the other is made by pcb optical scanning.

 by gerber file: the smt factory will request customer to get final gerber file which is exact mapping with instant pcb produced from 
 pcb maker not from customer's routing files. This approach is more significant to mass production without much
 variable tolerances.

 by optical scanning: the smt factory also can use the instant pcb sample to let stencil maker to do a scan to copy and produce it.

For Milkymist One RC1 we make this by file which is brought from New Heart Corporation. Thus is also this time RC1 pcb maker. For an EMS or SMT industry, they almost frequently build oriented-business with stencil makers like http://www.cksmt.com.tw or http://www.laser.com.tw/. In Taiwan for example the quotation about pcba assembly including outsourcing stencil made on smt factory side.

The stencil file is somehow the same as the layer SOLDER PASTE in gerber files generated in PADS, OrCad or Altiem Designer tools. Sometimes for economic goals in mass production effectively, gerbers in PANEL structures rather than single gerber. Eg., 2 pieces, 4 pieces, or even more pieces in one panel. That's the reasons that must use FINAL SOLDER PASTE layer film/file produced by pcb maker.

• Feeders

Before going for Pick & Place process, there's a step that to feed parts into AI machine. A feeder doing this bridge between parts and artificial intelligence arm. To achieve this, customer needs to provide bom with indicated different packages so that the feeders will be used differently. See follows:

• Pick and Placement

An AI(automatic insertion) file needed to be provide also to smt factory to let smt engineer write/code into AI machine. Milkymist One RC1 AI file is recorded for each smd part as row as follows:

 PartType       RefDes  PartDecal  Pins   Layer    Orient.    X         Y       SMD     Glued
 CAP	        C1	402	   2	  Top	   0	      120.142	36.576	TRUE	FALSE
 0805POL	C2	0805POL	   2	  Top	   270	      122.936	16.764	TRUE	FALSE
 0805POL	C84	0805POL	   2	  Bottom   90	      84.836	78.994	TRUE	FALSE

Using above row data, the AI machine can reference to optical origin point of the coordinates then do the pick and placement.

 high speed AI machine

 medium speed AI machine

 low speed AI machine

• IPQC

• Inspection before IR-Reflow

• IR-Reflow Measurement

The reflow soldering process for Pb-free components is very similar to the conventional eutectic solder reflow process. However, there are some important differences that must be taken into consideration for Pb-free soldering as the soldering material used for Pb-free soldering is different and higher reflow temperatures are required. The all critical parts on even top or bottom side of boards, with a measured temperature which coincides with the curve of recommended temperature is necessary. The most concerned one is Spartan-6 fpga's Pb-Free Reflow Soldering Guidelines. Be careful of the peak temperature(body) is 250°C max. The IR/Convection is 60–150 seconds (60–90 seconds typical) during temperature maintained above 217°C. The peak temperature(ball/lead) is 245°C typically. That depends on solder paste, board size and components mixture. So far we are at prototype stage, a bit 1~2°C over 245°C can usually solder well. This sometimes only done by customer requested. The smt factory will never take a risk to do this unless you had have confidence on this or strongly request.

 how to do this measurement?

• IR-Reflow

 how to compare with chip manufacturer's recommended profile?

• X-Ray Inspection

 how to make sure what kinds of shape look will have a good solderability?

• Through Holes Insertion (DIPs Process)

• OQA