Folks,

Here are the answers to the SMT IQ Test of a short while ago.

1. What does the “A” in SAC305 stand for?
   ANSWER: SAC stands for tin (Sn), silver (Ag), and copper (Cu). The “305” indicates 3.0 percent by weight silver, 0.5% copper, and the balance (96.5%) tin.
    
2. The belt speed on a reflow oven is 2 cm/s. The PCB with spacing is 36 cm. What is the maximum time that the placement machines must finish placing the components on the PCB to keep up with the reflow oven?
   ANSWER: Time (s) = product length (cm)/belt speed (cm/s) = 36 cm/2 cm/s = 18 sec.
    
3. In mils, what is a typical stencil thickness?
   ANSWER: In range of 4 to 8 mils.
    
4. BTCs are one of the most common components today; a subset of BTCs is the QFN package.
   1. What does BTC stand for? ANSWER: Bottom terminated component
   1. What does QFN stand for? ANSWER: Quad Flat Pack No Leads.
       
5. What is the melting temperature of tin-lead eutectic solder?
   ANSWER: 183° C.
    
6. In mm, what is the finest lead spacing for a PQFP?
   ANSWER: Most common is 0.4 mm. A few have 0.3 mm, but these smaller spacings are hard to process.
    
7. Are solder pastes thixotropic or dilatant?
   ANSWER: Thixotropic; the viscosity of solder paste drops when it is sheared (i.e forced through a stencil). Dilatant materials stiffen when sheared.
8. In stencil printing, what is response to pause?
   ANSWER: When stencil printing is paused, the viscosity of the solder paste can increase; this situation would be considered a poor response to pause. Pastes that have stable viscosities during pausing are considered to have good response to pause.
    
9. For a circular stencil aperture for BGAs or CSPs, what is the minimum area ratio that is acceptable?
   ANSWER: Typically greater than 0.66, although some solder pastes can print well a little lower than this.
    
10. What are the approximate dimensions of a 0201 passive in mils?
    ANSWER: Approximately 20 by 10 mils.