Man­u­fac­tur­ing of elec­tron­ic assemblies

Sequence of a typ­i­cal assem­bly production

The pro­duc­tion or the Elec­tron­ics Man­u­fac­tur­ing Ser­vice (EMS) of a print­ed cir­cuit board to be assem­bled can be divid­ed into the fol­low­ing work steps:

1. Pro­duc­tion of the print­ed cir­cuit board (PCB)
2. Depo­si­tion of sol­der paste on the future con­tact points of the com­po­nents. The high­ly vis­cous sol­der paste con­sists of tiny tin balls and flux.
3. Assem­bly of the print­ed cir­cuit board. SMD com­po­nents (SMD, Sur­face Mount­ed Devices) are now placed in the sol­der paste depots. They ini­tial­ly adhere there only through the adhe­sive effect of the sol­der paste. The assem­bly process is automated.
4. Sol­der­ing the SMD com­po­nents. The sol­der paste melts under the influ­ence of heat and firm­ly bonds the SMD com­po­nents to the cir­cuit board.
5. Assem­bly of the print­ed cir­cuit board with con­ven­tion­al, wired com­po­nents using THT tech­nol­o­gy (THT, Through Hole Tech­nol­o­gy). This is done by insert­ing the con­nec­tion wires into the holes made for this pur­pose in the cir­cuit board.
6. Sol­der­ing of the THT com­po­nents. The sol­der­ing is prefer­ably done with a robot, but can also be done manually.
7. Inspec­tion of the assem­bly result, visu­al­ly and elec­tri­cal­ly by spe­cif­ic mea­sure­ment tech­nol­o­gy. For com­plex assem­blies or series prod­ucts, pre­cise­ly designed test sys­tems are designed.

Appli­ca­tion of sol­der paste

Paste print­ing can be done in dif­fer­ent ways. Most­ly screen print­ing or sten­cil print­ing is used. Both process­es have in com­mon that where an open­ing has been cut in a stain­less steel mesh or sheet with a high-pre­ci­sion laser, the sol­der paste can wet the PCB. Each cir­cuit board lay­out there­fore requires a spe­cial­ly man­u­fac­tured screen or sten­cil. The pastes used are pre­cise­ly matched to the mesh count of the screen or to the stain­less steel sten­cil. The qual­i­ty of the print is checked by visu­al inspection.

Assem­bly of the print­ed cir­cuit board

The com­po­nents are set up on rolls, belts, trays or in bars on the pick-and-place machine. The com­po­nents are removed from the respec­tive pack­ag­ing by the head of the machine by means of a vac­u­um, cen­tered and aligned in the machine and then assem­bled to the cor­rect X/Y coor­di­nate of the print­ed cir­cuit board. This place­ment process is called “pick-and-place”.

Sol­der­ing of the components

For auto­mat­ic sol­der­ing of the assem­blies, we pre­fer to use vapor phase sol­der­ing, which is par­tic­u­lar­ly gen­tle on com­po­nents. Here, the heat­ing of the print­ed cir­cuit board is achieved by the con­den­sa­tion heat of a steam (Galden, LS230). The max­i­mum tem­per­a­ture that the assem­bly can reach with this sol­der­ing process is lim­it­ed to 230°C. The ener­gy require­ment for the assem­bly is auto­mat­i­cal­ly con­trolled by the sol­der­ing time. Dur­ing sol­der­ing, the sol­der paste melts, makes the elec­tri­cal con­nec­tion and the flux evap­o­rates. After the assem­bly has cooled and dried, the SMD com­po­nents are per­ma­nent­ly and elec­tri­cal­ly con­nect­ed to the PCB.

THT com­po­nents are sol­dered man­u­al­ly using a sol­der­ing iron or auto­mat­i­cal­ly using a sol­der­ing robot. This robot repro­duces the man­u­al sol­der­ing process by machine and thus achieves very repro­ducible results. Man­u­al work is lim­it­ed to han­dling the assem­bly and thread­ing THT components.

Check­ing the placement/assembly result

Sol­der­ing is not the end of the assem­bly process. In order to con­firm the qual­i­ty pro­duced or to detect defects at an ear­ly stage, an AOI (Auto­mat­ic Opti­cal Inspec­tion) check can be car­ried out. Here, an AOI sys­tem uses sev­er­al cam­era sys­tems to auto­mat­i­cal­ly inspect the print­ed cir­cuit board and com­pares the result with a ref­er­ence that has been rat­ed as good. If there are any devi­a­tions, the oper­a­tor is informed about the exact defect pat­tern, who then cor­rects the defect or rejects the PCB.
Much more impor­tant, how­ev­er, is the elec­tri­cal test of the man­u­fac­tured assem­bly. For this pur­pose, the assem­bly is con­nect­ed to spe­cif­ic test equip­ment and usu­al­ly test­ed automatically.