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Populating a PCB

Once we have the PCB made we have to populate it with components. To do this we need the following;

BOM (Bill of Materials)

Swan are happy to work with your BOM in whatever form you may have it. However some things to consider:

Obsolescence. Parts go obsolete more often than they used to as suppliers cut lines, check that the part specified can still be bought. Swan will consider lifetime buy and can look at alternate parts for you.

 

Long Lead Time. Even if the part is available, capacity is still not what it was in 2008 and may suppliers are still making to order. Some parts can have long leadtimes, from a few weeks to in some instances over a year. Parts particularly at risk are microchips and some capacitors. Swan can look at alternatives on request, but this is something to check early. If possible highlight alternates on you BOM if you have any.

 

Grades. Many parts have a range of temperature grades such as commercial (0-70C) and industrial (-40 to 85C), if you need a particular grade please make sure you specify it correctly. Other example of grades include LEDs with different brightness and colour ranking grades, which can vary significantly.

To help see an example BOM below:

 

 Qty

Part ref Description Part No Manufacturer Distributor Part number ROHS Temp range Note 
5 R1-5 1206 200R CRCW1206200RFKEA vishay Farnell 1653-089 yes  -55 to 155
2 C1,C2 0.1uF 25V 0603 06033C104KAZ2A AVX Farnell 1833-840 yes  -55 to 125  
1 D1 1n4148 smt 1N4148W-V-GS08 vishay Farnell 1469-425 yes  -65 to 150 sod123
so on.. ... ....
....
....
...
...
...
...
....

It is very helpful in any BOM to advise what side of the board particular parts go on.

 

Part Location

Once we have what parts we need, we need to know exactly where they go, this can be done two ways.

 

Engineering Drawing

Sometimes this is as simple as the silk screen with parts numbered on the silk screen and the polarity being clearly shown on this silk screen. Often more detail is required, particularly for surface mount and components like inductors with many pins that can be put the wrong way round. Other details like label locations, on board spacers can be included on such drawings.The below is part of a PCB layout drawing as an example, showing which way round diodes go, which is positive on electrolytic capacitors and where pin 1 (red dot) is on the chip.

PCB Layout Info 

 

Part Location List

Surface mount machines require a part location list to be produced, this can be generated by hand, or most PCB software will generate a component locations report. This of course only work if your component references tie up with your BOM references. An example is shown below:

Component Positions Report
--------------------------

Report File    : xxx-091211 (PCB - Component Positions Report).txt
Report Written : December 09, 2011
Design Path    : C:\Users\xxxx\Desktop\xxx\xxx-091211.pcb
Design Title   :
Created        : 09/12/2011 21:53:22
Last Saved     : 09/12/2011 13:23:45
Editing Time   : 3810 min
Units          : mm (precision 3)

Name   Component          Side   Centre              Rotation
-------------------------------------------------------------
C1     C0805              Top    (515.399,505.013)   90
C2     C0805              Top    (517.657,522.970)   0
C3     C0805              Top    (508.133,522.701)   0
D1     d-minimelf         Top    (514.774,525.178)   45
R1     R0603              Top    (511.296,525.245)   0
.... 

 

Additional Population Information

Some parts on a PCB may need particular attention in how they are soldered on or located for creepage and clearance and mechanical durability reasons. If you have particular instructions for a part then put it on any parts location paperwork or in the BOM. If you have drawings or photos this is useful, Swan EMS can often identify such issues during the first off build for new products.

 

Programming Information

If you have devices that require either pre-programming or on board programming, we will need the programming information and any applicable instructions. Swan EMS treat such information in strict confidence. 

An example of the such assembler code is below:

#include <p16F690.inc>
     __config (_INTRC_OSC_NOCLKOUT & _WDT_OFF & _PWRTE_OFF & _MCLRE_OFF & _CP_OFF & _BOR_OFF & _IESO_OFF & _FCMEN_OFF)

     cblock     0x20
Display
     endc

     org 0
     bsf       STATUS,RP0
     movlw     b'00000111'    ; configure Timer0.  Sourced from the Processor clock;
     movwf     OPTION_REG     ; Maximum Prescaler
     clrf      TRISC          ; Make PortC all output
     clrf      Display
     bcf       STATUS,RP0 

 we will also take hexdumps (such as shown below):

 :020000040000FA
:10000000FF3085008316F83085009F0105308100A0
:100010009516151696168312073099008B15203009
:100020008400DF008001840ADF08031D12287D2080
:10003000B33081000B110A30B0006620031D222866
:10004000A50A1D28F120A600250203182A2803016D
:10005000A600E520260824060319302866202A2851

......

Test Information

This is the information that the customer provides that enables Swan EMS to carry out functional tests on the completed boards. This could be powering up and measuring voltages or waveforms etc. Many customers have provided test rigs for this purpose and this is a time and cost efficient way of carrying out such tests. Please discuss your test requirements with Swan EMS.

 

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